Chapter 10 The detail of the Mona Complex

This will be dealt with region by region, district by district. And, as the detail here given is intended for the most part as a local guide to those who will carry on research in future, matter of a theoretical nature will be avoided as much as possible, or confined at any rate to that which can be inferred with confidence. In other words, in references to tectonics, the major secondary and lesser structures only will be discussed. The views of the present writer as to the primary recumbent structures will be excluded; except in one or two places (the Breakwater tract at Holyhead, for example), where discussion of them in Chapter 8 would have encumbered that chapter with too much local detail.

At the close of Chapter 6 (p. 169) the reader was referred to this chapter for the detailed evidence as to the sequence of the minor sub-divisions of the Complex. A summary of this, with page-references, is therefore given on pp. 383–5.

Holy Isle

This will be described in the following order:

1. Tracts west of the North Stack fault, from north-west to south-east.
2. Country between the two main faults, from Rhoscolyn to Holyhead Mountain.
3. Tract east of the Namarch fault from Tre-gof to the Breakwater coast.

Tracts west of the North Stack Fault

The North Stack is composed of true white quartzite, but good even bedding, dipping southward, can be seen from the sea on the north face, so that it is taken to be the very base of the Quartzite, immediately above the South Stack Series, the fault passing between it and the land (see p. 207). The fissure of the great fault, on the mountain-side above Gigorth Bay (Plate 16), can be got at from above, but fallen blocks roof it, making a sort of cave at the head of the chasm. Through chinks in this roof one can look down and see the breccia, but by going round outside the blocks at the very edge of the cliff (seeing to it carefully not to trust to crumbling foothold) a much better view can be obtained. From the same view-point some ten or more parallel faults can be seen in the great quartzite cliffs, all hading to the north-east, and producing an overhang at the cliff's foot. The south walls of Gigorth Bay are determined by the vertical foliation, and are only strike-sections, but the chasms into which they bend show, something of the structures.

The Stack Moor — The surface of the Stack Moor is composed of the massive beds, which at the rugged western end (E6076) [SH 208 826] are very well exposed. At the cliff's foot, north-west of the tarn, where a buttress runs out, thinner beds appear, showing that the Liwyn beds are beginning to rise. The highest inembers, brought in by the pitch towards the eastern end (E10658) [SH 213 829] are felspathic, decomposing readily. The two little outliers of the Quartzite are taken in on sharp infolds. The western, a double one, forms a conspicuous craggy knob, which rises to the 500-foot contour, and its relations are tolerably well exposed (Figure 28). The eastern one forms only a gentle dome, but immediately to the south of it dips of 60°–90° are seen in the narrow quarry 50 feet below, so its infold must be quite as deep and sharp. They are parted by a fault with a downthrow to tile east, along whose course are the more-said quarry, and a savage narrow gully that runs down to the sea, revealing several folds upon its walls. Scolithus may be seen in abundance [Af. 3681–4] on the crags between the tarn, Goferydd, and the coastguard flagstaff. The burrow-castings have been deformed, and have also been driven somewhat out of their original position (Figure 103) of verticality to the bedding, thus affording a measure of the deformation of the rock.

The South Stack — Rugged as is the summit of the Moor, it gives no hint of the amazing revelation of structures that awaits the geologist who descends to the South Stack. The folding-phenomena Frontispiece (Plate 1) and (Folding Plate 1) have already been studied on p. 183, and need not be re-described here. The folding can be seen in its full grandeur only from a boat (and seldom can a boat be brought round through the tide-race to the foot of these wild cliffs—see p. 183). The northern folds, however, are seen finely from the Stack, the approach to which is by a flight of steps, carried zig-zag down the cliff, at whose foot is a suspension bridge. Permission to cross to the islet can be obtained only from the Honourable Corporation of the Trinity House. Geologists wishing to study the folding should therefore apply to the Secretary, Trinity House, Tower Hill, London, E.C., for a permit, enclosing a note of introduction from an officer of H.M. Geological Survey, or from a member of the Council of the Geological Society of London.

The finest view of the major folding to be had from the Stack is that from the foot of its southern crags. The relations of minor to major folding (Figure 38) can be seen (especially with the aid of a small glass) from the western end of the bridge. The relations of folding to foliation, and also the secondary thrusting with folding of the foliation (Figure 88) are admirably shown (Plate 24) at the outer end of the islet, outside the walls, both north and south of the lighthouse. The Stack is carved out of two large anticlines, visible together only from the sea. It will be seen that the thin Llwyn beds rise on the pitch. In them, low down on the southern escarpments of the Stack, were obtained [Af. 3670–80] the finest specimens of Planolites and the forms referred to Archaeocyathus or Archaeoscyphia. Some of the grits (E10130) [SH 203 823] contain a good deal of tourmaline; and there is a coarse one (E10570) [SH 203 823] full of blue opalescent quartz, and with fragments of old grits (one containing tourmaline), granites, granoblastic rocks, and mica-schist.

But even without crossing to the islet, fine parts of the great section can be seen from the steps. The southern of the two Stack anticlines is disclosed in a chasm that cuts into the south side of the islet, with the incessant ripple of the minor folding running round it. At the last zig-zag above the door, there is a fissure in a crag, through which, as through a window, one of the major synclines, with a vertical amplitude of 330 feet, can be seen sweeping up the cliff. A still better view of the same syncline, with part of an adjacent anticline, can be obtained by getting over the wall and creeping (with careful regard to foothold) along the top of a buttress that looks into the chasm. Curious details of the nature of the bedding (Figure 104) may also be seen in some of the massive grits. The remainder of the great section, from the South Stack to Henborth (except for a glimpse at Pen Las Rock) is visible only from the sea.

The Stack Moor Syncline — Beyond the monument called on the six-inch maps Ellen's Tower', the base of the Stack Moor beds rises once more, and runs inland, by the Cyttiau, to the fault. Referring again to (Folding-Plate 1) (where they are easily distinguished from the thin and flaggy Llwyn beds) we shall see that these beds are sinking into a broad and deep syncline, of which even the tremendous visible folds are but the crumplings. Remembering, further, that their base rises just along the surface of the South Stack, it becomes evident that the whole of these beds of the Stack Moor are taken in on a vast crumpled infold, some 600 feet deep at the coast, which, deepening inland on the pitch, just admits of the survival of the two little infolded outliers of the Quartzite on the crest of the moor, and is then cut off against the North Stack fault at the crags below the summit of Holyhead Mountain. Where it is thus cut off, the Stack Moor syncline must have a depth of 1,200 feet.

By combining together (Folding-Plate 1), (Figure 28), and the one-inch map, the succession, given in the table on p. 164—

Holyhead Quartzite,

Stack Moor Beds,

Llwyn Beds,

becomes perfectly evident.

Henborth and the Llwyn-y-berth Promontory — Returning to the coast, the folding has now become completely isoclinal, and another fine glimpse of it is to be had in the chasm of the great dyke (Figure 36). On the Henborth cliffs, where a group of thrusts appears, the amplitudes are still very great, but the major folds are not yet (save details such as (Figure 105)) well seen from the land, though some of the southern isoclines can be made out from the side of the long promontory. Nevertheless, with these great folds are found the smallest yet detected in the Complex (Plate 21), Fig. 3. Planolites [Af. 3694–98] was obtained in moderate preservation at Henborth beach, but fine specimens, almost as good as those of the South Stack, are tolerably plentiful [Af 792, 3685–93] on the cliffs west-north-west of Gors-goch.

The coast of the Llwyn-y-berth promontory, from Henborth to Porth Dafarch, affords a series of sections, three miles in length and 70 to 100 feet in height, that are unsurpassed in the Mona Complex, and can seldom be surpassed in Great Britain, for the study of isoclinal folding and its relation to foliation. The principles of this have been set forth on pp. 185–9, 201–3, so little detail need be added here save an indication of the most instructive sections. In the south cove of Henborth are fine examples (E10158) [SH 215 815] of the extreme encarsioblastism that gives rise to prismatic fracture (p. 44), and at the creek west of Gors-goch are excellent encarsioblasts that appear as green spots of biotite (E10139) [SH 212 805] (Plate 2), Fig. 1. This cliff is a steep foliation-dip-slope, but, at the headland, pitching isoclines and foliation with changing angle are well seen all together (Figure 48), (Figure 106). Fine sections continue to Penrhyn-mawr, where, looking back north-eastward, the relations of minor to major isoclines, and of the former to vertical bedding-dip, are displayed (Figure 107) in a cliff some 70 feet in height. From Penrhyn-mawr to the next headland is a good deal of the softer greener type (E10133) [SH 212 795], (E10141) [SH 213 796], (E10141) [SH 213 796], the only material in the series that may contain volcanic dust, and the pitch is well displayed in the long chasm at that headland. A little before the small dyke is reached are good structural sections (Figure 108), strain-slip and minimum corrugation being finely seen (E10143) [SH 214 796] in lepidoblastic partings; and on the western side of Porth Ituffydd a thrusting along the foliation produces incipient lenticular isolation of the grits, the same structure as that of the Autoclastic Mélange developed on a regional scale in the Gwna Beds. But the structures for the most part preserve the bedding well, with sharp isoclinal folding in the lepidoblastic partings. At the cove, however, the foliation of the grits, usually rectilinear, is folded sharply (Figure 85), and the soft beds thrust between the foliation-laminae. The folds are then longer limbed and smoother for some way, but on the headland between Clybyddiad and Porth Rhwydan sharp minor isoclines again set in. The structures are splendidly displayed ((Plate 18) and (Figure 89)), and being easy of access, this is perhaps the best locality for a study in detail both of rock-types (E10134) [SH 227 796], (E10142) [SH 227 796], folding, and foliation. From Porth Rhwydan to Porth Dafarch a series of beautiful cliff-sections about 80 feet in height (Figure 37), show strong flaggy grits with lepidoblastic seams, dipping almost vertically, but corrugated by rapid minor isoclhies with moderate inclination, and traversed by foliation generally parallel to the isoclinal axes. Beyond Porth Dafarch a larger fold appears. The North Stack fault emerges in a chasm on the side of Porth y Post. Its fissure is concealed by fallen blocks, but the chasm-wall hides north-east at about 15°. Similar structures are seen on the shallower sections inland, which are good on the slopes of Penrhos Feilw (E6071) [SH 230 806], (E6072) [SH 230 806], (E6073) [SH 230 804], (E6074) [SH 225 811]. The thin coarse grits. may be found a little to the west of Porth y Post, on a steep escarpment some 600 yards south-west of Plâs Meilw (E10135) [SH 224 806] (where they contain a fragment of coarse muscovite-biotite-gneiss), at spots 30 to 100 yards to the south-east, on the cliff-top west of Gors-goch, and at other places.

Rhoscolyn

Passing to Rhoscolyn: The fault at Bwa-du (which is evidently the North Stack fault) passes into a little chasm. Debris hides its breccia, but the north-easterly hide is clear on both sides. In Borth Wen it runs out upon a shallow beach. There are several slight changes of facies at Rhoscolyn. The Quartzite is not quite as hard and siliceous as on Holyhead Mountain, being (E10159) [SH 263 753] more like that of analysis No. 3, on p. 41. It is also rich in zircon and other heavy minerals. Bedding is more often perceptible in it, besides which there is a strong bed of lepidoblastic mica-schist. The South Stack Series is thinner, hardly exceeding 1,600 feet. The flaggy Llwyn portion (E10160) [SH 26 75] is unchanged; but the Stack Moor part is represented only by the massive beds near the Quartzite, and is greatly reduced. At Borth Saint it is bright red and brown, but this is only staining (for which no cause can be assigned at present), as the colour changes rapidly along the same horizon. A coarse grit (E10161) [SH 267 748] is seen at the 'b' of 'Lifeboat', in which are fragments of quartzite of Gwna type.

The section given in (Folding-Plate 2), and quoted on p. 156, in evidence of the succession, was drawn from a boat (which is necessary in order to grasp its true proportions), but the details (from which minor thrusts and faults are omitted) were filled in from the cliffs, which, except under the rugged Rhoscolyn Hill, are accessible most of the way along. From the 'White Arch', south of Bwa-du, called Bwa-gwyn on the six-inch map, to Borth Saint, are excellent sections in the Quartzite, showing the corrugated seam of mica-schist very clearly. On the north cliffs of Borth Saint the Quartzite is faulted against the reddened South Stack Series; and where the steep track descends to the beach the core of the large Borth Saint anticline can be seen. Rhoscolyn Head is a fine section through a major isocline of quartzite, with its thin mica-schist. On the cliffs opposite Maen y Fran, the characters of the South Stack Series are admirably seen. The minor isoclining is most beautiful, and the folding of the vertical foliation of the thick beds (Figure 86) close to the corrugated fissile partings can be better studied here than at any other place. At the two great fault-chasms the minor isoclines can be seen to maintain their direction in spite of the southerly clip as the beds pass below the quartzite. Where the rugged, mountainous hill descends to the sea, the corrugated seam of mica-schist within the Quartzite ((Figure 39), c) is revealed in a wild gully; but the great doubling back of the Quartzite is perceptible only from a boat. Bedding can be made out on the summit of the hill. Where the South Stack Series comes on again there is a small crush, but it leaves the junction intact in part of the cliff. The beds are here clipping at nearly 90° S.E., yet the minor isoclinin of the lepidoblastic seams ((Figure 39), a, b) is unaffected, and retains its persistent inclination and direction. Massive bedding as of the Stack Moor beds prevails next the Quartzite, but the flaggy Llwyn type develops rapidly as we pass to the south-east.

Just before we reach the cove that runs out south-westward from the Lifeboat Station there is a chasm, and on its north-west cliff the passage from the New Harbour Beds to the South Stack Series is laid bare, with the little spilitic tuff-schist just within the New Harbour Beds. It is not so green as usual, the ferro-magnesian matter being densely studded with the granoblastic felspar, which is albite-oligoclase. The band is repeated by the minor isoclines (Figure 31). The islets at Rhoscolyn Beacon are typical New Harbour Beds, thinly foliated and evidently the Celyn portion, folded on beautiful minor isoclines.

The country between the two main faults

The Rhoscolyn Anticline

The coast from Borth Wen to the entrance of the Strait of Holy Isle is a very fine strike section of New Harbour Beds with beautiful pitching minor isoclines, often overdriven at very low angles or even horizontal'. Similar sections abound in the rugged country about Pentre-gwyddel and on the moor that rises between the Strait and Bodior dyke. On a boss 150 yards south-east of Bryn-y-bar one of the quartz-knots with albite contains well-foliated chlorite (E10164) [SH 288 756]. In the Rhoscolyn anticline (which is bisected by the North Stack fault) Llwyn beds of the South Stack Series are well exposed north and east of the alluvium and around the eastern margin. Where the basic band so often . alluded to (pp. 157, 264–7, 270, &c.) appears on the beach of Borth Wen, it is much thicker than usual, and closely resembles, moreover, the spilites of the New Harbour Beds in the Llanfwrog and Amlwch districts, retaining even traces of ellipsoidal structure, and (E10165) [SH 275 751] is highly epidotic, with ternary albite. Everywhere else in Holy Isle the band is to be regarded as a thin spilitic tuff, but here it is evident that a lava was poured out upon the same horizon. The junction (folded) with the green-mica-schist is exposed upon the upper side. Rapidly thinning, and resuming its usual characters, it is to be seen on the bosses between this and the dyke, just within the green-mica-schist, with which it is folded several times. Just to the south of the'a' of 'Ingo' a cottage (Ty-dudur of the six-inch map) stands on the edge of the South Stack Series; and 50 to 1.00 yards southeast of this cottage, those beds are seen within a yard of the green-mica-schists, pitching under them. True to its precise horizon, the basic tuff, here only a foot thick, appears just within the green-mica-schists. The junction of the groups is exposed 170 yards south-east of the cottage, at the foot of the escarpment, and the basic tuff, here six inches thick, lies in the green-mica-schist a yard from the junction.

New Harbour Beds, all, so far as is known, of Celyn type (E6081) [SH 277 760], are also very well seen about Cromlech farm, whence came the partly analysed specimen (E10162) [SH 264 767], and 350 yards to the south is an unusually coarse grit (E10163) [SH 264 763] in which composite fragments may be looked for, though only doubtful quartzites have been identified. Indeed, all about Rhoscolyn, clastic grains are more easily seen than in the corresponding beds at Holyhead.

Rhoscolyn to the Tre-Arddur Gap

Details of the Serpentine-suite of intrusions will be found on pp. 274–7.

Strike and dip, isoclines and pitch, are normal almost as far as the epidosites that behave as a thermal halo to the serpentines, but a deflection begins to be perceptible along Cromlech alluvium and about Bodior, becoming marked in the Fadog promontory, where dips are vertical, as they are in the epidositic zone. Between Cae'r-sais and Four Mile Bridge the strike swings round to northwest. But it is interesting to note that between Cae'r-sais and Porth y Garan, on the north-western side of a curved line that very nearly coincides with the parish boundary (i.e. the dotted line), the normal structures are suddenly resumed, even where the green-mica-schist actually comes against the intrusions. There is no thermal alteration at that junction, and the line (all along which is a sharp escarpment) is undoubtedly a southward overthrust at a low angle, which we have called the Garan Thrust-plane (Folding-Plate 3). The Garegf-lwyd fault which cuts off the intrusions towards the south-west runs up to meet the Gann thrust-plane at an angle of 45°. Thrust, fault, and intrusions leave between them a little triangle of ground, and in this, accordingly, we find the marginal vertical dips and disturbance. The fault seems at first sight to cut off the thrust opposite Porth y Gann, but the features indicate that, executing a turn, the thrust escapes, and runs alongside the fault, only about 25 yards from it, until they both pass out at the cove a quarter of a mile away, where there is a crush at the foot of the escarpment.

From the North Stack fault at Bwa-du to Tre-Arddur Bay the coast is a splendid series of sections in the Celyn part of the New Harbour Beds; and those on the rocky land that extends almost all across the Isle, are scarcely inferior. The Soldier's Point beds (which were not recognised as an horizon at the time this was surveyed) should be looked for by future investigators, flaggy beds having been noted about Castell, as well as in the disturbed ground about Four Mile Bridge and Fadog. To establish either their presence or their absence would throw great light on the nature of the major folding, and perhaps on the form of the intrusions.

About the Tre-Arddur Gap, the Soldier's Point beds (flaggy, and with definite thin grits) have been observed at the coves of Isallt, at Isallt-bach, and (E10282) [SH 260 795] along the road to Tre-gof, especially just where it leaves the alluvium. But they should be sought for also among the alluvium a little to the south. Near Towyn Lodge and Castell the minor isoclines are often quite recumbent. In the vicinity of the Tre-Arddur Gap there are innumerable planes of thrusting (Figure 62), (Figure 63), which are at the same time true foliation-planes, and at the cove west-south-west of Isallt-bach are seen to be folded on the minor isoclines. (Figure 109).

The Gap is undoubtedly a zone of thrusting, and in these planes we see some of the minor thrusts themselves. Albite is found in venous quartz (E10227) [SH 261 798] 117 yards south-east of Tre-Arddur garden, most of it untwinned; and southeast of Towyn Lodge, at the semi-ring of a pitch-arrow, similar albite has a nemablastic foliation (E10228) [SH 259 782].

The country about Holyhead

The coast from Tre-Arddur Bay to the North Stack fault at Porth y Post is another fine series of sections in the Celyn division<ref>If the Soldier's Point beds appear at all, it would be on some of the larger anticlines, but only close to the North Stack fault.</ref> of the New Harbour Beds, which are almost as well exposed inland on the wide tracts that extend, with but little interruption, all the way to Mynydd Celyn. A variety (E10152) [SH 230 810] poor in muscovite, and therefore unusually green from the green biotite on its foliation-planes, occurs north of the Cyttiau. A quartz-albite knot (E10147) [SH 408 823] with foliated chlorite was found 100 yards south-west of Mynydd Celyn farm. Thin haematite-mica-schists (E6078) [SH 236 816], (E10148) [SH 241 803]–(E10149) [SH 232 819], (E10557) [SH 236 816] that may be recrystallised jaspery phyllites occur in the Celyn beds north of the 'rc' of Dafarch', at Mynydd Celyn farmyard, at the foot of the crag a quarter of a mile east (and south) of Tre-Wilmot, and at some other places. Lines have been drawn for the most definite of them. All the characters and structures of the Celyn beds, the foliation-types, the successive generations of segregated quartz with incorporation of the older ones (p. 48), and especially the minor pitching isoclines (Figure 47) are splendidly displayed on the high rugged moor north-west of Mynydd Celyn.

The South Stack Series is represented only by the Llwyn beds as far as Capel Gorllas. They are well exposed at Kingsland on both sides of the road, thence to the great dyke, at and south-west of Stryd, and form a very rugged tract about Capel Gorllas (E10132) [SH 233 822], and are everywhere perfectly typical, the usual fine white-weathering thinnish grits with lepidoblastic partings. Encarsioblastic structure (E10083) [SH 244 813] is well developed at Kingsland. At Capel Gorllas, and thence to Tre-Wilmot the beds become very massive, and must be regarded as the passage to the Stack Moor Group. About 150 yards east of Tre-Wilmot is a thin coarse bed with quarter-inch pebbles of granite and Gwna quartzite, as well as plates of old gneiss-muscovite. Just east of the'a' of Pont Hwfa', close to a tall house with a tower, which is at the end of what is called Gors Avenue, is an interesting greywacke-like grit (E10131) [SH 217 805]. No slide is richer in felspars (chiefly albite) that show the old weathering decay with reconstituted margins (pp. 44, 145, and (Figure 27)). A few of them contain small ancient fresh cores as well, though none so clearly as the one in (E10151) [SH 241 829] (Figure 27). It is also the slide containing an old mica-schist with a large porphyroblast of tourmaline, besides which there are fragments of hommoblastic albite-quartz-mica-schists, though none of these lie across the foliation of the grit.

The country south and west of the town of Holyhead is of great stratigraphical importance, because of its four miles of hardly broken junction between the Celyn part of the New Harbour Beds and the Llwyn division of the South Stack Series. With the unfailing little spilitic tuff-schist always about a yard within the New Harbour Beds (see p. 157), the junction is actually cut across at nine sections, and four more only fail to do so by an interval of three or four paces of covered. ground. Probably the tuff might have been drawn almost continuously along the junction all the way from Tre-fignath to Tre-Wilmot, but as it may be cut out by thrusts here and there, it has been coloured only where actually seen. Perhaps the clearest sections are on some ridges half-a-mile west of Tre-fignath (in ground that is now part of the golf-links). Stronger white-crusted grits come on between the fissile beds, which become at the same time less green. The spilitic tuff, about a yard thick<ref>This beautiful ridge of tuff, the finest in the Isle, has unfortunately been destroyed for the purpose of smoothing the Golf Links. The Committee, however, consented to retain two small exposures, one at each end of the ridge, by which its position can still be identified.</ref>, is just within the New Harbour Beds, and is repeated on 10 minor isoclines. It is (E10556) [SH 251 806] highly epidotic, with chlorite that seems to be after actinolite, and is rich in ternary albite. It appears again, twice, at Bodwradd, the junction of the groups being seen on the crag-face by the dyke-hollow, along which there is local disturbance of the pitch. The Green-mica-schists, with the basic band just within them, pass, folding, under the Llwyn beds. Jasper-like aggregates (E10157) [SH 241 805] occur in the basic band. Two small bosses in a hollow, 125 yards to the north, again show the basic band close to the junction; and so does another, 300 yards to the west of them. Then, on a high boss, overlooking a watercourse, about 160 yards from the road, the section shown in (Figure 30) can be made out. On its escarpments, the passage from green-mica-schist to basic schist is clear and unbroken, though rapid. Thin jasper-like seams (E10145) [SH 219 829] are present, but they are of doubtful nature, as they contain epidote and albite. About 50 yards further on is a lower boss on whose south-west end are two nips of good solid basic schist lying in thin-seamed mica-schist. Then the high boss next the road has two more basic nips on its southern escarpment. The base of the Llwyn beds keeps on zig-zagging across the strike a few yards to the north-east. The basic nips just described are all ranged along the north-eastern side of a deep hollow (into which the water-course has turned) which runs on south-eastward to join the strong dyke-feature, an arrangement clearly indicating a small cross-fault. Accordingly, we find alongside this feature a local disturbance of the otherwise persistent north-easterly pitch.

The junction is again cut through at the crag by the Porth Dafarch road, close to where that road crosses the great dyke. There is a perfectly conformable passage from group to group, seven yards north-east of the turning that leads to Kingsland Windmill; and in spite of a little vegetation, the base of the folded Llwyn beds, pitching off the New Harbour Beds, can be fixed to an inch, where the first hard grits begin. Below them come about a yard or so (when allowance has been made for thickening due to minute thrusting and corrugating) of soft green ashy-looking passage-beds, which are found all over this district. True to its horizon, the spilite-tuff, a foot or so in thickness and rich in granular albite, immediately underlies them. It is repeated by folding in the cliff, and at the top occurs again about four feet below the South Stack Series. The Celyn beds adjacent to it have a purplish tinge, from scales of hoematite, and are very rich in granular albite [A.P. 289], a peculiar type that, round about Holyhead, heralds the appearance of the tuff, into which it rapidly graduates. The junction still zig-zags between the exposures, but is not visible for some distance. Just before it is crossed by the Stryd road, however, is an isolated exposure of the basic tuff, immediately beyond which is a craglet of the South Stack Series, with room for four feet of beds between, showing that the tuff keeps to its horizon. A fault, with downthrow to north-east, now breaks the folded junction. On the downthrow side, 370 yards from the Stryd road, what seem the green passage-beds occur on the top of a boss (Figure 110), thrust on to the South Stack Series, probably the core of a hidden anticline carried a little to the south-east.

A large anticline then rises and runs all the way to the Namarch fault. At its north-eastern end, the foot of a stroll, escarpment of grits just reveals the passage-beds rising on the crest of a minor anticline that fails to bring up the tuff (Figure 111), but shows the junction clearly. South of this, in the middle of the major anticline, a subsidiary syncline is exposed north of the 'S' of Stryd', on a low boss, but a wall running along it has been built just upon the junction. On the north-west side of the wall are the South Stack Series, on the south-east side appear a yard of the green passage-beds, and then, true to its horizon, the basic tuff itself, here a yard in thickness, with the purplish beds upon its other side. South of this, across the hollow, is a long wall, on the other side of which a footpath runs, and at the hither foot the junction of the groups is again seen (Figure 112), a nipped outlier of the grits lying on the green passage-beds, here piled up by packing and foliation-thrusting. The section is a foliation-dip-slope, and the tuff is not seen, though there is room for it under some bushes towards the south-west end. Beyond the footpath, on a low boss, the same junction is once more exposed, but there is no room for the tuff on the exposure. At the western end of the major anticline, between the dyke and the fault, a steep escarpment, 50 feet in height, looks down upon a little farm (Cae-allt-wen of the six-inch map). It is a complete section through the junction, the South Stack Series forming its brow and the Green-mica-schists its foot, with the passage-beds and the tuff between; but is greatly complicated by folding, which is shown, considerably simplified, in (Figure 29). A purple phyllite (E10144) [SH 232 820] occurs in the zone of passage here. About 150 yards northwestward along the fault, a line of crags (at whose foot is a marshy tarn) evidently a strike-fault, is seen to be cut off by it; and between this fault and the South Stack Series, towards Tre-Wilmot, eight small infolded outliers of the spilitic tuff (E10146) [SH 232 820] have been mapped. The probable structure is shown in (Figure 113). The suite of sections at Holyhead and Rhoscolyn demonstrate the remarkable precision of the horizon indicated by this thin spilitic tuff-schist, which is comparable with that of a graptolitic zone.

Holyhead Mountain

Beyond the grits of Capel Gorllas and Tre-Wilmot a valley, choked with boulder-clay, runs along the foot of Holyhead Mountain, and must be occupied by the outcrop of the Stack Moor beds. These are exposed alongside the road, north of the 'o' of 'Reservoirs' by a chapel, and (less clearly) at the foot of the green drive that goes up the hill near the 55° dip-arrow. They-are felspathic, with lepidoblastic mica-seams, very massive, typical Stack Moor beds in fact, and (by the chapel) are seen to graduate up conformably into the base of the Quartzite.

No rock in Anglesey is laid bare in such a series of exposures as the Holyhead Quartzite. Its coast is uninterrupted rugged cliff, which under the Old Telegraph Station is 500 feet in height (Plate 16), the greatest cliff in Anglesey; and where the North Stack fault crosses the mountain, the Quartzite looks down upon the South Stack Moors in a line of crag of nearly 200 feet in places, a feature that is conspicuous for many miles; besides which the great quarries from which the Breakwater was made have cut into it on a scale second only to that of nature; and the surface of the mountain, 720 feet in height, is nearly bare of cover. The whole is a solid mass of quartzite, interrupted only by a foot or so of white mica-schist (like the seam at Rhoscolyn) at the north end of the old quarry that is east of the large dyke, and by a crushed nip of similar schist in the quarry above Porth Namarch. The massive unfoliated type (E10127) [SH 220 832] (in which little scarlet grains are frequent) is well seen all along the southern crags above the road. The pebbly quartzite (E10128) [SH 218 825]–(E10129) [SH 218 825], comprising five slides] is found about 140 yards south-south-west of Twr, on the face of the spur just above the road, as well as about 50 yards to the north-west of this crag. It contains good fragments of scarlet Gwna jasper, with the characteristic haematite-mottling, (though no good ones were cut through in the slides), of micacised igneous rocks, old granoblastic rocks, and mica-schists with original foliation. The foliated, which is really the normal, quartzite (E6068) [SH 217 826], (E6068) [SH 217 826], (E6068) [SH 217 826], (E10126) [SH 226 834], (E10600) [SH 227 832] may be studied almost anywhere, but specimens are best obtained in the great quarries, where the internal whiteness is remarkable. Objects that may possibly be Scolithus occur on the 400-foot contour, 150 yards south-south-west of where the small dyke is cut through in the large quarry.

No bedding has been made out upon the surface of the mountain, or in the quarries (for some planes dipping at moderate angles in both directions there appear to be due to movement). Undoubted bedding, however, is visible over the vast caverns at the North Stack. (Figure 114) was drawn from the lifeboat, but the section can be seen by looking southwards from the Signal Station. In a favourable light, looking from near Phis Nico, planes that seem to be bedding, dipping at various angles, may be made out on the great fault crags that cross the mountain. And if we stand on the north side of the moorland, somewhat east of the tarn, we shall have no doubt (especially if the light be good) of true bedding on the great cliffs (Plate 16) between the-North Stack and the old Telegraph Station. But the section (Figure 91) is the result of several re-drawings in different lights, and its dotted line parts are conjectural. The foliation is vertical over most of the mountain, but a slight fan-like divergence can be made out. Sills of venous quartz often lie along it. There is a good deal of slight undulation horizontally, with many small cross-wrenches. If the section be correct, the base must be about to rise at the North Stack Signal Station, so that the bedding there seen is probably a herald of this.

District north-east of the Namarch Fault

Stanley Gate to Soldier's Point

Except in the Breakwater tract, only Soldier's Point beds are known. From where the fault runs out south of Tre-gof to Gorsedd-y-Penrhyn the structures are not pronounced, and there is a local reversal of the pitch. But along the low cliffs north-east of Penrhos house is a curious exception to the general structural law — a local foliation-dip to south-east on unfolded parts, between which are minor isoclines that face in the usual direction (Figure 115) and are thus in conflict with the dip. The normal relations are resumed on the Llanfawr coast.

Fine sections with isoclinal folding in flaggy beds are seen on the crags by the roadside below the Monument quite near the station. The banding is well marked: only a slight change of facies would give the Garn type, and a moderate one (especially if less foliated) the Amlwch type. There are a few thin white saccharoid quartz-schist seams. The sections on Salt Island are on its western side; the beautiful structures already described (p. 192 and (Plate 20)) being seen about 100 yards north of the bridge that leads the road and railway to the mail-boat pier, on a stack of the foreshore that is isolated for a while at high water. From a few yards away only isoclinal folding catches the eye, but the peculiar strain-slip appears on approaching near, in fissile bands between horizontal hard ones, especially at the foot of the stack on its outer side, and perhaps even better at the summit. The rock is beautifully crystalline, and the lepidoblastic seams are encarsioblastic, with a great development of green biotite, and a good deal of apatite. The sharply curved quartz-seams appear to be later than the folding (E10154) [SH 252 830].

Some of the grits are unusually coarse (E10155) [SH 252 830] and contain clastic microcline as well as albite. On the foreshore opposite the Coastguard Station ('C.G.S.') is a very fine display of isoclinal folding; and, a few yards further, below the house called on the six-inch maps New Harbour View, are the coarsest grits known in the Green-mica-schists (E10156) [SH 249 834], with small rolled fragments of quartzite of Gwna type, and salmon-tinted felsitic-looking scraps like those common in the Skerries Grits, drawn out along the foliation.

The Coastguard shore is the only place in these beds where planes that suggest a second foliation in hard beds have been seen, dipping at a lower angle than the dominant planes. Fine sections begin again at the Trinity Stores (the large building on the shore below Porth-y-felin), and isoclinal folding, sometimes with a strain-slip foliation in the lepidoblastic (not noted in the hard granoblastic) seams (Figure 116), may be seen anywhere as far as Soldier's Point. The Soldier's Point beds are also finely seen in the roadway-cutting at Government House, where they are unusually massive. In the cutting on the upper road, just after it has been carried over the curved embankment, is perhaps the finest display of minor isoclines in the whole Complex, beautifully smooth, with amplitudes of about six to ten feet. A few of them are shown in (Figure 44). The thin seams of saccharoid quartz are well seen also. The rocky land between the town and the Breakwater railway shows the same beds very well, the type specimens that were analysed (E10151) [SH 241 829], cf. (E10282) [SH 260 795] having come from a little north-west of the new church with the spire (called St. Seiriol's on the six-inch maps). It will be remembered that (E10151) [SH 241 829] is the specimen containing piedmontite, which should therefore be further sought for in the lepidoblastic bands at this place. It also shows very well the ancient stages of decomposition in clastic albite with reconstituted margins, and one ancient undecayed core as well (Figure 27) (Plate 2), Fig. 2. The ground whence it came is now being formed into a public park for Holyhead, and the topography of the six-inch map will be altered. The most massive beds known in the Soldier's Point beds, a yard or more in thickness, are to be seen at and about Porth-y-felin Mill (E10150) [SH 235 830]. They contain small fragments of granite, and large twinned albites that show the cores of old sub-aerial decay and reconstituted margins very well. Two hundred yards north-northeast of the Mill the rock (E10153) [SH 235 833] has a bluish tint, but the source of this is not apparent in thin section. The folds near Porth-y-felin have long horizontal and short inclined limbs (Figure 53), so that the strike often seems indefinite for some yards of outcrop.

The Breakwater Coast

The Celyn beds (E6077) [SH 250 842], cut down to only about 70 feet, are seen with beautiful isoclinal folding (Plate 19) and (Figure 46) on the bosses that are partly built into the western face of the Breakwater, just outside the cove, and at the cove itself the little spilitic tuff appears in them, here only a few inches thick, and (E10555) [SH 236 838] rich in ternary albite. Beyond the Celyn beds thin-bedded greenish grits of South Stack (Llwyn) type, of which there is not room for more than about 80 feet, appear on the beach; and, then, at the beach's end, are typical felspathic Stack Moor beds, which may reach 280 feet, and range westwards all the way to Porth Namarch. They are often blue-hearted, and close to the Namarch fault are a good deal reddened. The bedding is distinct, though sometimes massive, and there are some lepidoblastic seams. At Porth Namarch a rusty bed on the foreshore contains pebbles one-third of an inch across of a quartzite of Gwna type. At the Trinity Magazine, just corinmi brid oin over the railway, is a rude conglomerate, containing rolled fragments of Gwna quartzite, mica-schist, and paragonitised albite (E10137) [SH 229 834]. It is thoroughly schistose, with bedded partings of corrugated mica-schist, and belongs without doubt to the South Stack Series. Scolithus is abundant at the Breakwater cove, and on the coast 300 to 400 yards east of the Namarch fault [Af. 791, 3662–9]. The beds dip for the most part at moderate angles to the south-east, occasionally rolling over northward. Yet between the great dyke and the Namarch fault, and also just north of the Breakwater cove (Figure 117), the fissile partings are vigorously folded in sharp isoclines with a strong southward impulse, close to which the foliation of the massive beds becomes likewise folded. The dynamics are therefore the same as those opposite the South Stack, and among the southerly dips at Rhoscolyn (pp. 186, 207). The Quartzite is neither quite as white internally, nor quite as massive as it is on the Mountain, or even at Rhoscolyn, so that there is a slight change of facies. Yet it is easily distinguishable from the Stack Moor beds, which are blue internally and well bedded, though the change from the one to the other comes on almost imperceptibly, especially where the vertical foliation masks the bedding of the passage rocks, as is well seen on the coast south of the 'e' of 'Wellt.

The Quartzite rises at a nook beyond the Breakwater cove, and forms Ynys Wellt Point, where it is cut off by the Wellt fault. It rises again 250 yards to the west-south-west, and runs, underneath the conglomerate, as far as another fault. But the South Stack Series has, in the meantime, risen from below this quartzite along the cliff, so as to both underlie and overlie a cake of it (Figure 93). A massive rock rises beyond the fault, and runs to the dyke, but does not seem clean enough for the Quartzite. Beyond the dyke, however, quartzite appears once more, but ig cut off by a small fault, so that it fails to run on to the Namarch fault. It rests upon the South Stack Series, and they appear also above it just before it is cut off (Figure 118), (Figure 119). There is even a suspicion of it below them on the buttress against the Na-march fault, north of the beach (Plate 17). The masses of quartzite along this coast vary a good deal in thickness, the one to the north of the 'Magazine' conglomerate being about 270 feet, that west of the dyke only some 30 feet, so that one at any rate of the junctions (apparently the lower one) must be ruptured. The rupture is laid bare in the cliffs north of the western bridge, and also some 70 yards east of the Namarch fault, where it behaves as a southward overthrust (Figure 120). This must be one of the primary thrust-planes ancillary to the Breakwafer fold itself.<ref>References to the general theory of the Tectonics that was set forth on pp. 171–82 have been (see p. 256) excluded from the descriptions of the Detail. An exception to that rule has been admitted here, as it is almost impossible to discuss the sections along the Breakwater coast without it.</ref> Those which cut out all but some 70 feet each of the Celyn and Llwyn beds are not exposed, but their displacement by the Went fault (which is a downthrow to the west) shows that they must have a southward inclination. This could not be the case with secondary thrusts cutting the whole Breakwater core and driving it up. They must therefore also be primary thrusts, dipping with the axis of the folded Breakwater core, which must be rising on the south limb of a secondary major anticline, whose northern limb has been destroyed by the sea.

The Serpentine-Suite about the Strait Of Holy Isle<ref>In order to keep the descriptions of these rocks all together, those on the main Island will be given here as well as those on Holy Isle.</ref>

1 Mainland and islets

Four-Mile-Bridge to Cymyran

In the large intrusion east of Four-Mile-Bridge the gabbro is well exposed in its high western boss (E10644) [SH 281 786], which has been quarried, and in many bosses throughout its area. It is not severely deformed save along certain lines, especially on the margin west of the dam, but contains compact seams that may be cataclastic. The serpentine is not well exposed, except on the islets at the south end, where it is schistose. North and south of the high gabbro boss it is spherulitic, but the northern exposure is now rather poor. Tremolite-schist occurs on the north shore of the large islet. The islet south of the channel is composed of green-mica-schist, ophicalcite, gabbro, and serpentine. Serpentine is driven on to mica-schist on the shore south of the high gabbro boss, and some, crushed pyroxenite, is seen close by.

The gabbro of the next large intrusion is well exposed west of the church, and is locally schistose. Where a tongue of it runs into a boss of serpentine by the east-and-west lane, highly crystalline tremolite-schist (E10319) [SH 295 809] is found at the junction. The ophicalcites (which are complex) and serpentines are but moderately exposed.

On the north edge of the Felin-wen intrusion is a little pyroxenite, and the serpentine is very schistose. On its south margin the New Harbour Beds are slightly hornfelsed. At the junction, and about 20 yards to the south, is a little tremolite-schist; and then follows a complicated zone of schistose gabbro, serpentine, epidotic schist, and ophicalcite, sonic 20 yards wide; beyond which thin bands of tremolitic and epidotic rocks appear in the New Harbour Beds, doubtless the recrystallised edges of sheared basic masses, and at the little headland is much epidote-hornfels (E10320). Islets of gabbro lie off the shore.

The most southerly of the intrusions is the gabbro at Cymyran (E10321) largely converted into an epidote-amphibolite; but if a mass of gabbro-schist on the beach east of 'Bay' be in situ there are some still further south. The gabbro of Carnau contains, at the 's' of 'Reservoir', the largest plates of diallage yet seen. Epidosite is found among the blown sand west of Harlech cottage.

The Lakes

At the north end of Llyn Dinam the serpentine (which is schistose) is poorly, the gabbro well exposed. At the islet west of 'Llyn' it is massive, though schistose on the western side. A little reef by the alluvium between this and Dinam farm is composed of gabbro and fine, banded, epidote-hornfels (E10292) [SH 309 778], and the junction is well exposed, the gabbro behaving as a sill, slightly transgressing the banding of the hornfels. There is no chilled selvage, but the gabbro at the junction is a rather fine schist. Between the lakes, and on the island in Llyn Penrhyn, the gabbro, locally schistose, is finely exposed. The basic schist north of the stream that comes out of Llyn Dinam seems to. be a deformed spilite. The serpentine associated with this gabbro is mainly schistose: chlorite-rock occurs close to it by Llyn Penrhyn, south of the stream; and a wedge of siliceous hornfels occurs in a nook between gabbro and serpentine by the lake-edge. The chief interest of the tract by the railway south of Treflesg, apart from a variegated ophicalcite, and a small chlorite-chromite-magnetite-rock north-west of the cottage on the promontory in the alluvium, is the schistosity of the gabbro. On the western side of the promontory, near the parish boundary, this attains a maximum, the gabbro being converted into a fissile epidotic pale-amphibole-schist (E10295) [SH 307 768] (Plate 9), Fig. 4 with minute corrugations.

Porth-delisc

The exposures are all on the foreshore, and can be studied only at low water. At the north end, a sill of highly altered gabbro (E10486) [SH 280 845] lies between calcareous mica-schists. The pyroxene is represented by large plates of pale hornblende, the felspar by saussurite, needles of actinolite also streaming along the foliation. A similar gabbro is to be seen in the middle of the little bay, but more interesting here is a beautiful talcose calcite-marble (E10487) [SH 281 844] containing chromite. Another like it (E10488) [SH 283 850], (E10501) [SH 285 847] at Cliperau is richer in talc, and the carbonate is either dolomite or magnesite.

The Strait Islets

Three of the islets between the bridges are composed of gabbro, a little hornfels adjoining it on those near Tre-gof. On the islet north of the Cyttiau, some of the gabbro is a fissile schist. With these islets may be placed the peninsula west of Four-Mile-Bridge, on whose shore, by the 'B', a bright green antigorite (E11388) [SH 272 783] may be seen, as if serpentine were near.

Holy Isle

Many parts of the large intrusion are so complex that they can be shown only on the .0004 (' 25-inch') scale. Its boundary to the south-west is clearly a fault, called the Gareg-lwyd fault, the foliation of the New Harbour Group striking at that of the serpentine. Towards the north-west, where there is a strong escarpment for about half a mile, the New Harbour Beds are in their usual condition, and are undoubtedly driven over the intrusions on the Garan thrust-plane (pp. 208, 263), Along its other margins, especially to the north and south of are hornfels, and epidosite (in which are outlying strips of serpentine and gabbro), and it is clear that these are the original intrusive margins, cut out locally by ruptures.

Between the Strait and the Alluvium

Where the basic rocks first appear on the shore of the channel south of Four-Mile-Bridge, there is evidently a thrust, for the adjacent schist is normal while the basic rocks are schistose, and so carded together that there is room for some thirteen bands of gabbro, serpentine, and calcareous chloritic-schist along the little creek. At its northern point, some spherulitic serpentine (with a diallagic variety) is to be seen on the beach. On the islet off the mouth of Rhyd-bont (large) creek is enstatite-pyroxenite, chiefly at the south end and on the western beach, apparently veining serpentine. On the shore opposite this islet, pyroxenes appear in the serpentine itself; and along the north shores of Rhyd-bont creek veins of both pyroxenites as well as websterite keep on appearing. South of Rhyd-bont the serpentine itself contains the great plates of diallage, enstatite, and bastite mentioned on p. 100, the coarsest rock known among these intrusions. Between Rhyd-bont and the creek the serpentine is well exposed, often with good massive cores. Some of the most richly spherulitic serpentine is found on the eastern side of the road, 170 yards south of the School (E6423) [SH 275 776]<ref>The best slides of the spherulitic serpentine, however, are those cut by Prof. Bonney and Miss Raisin, and figured in their paper.</ref> but only for a space of some 10 square yards. A complex ophicalcite with tremolite and chromite (E10215) [SH 290 793] occurs about 80 yards further south.

The tract beyond the creek (Figure 121) is extremely complex, even the high gabbro boss (E10645) [SH 280 773] (E10646) [SH 280 773] containing four tongues of serpentine, while 233 yards south-east of the dam a half-inch vein of tremolite cuts a serpentine, the fibres being set across the vein. Many strips of modified New Harbour Beds occur also as inclusions. East of the dam, the shore for 150 yards consists largely of hornfels (E9759) [SH 278 773], (E10222) [SH 278 771] with needles of actinolite: and there is a small chlorite-rock adjoining serpentine. Tremolite-schist occurs on the creek-ward face of the lofty boss, at a junction of serpentine with gabbro; and four other occurrences exist in the field south-east of that boss. Brucite (see p. 104) is found in a talcose rock on the islet north-east of the lofty boss [A.P. 281]. The enstatite-gabbro described occurs as a dyke in serpentine on the shore between the lofty boss and the little farm to the east of it. The remaining details of this tract, whose basic rocks are usually more or less deformed, could not be described without reference to the .0004 maps.

The small outlying masses about Fadog are tolerably normal, and the serpentine 150 yards east of the farm is unusually massive. The islets in the creek are mostly serpentine. The chief subjects of interest here, however, are the metamorphic rocks. A fine white tremolite-schist occurs where Fadog lane comes down to the creek, at a junction of serpentine with gabbro, and trcmolite-epidote-schist (E10294) [SH 285 769] on a reef 600 yards east of Fadog. The reefs on the south side of the inner reach of the creek are dark, platy actinolite-schist (E10218) [SH 288 770], (E10219) [SH 288 770], (E10220) [SH 286 770], probably at a junction of New Harbour Beds and serpentine; and 465 yards west-south-west of the house a similar schist with magnetite octahedra lies against a gabbro. In the last field of the promontory are little radial groups of actinolite.

West of the Alluvium

Passing west of the alluvium spherulitic serpentine is recorded by Prof. Bonney and Miss Raisin a few yards east of Pwll-pillo. The long curving ophicalcite along the northern margin is not well exposed except at an escarpment 140 yards from the School, where a foliated white quartz-chlorite-marble dips under green-mica-schist. Between this and the road south of Cae'r Sais the New Harbour Beds are baked. West of this road the concentric disnnsifinn of the vabbros (E6424) [SH 269 774] is remarkable (Figure 122), and they are unusually free from deformation, especially at the high boss by the lane (whence came the specimen analysed (E10213) [SH 270 772]), veins in which contain one-inch plates of diallage. The crescent of gabbro north-west of this ring has a nook on its outer side (north of the 'w' of 'Pwll'), and in this nook, serpentine being close by, an eight-foot wedge of tremolite-schist runs into the gabbro. This is the beautiful tremolite-schist that was analysed (E6425) [SH 267 773], (E10226) [SH 267 773] (Plate 9), Fig. 5. About 130 yards to the west-south-west is an obscure actinolitic rock associated with an inclusion of New Harbour Beds of which it seems to be a thermal modification. About Cerig-moelion (six-inch map, the farm west of the 'P' of 'Pwll' on one-inch map) the serpentine (E6422) [SH 269 772], (E9899) is well-preserved in places, with spherulites a few yards to the north and east-north-east, and diallage a little to the south.

The beautiful calcium-ophicalcite east of Cerig-moelion analysed [(E10214) [SH 266 773] and analysis] is a thick band about 110 yards in length, striking west-north-west to east-south-east conformably to the local foliation of the serpentine, tongues of which run into it. On the south-west it passes into calcareous serpentine, but is sharply bounded on the north-east. It is exposed in three natural bosses, and there is a fine section at the old marble quarry in the middle one, where it has a massive slabby structure dipping steeply to the northeast. On the north-western boss (E10216) [SH 26 79] it is a white rock with crystals of what seem to be diopside. At the top of the quarry it is cut in all directions by innumerable veins of pale actinolite whose needles are set at right angles to the vein-face. As these veins are later than the carbonate (from which they have evidently subtracted the magnesium), it is clear (see p. 106) that the ophicalcites belong to the period of the Mona Complex.

The large limestone south of Cerig-moelion is the dark hard one that was analysed (E10217) [SH 265 770], south of which again is a reddish ophicalcite with magnetite, and on the boss east of this a foliated serpentine is folded. East of these limestones are extensive exposures of good serpentine. A few yards to the south-east of Gareg-lwyd house, in serpentine but close to a small gabbro, is the beautiful tremolite-rock (E11392) [SH 261 770], and the tremolite-marble (E10563) [SH 260 770] that was analysed. The exposure is inconspicuous.

The Southern Margin

Along the southern margin of the great serpentine, from the Gareg-lwyd fault as far as Bodior woods, ranges the zone of epidosite described on pp. 108–9 (E9752) [SH 270 768], (E9753) [SH 270 768], (E9754) [SH 267 768], (E9755) [SH 264 768], (E9756) [SH 270 768], (E9757) [SH 272 769], (E9758) [SH 272 769], (E9761) [SH 272 767], (E9763) [SH 265 768], (E9764), (E10166) [SH 269 767], (E10167) [SH 29 76], (E10221) (Plate 9), Fig. 6. It is full of minor intrusions of serpentine (in the largest of which a foliated modification (E9762) [SH 271 768] is very sharply folded), gabbro, and chlorite-chromite-magnetite-rock; and, as the epidosite itself varies rapidly, its details cannot well be described without the .0004 and six-inch maps. It is best studied in the tract that is nearly enclosed by the large alluvia. The more normal epidosites are best seen on the high bosses east of the Rhoscolyn road; and the actinolite-epidosites by the alluvium west of the 'D', as well as on the crags just west of the great dyke nearly north of Cromlech farm. At the great bend of the alluvium, near the large 'D', some epidotised massive rocks that retain traces of igneous texture occur in the zone (E9760) [SH 272 767]. Quartz occurs in them only as veinlets in the larger epidote aggregates, between which are wisps of a pale amphibole, and west of the serpentine they seem to pass marginally into sharply contorted seams of the same amphibole (E10221). There is a suspicion that they were in some degree deformed before the epidotisation, and if so they must be older than the serpentines.

The typical talc-schist (E10168) [SH 269 767] and chlorite-chromite-magnetite-rock (both analysed) are exposed in a quarry facing south near the eight-foot level 250 yards north of Bronddel (near Plâs-coch of the six-inch maps), overlooking a little channel of alluvium. The chlorite-chromite-magnetite-rock appears to have ellipsoidal structure, but some broken ellipsoids show 'cores' of hard epidosite, and may possibly be the fronts of anticlinal folds. White tremolite-schist, in which are later needles of actinolite, occurs along with them. The quarry is on the outer margin of the epidosite zone, and the still clastic epidosites occur on the platform just above it.

A feature of all these intrusions is the way in which they have interfered with the dynamics of the district. Away from them, the overfolding-dip, strike, and pitch are wonderfully steady all over Holy Isle; but, round about them, are quite irregular. The strike of the igneous masses themselves, and of their own planes of schistosity, partakes of the same irregularity, especially about the gabbro rings and crescents near Cerig-moelion.

Western Region

The New Harbour Group

The Sea-Board

The coast of this district is low, and the section interrupted' by superficial deposits in many places. From the Cymyran gabbrol to the Carnau water, where a branch of the Na march fault probably runs in, the Green-mica-schist is tolerably normal both in character and structures. Opposite Ynys-Els, and on the adjacent bosses, it is rough and quartzose, and contains many basic bands quite unlike the gabbros. They are albite-chlorite-epidote-schists (E10310) [SH 297 763] and probably belong to the spilitic suite. At the little headland north of Ynys-Lâs one of them contains an inclusion of bedded jasper, so they may have been albite-diabases, a type not yet found undeformed in this series. A good many purple phyllites follow, but this reach is full of exceptional types due to the serpentines. From the creek to the damn the mica-schist is fairly normal, but with variable dip and pitch, as is usual near these intrusions. This is the case also about Four-Mile-Bridge and Valley, though the north-westerly dip is there still dominant. About the station and village thin hard grits are frequent, foreshadowing the Amlwch facies. The New Harbour Beds are, indeed, so far as is known, represented only by the Soldier's Point division throughout this region.

The important section along the foreshore from the Embankment to the Alaw's mouth has already been partly described in connexion with the change of structure (p. 196, (Figure 71)). The nature of the rocks is also important. The green-mica-schist is in much the same condition as in Holy Isle, but purple scams appear in it in a few yards, increase and thicken, till, half a mile from the Embankment is the best section in jaspery phyllite of the region. Thin bands of hard pale jasper are also present, and a basic rock is visible at low water. On Traeth-y-gribin sand similar beds reappear, with a spilite at the north end of the reefs. A similar lava, with good surviving ellipsoids, is exposed at Bodlasan.

Rock is next seen at Penial, and is a fine mica-schist at the south end of the section under the drumlin. At the north end is a compact rock with wriggling veinlets (E10491) [SH 291 832]_. like the Church Bay Tuff. It contains clastic felspar and quartz, with small sheared lapilli, is full also of epidote, sometimes almost crypto-crystalline, but in general more reconstructed than the Tuffs of Church Bay. There is a clear passage from its schistose margin into the mica-schist.

The serpentine-suite of intrusions has been described along with those of Holy Isle (pp. 272–4). But whether this Penial tuff be a nip of the main Skerries Group, or a volcanic band in the New Harbour Group, is not yet known. Crystallisation then waxes, until, in the little cove beyond Porth Penrhyn-mawr, a pale rose-coloured schist, brilliant with white mica and haematite, is as highly reconstructed as anything at Holyhead. Just beyond this, at the south end of Porth-delisc, is a recrystallised coarse quartz-albite grit (E10495) [SH 281 843].

The basic bands in Porth-delisc ('the sea-weed cove') are evidently the sheared ends of the large ellipsoidal variolitic spilite which is well exposed about the farm west of Plâs-y-glyn. Probably the best section in the spilitic lavas of the region is in the mass north of Llanfwrog Church, a quarry just west of a roadside chapel, close to the ring of a glaciation-arrow (E10474) [SH 301 844], (E10545) [SH 301 844]–(E10546) [SH 301 844]. All the characters of the New Harbour spilites can be studied here. At the north end of Porth-delisc there is a beautiful calc-mica-schist (E10494) [SH 280 845] after which tolerably normal types, with variable dips, and some bedded jasper; recur as far as the remarkable little complex of Cliperau. There is not space in this volume to describe the intricate and interesting details of this group. It may be summarised as a flow or flows of ellipsoidal spilite (E10478) [SH 283 850], beautifully variolitic in parts, resting on flaggy calcareous chloritic schists which graduate into the ordinary New Harbour Beds. These green schists (E10499) [SH 289 852]–(E10500) [SH 285 849] are full of albite and are evidently reconstructed spilitic tuffs. The spilitic lavas have been sheared and their internal textures destroyed even in the hard ellipsoids, while the dark inter-ellipsoidal skins have passed into fissile chloritic schists which have begun to fold. As well as the spilitic suite, there is a beautiful talc-marble (p. 105), so the peridotite-intrusions here must have been precisely on the horizon of the spilitic lavas.

At the north end of Porth-tywyn-mawr, and thence to Porth-y-defaid, are silvery albite-green-mica-schists with fissile partings like the Soldier's Point beds of Holyhead, and containing a little opalescent quartz. They are overfolded from north-north-west, but the longer limbs and strain-slips are at very low angles. Over-riding these are later thrusts at angles not quite so low, coming from the south-east (Figure 123), which belong to the movements that soon dominate the Western Region. Close to the junction with the Church Bay Tuffs is an epidotised igneous rock (E1544) [SH 289 857]. It is distinct from the Palaeozoic dyke; and probably belongs to the spilitic suite. The foreshore section is complicated both by thrusting and cross-faulting, which confuse much of the junction with the Tuffs.

The interior

The description of the interior will be best begun at Valley. From the alluvium to the Treflesg road there are fine exposures on the heights that overlook the railway from the east; and opposite the church are the remarkable structures described on pp. 197–8 (Figure 72), (Figure 73), (Figure 74), (Figure 75). The nature of the material is also of importance, for here in the Soldier's Point division of the New Harbour Beds appear the thin hard grits which become so marked a feature in the Amlwch facies.

About Caergeiliog Windmill is a considerable development of the bedded jaspers, with which are basic epidotic schists, evidently the spilitic lavas highly deformed. A similar group recurs at Llanfihangel-yn-nhowyn. Between Caergeiliog and Bodedern the lavas are less deformed, retaining traces of their ellipsoids. The large Palaeozoic dykes contrast interestingly with them. About Llanynghenedl and Bodedern exposures are scanty, but enough to show that the same types continue. The spilite of Ty'n-llan, Llanllibio (E10476) [SH 330 818] is variolitic, with remains of ellipsoids, but highly amphibolised. About Llywenan the flaggy mica-schist (E10489) [SH 338 818], though fine, is well crystallised, sometimes with enearsioblastic seams; and an apparent falling off in crystallisation at Presaddfed is not really such. The folding in this district (Figure 124) is usually on vertical axes, but good over-folding from the north, with lineation at right angles to the strike, which rolls over on the folds, is well seen on the little moor north of the Woollen Factory. At Penllyn, and still better on the high knobs three-eighths of a mile to the north-north-east, the same phenomenon as that by The railway is to be seen, the old folding surviving, cut by later strain-slip, this folded in its turn, sometimes isoclinally (Figure 125). These planes are really the dominant foliation of the district, dipping for the most part south-east, the dip being, however, not plane but corrugated (Figure 126), and the strike yery variable all over the Llanfachraeth and Bodedern areas. The folding must therefore be regarded as a corrugated strain-slip. Thence to the Tomb of Bronwen the same types are scantily exposed, in positions that may be found by the drift-lines.

Good exposures are frequent all along the Vale of Alaw. At Mynydd-yr-eithin is an unusually massive grit, with seams of purple phyllite. The large spilite of Llanfigael (E10476) [SH 330 818] is best exposed east of the church, where it is very pale, and highly amphibolised. At its south-west end, in the curve of the alluvium by Erw-goch, it is schistose, and interdigitates with mica-schist.

In a smaller spilite (E10477) [SH 335 832] on the north bank of the river, west of the 'f' of 'Afon', ellipsoids, with little white spherulites, are unusually well preserved, but even this one is amphibolised, though the felsnar-brushes have survived as at Llanfwrog. Thence to Treffynnon, the mica-schist is well seen along the little rocky ravines. At a steep islet in alluvium north of the n' a thin quartz-seam which passes across the foliation at an angle of about 30°, and is nearly vertical, is (Figure 127) cut and shifted by a series of little thrusts from the east-south-east. Yet these do not shatter the rock, which is 'healed up' along them, so they are movements of the Complex. On a southern slope 633 yards south-west of Llantrisant old church is a small epidote-actinolite-schist (E10490) [SH 343 842] with seams of clear granoblastic ternary albite, evidently a highly reconstructed spilitic lava. Along these reaches of the river are many bands of jaspery phyllite, which at the old mill east of the Smithy are lustrous haematite-mica-schists. Purplish mica-schist, folded as in (Figure 128), is well exposed also at Meiriogen, and along the south side of the boundary at Brwynog this alternates with what seems to be re-crystallised Church Bay Tuff, so that the zone of passage is evidently reached (pp. 159, 282, 286). East of Glan-alaw there is a bend in the strike, which brings the Mona Complex into pseudo-conformity with the Arenig conglomerate. Returning sea-wards, the massive pale epidosites east of Llanddeusant Church closely resemble the Church Bay Tuffs and are probably a nip of them. By the crossroads at the south-west end of the village strain-slip can be seen, though less distinctly than at Penllyn and the railway. The large masses of spilite along the Llyn-on Water are best exposed north of Llyn-on Hall, at Bryn-palma, and at Pen-yr-argae. All are more or less schistose. Finally, it is noteworthy that the contents of the Arenig conglomerates of Bod-Deiniol show that a tract of much less altered Gwna Beds must come on to the east, under the great Ordovician syncline of Cors y Bol.

The reef called Ynys Feirig contains many small nips of basic schist that probably belong to the spilitic suite. The outermost reefs at Rhosneigr consist of green and purple schists, rising from beneath the Arenig conglomerate. They also contain basic bands, and appear to belong to the New Harbour Group.

The Gwna Group

Every member of the Gwna Group is developed in the Western Region. It will be convenient to describe the Llanfaethlu country first, then Llanrhyddlad, then the high land about Foel.

Llanfaethlu to Brwynog

All about Llanfaethlu the Gwna Green-schist and Mélange are exposed in many places. In Gareg-lwyd Park are fine sections in mélange, full of lenticles of grit, with a few knots of limestone. On the south side of the woods the matrix is epidositic, evidently in great part volcanic dust, of the same nature as the adjacent Church Bay Tuffs, but highly schistose. The rock should now be described as 'Gwna Green-schist' (E10498) [SH 315 873], and at Bryn-maethlu is almost as crystalline as in the eastern Aethwy Region, with much chlorite, mica being fairly developed, but many clastic grains surviving. Passing south-eastwards from Llanfaethlu, the main divisional planes grow more micaceous, but the mosaic between them does not become saccharoid as are the New Harbour Beds, and the structure is lenticular, not flaggy. There is an absence of sharp folding, the foliation having a tolerably steady dip at moderate angles, with continual small undulations, and 'hitches' at intervals.

The rocks are well exposed at Bod-wyn, in Gareg-lwyd Park, all over Llanfaethlu hill, between the Maen-hir and the southern boundary, around Clwch-dyrnog, and at Brwynog. The quartzites, well seen about the Smithy, differ from those of Gynfor only in that any little schistose films traversing them are here well coated with fine white mica. The leading feature of the Llanfaethlu_. rocks is the limestone, 16 outcrops being laid down upon the six-inch maps, besides which are many too small for that. There are fine sections in the old quarries at Bryn-maethlu. The rock (E10482) [SH 315 875] is massive, grey, fine, calcitic, and sometimes mottled, as in Gynfor, but is slightly saccharoid, and where schistose films traverse it, they are bright with mica-, the Gwna Green-schist adjacent being also more crystalline than elsewhere. In the large quarry a massive bed is cut off at the top by a curving slide, above which the structure is more lenticular. Bedded limestone and graphitic phyllite (E10483) [SH 315 875] like that of Gynfor appear on the western wall of the quarry, and must have been better exposed in the lesser quarry, for a wall along a footpath near the 'a' of 'Llan' is built of them. The bodies that resemble foraminifera are just above the massive limestone, at the south end of the pool in the large quarry. The only basic rock of interest is a six-foot band of heavy chlorite-schist in the large quarry by the road, 360 yards west of Fadog-frech (E10480) [SH 315 867], which contains abundant undeformed cubes of pyrite that are often half an inch in diameter, enclosed in shells of fibrous calcite. No other such rock is known in Anglesey.

About Clwch-dyrnog, in spite of the survival of lenticular cores of grit, the green-schist is decidedly crystalline, the quartzites are granoblastic, and the limestone (E10484) [SH 340 860] shows a distinct access of anamorphism. It is full of scams of graphitic schist. The same rise in anamorphism continues to Brwynog, where nevertheless, clastic cores also survive. The passage to the Church Bay Tuffs (see p. 159) is discernible on the slopes towards the little river, but the Gwna Beds are undoubted, and are defined by quartzites. These are too small to be shown upon the maps, but are of good Gwna type. Some of the dips about Brwynog House are to the north-west, others to the north-east, indicating (like those in the New Harbour Beds near Meiriogen) an anticline pitching to the north-east, which doubtless brings on higher tectonic horizons of the Complex in the Sub-Ordovician floor.

Llanrhyddlad

The Gwna Green-schist becomes less crystalline as we pass northwest from Gareg-lwyd, here and there taking on some of the characters of the adjacent tuffs, a rock at Rhydwyn Windmill being a felspathic gritty tuff, not severely sheared. Thence along the little valley there are good exposures, and near the other windmill some lenticular jaspers (evidently stripped from a basic rock) appear in the Mélange. On the slopes below Llanrhyddlad Rectory are important exposures of sheared ellipsoidal spilite (E10429) [SH 32 88]–(E10430) [SH 32 88] with nests of jasper, now partly bleached and slightly schistose. Variolitic brushes Of decomposed felspars are still to be seen in the spilite; which, however, is highly epidotised, and some needles of pale amphibole have developed. In Porth-swtan the Gwna Beds are extremely decomposed, even the quartzites decaying at every possible point, and what was the Green-schist being now (E11070) [SH 300 896] soft and bright straw-yellow. At the north end of the beach the rocks are again fresh, and for 200 yards along the foot of the high cliffs there is one of the best sections in the Island of Gwna Green-schist with large lenticular masses of quartzite and dolomite. The upper part of this cliff is obscure from decomposition, but seems to consist of Gwna mélange. The good Gwna section ends at a fault beyond which only a little mélange and green-schist is left, which passes by interdigitation into the Church Bay Tuffs of the great cliffs.

Foel High Land

Foel hill itself, 450 feet in height, is composed of dull Gwna Green-schist (E11069) [SH 305 896], retaining near the summit a little of the original grey tint of the sediment, and sometimes the old bedding, but for the most part full of small hard phacoids, with some of quartzite a yard or so in length. On the south-west brows that overlook the cliffs just described, it becomes much more ashy-looking, and was at first regarded as Church Bay Tuff, but, on account of the presence of numerous lenticular quartzites, must be placed in the Gwna Group, though evidently a zone of passage. A long tongue of schistose tuff now runs inland, beyond which is again Gwna Green-schist, with. five quartzites and three limestones large enough to be shown upon the six-inch maps. The largest limestone (E10377) [SH 303 900], (E10378) [SH 29 91], (E10379) [SH 298 931], (E10380) [SH 302 900] has been quarried alongside the grassy road; it is grey, dolomitic, and massive, with compound oolitic grains. With it are excellent black phyllites, shaly-looking, with minute clastic quartz, and pyritous. The horizon is therefore undoubted. Near the sea these Gwna Beds are quite distinct from the Church Bay Tuff of the high rocky tops of Clegyr-mawr, but as the line runs inland along the moorland by Orsedd-goch, it becomes, in spite of good exposures, extremely uncertain (see p. 159), gritty matter and flaser-structure appearing on the south-east and ashy matter on the north-west side. Doubtless it runs through a zone of alternation, complicated by broken folding and repetition, and still further obscured by the old silicification of the tuffs, which when sheared into augen simulates closely the Gwna mélange and even the Quartzite. A little to the west of the large limestone, among the little farmsteads, are three small nips (E10369) [SH 302 900] of crushed albite-granite. The largest and most north-easterly, which adjoins a building 25 yards from the road, is well exposed, and actually in visible foliated contact with Gwna Green-schist. The granite is coarse to the margin, the schist shows no sign of thermal alteration, and the junction is evidently a slide-plane. Being albite-rocks, there is little doubt that they really belong to the Gneisses, and are shattered nips like those of Mynachdy and Mynydd y Wylfa (pp. 296, 307).

The Church Bay Tuffs

Except the rugged summits of Clegyr-mawr, 320 feet in height, there are no inland exposures of importance; but along the coast is a continuous section for a distance of three miles. The rock that rises on the steep slide of Yr-ogo-goch (Figure 203) is a uniform dull-green porcellanous tuff, slightly schistose. About 100 yards to the south, where the cliff turns a little westward, is one of the best sections in bedded material known in the formation, thin green grits and evenly banded pale jaspers being interstratified for a short distance with the fine porcellanous tuff. After this, banding decreases; and 400 yards from Yr-ogo-goch, a massive green ashy grit of Sherries type appears (E9681) [SH 293 906], (E10371) [SH 293 906] and runs out into a little headland. The section here is of the first importance, for it yields the pebbles of keratophyre, quartz-felsite, granite, quartzite, and scarlet jasper mentioned on pp. 57, 166, which are found on a greater scale in the Sherries Grits, and are therefore so valuable for correlation. Those of jasper are quite small, the others usually about half an inch, and occasionally two inches in diameter. Bedded jaspers (E10372) [SH 293 906] occur close by (with short thin bands of basic rock), and resting on the pebbly grit are bands of porcellanous epidosite which pass locally into groups of clots, a phenomenon seen here on a greater scale than at any other place. Uniform and massive green tuff extends to Porth-y-dwr, where there are some bedded pale jaspers, and a thin basic bed with a suspicion of ellipsoidal structure. Old silicification (E11341) [SH 293 905] torn out into augen is to be seen on the high moor above the cliffs. These hard augen contain also veinlets of secondary clear albite. The two little limestones are of the late metasomatic type (Chapter 19). The massive tuff now shows no special feature as far as Ogo Lowry, but from the foot of the cliff north of the chasm signs of bedding can be seen. Beyond that the tuff is still very massive, but streaks (rather than beds) of coarser matter are appearing as we round the headland, and where the cliffs look- southward there is a good deal of this (E10370) [SH 298 898], containing fragments of keratophyre and old mica-schist, and with short thin bands of a fine tuff in it, though the headland shows no bedding train a distance. Overlooking this head-hind are the high rugged summits of Clegyr-mawr, which (E9315) [SH 300 901], (E10316) [SH 299 900] furnished the specimen analysed. Close at hand only a short thin banding can be seen, but from the coast in a good light the whole crag shows bedding, dipping at 20° to 30° as if to pass under the Gwna Beds. Incipient old silicification may be seen in places (E11342) [SH 299 900]. As there is no shearing this is the best locality for a study of the tuffs. At the south end of the great ivied sea-cliffs the tuffs are brought down by a fault, and the Gwna Beds occupy the cliff to the end of Porth Swtan, but the section is even here continuous, for the tuffs occupy the foreshore. The Swtan sections demonstrate (see p. 159) the stratigraphical passage from the here overlying Gwna Beds. At their south end there is another fault, and the tuffs once more form the cliffs.

In the cove south of the roadway and old brickworks (E10432) [SH 299 891] there is again good bedding in the tuffs, but never running more than a few yards, or imparting a bedded aspect to the cliff. One grit seems to cut down into bedded epidosite, suggesting that, locally, there is no inversion; but the bedding, when examined, is so full of abnormalities, little faults, old brecciation that does not break the beds above and below, beds cutting across each other, and other such phenomena, that no reliable inference can be drawn. The conditions of deposit of this great pyroclastic accumulation are far from being fully understood. Local steam explosions, for example, such as those described by Anderson and Flett in St. Vincent<ref>Rep. Phil. Trans. Royal Soc. 1903, p. 423, Plate 23, Fig. 1.</ref> might erode and disturb the deposits, fine bedded dust then accumulating in the hollows, in which case the whole can after all be inverted (see below). It is quite likely that many of the small normal faults may be far older than the great earth movements—they may he due to settlement not long after the eruptions.

Cliff-sections in brown-weathering tuffs, massive and igneous-looking, follow as far as Crug-mor streamlet, after which is only a foreshore section to Porth Trwyn. In the first little cove beyond the sandy beach is the epidosite-clot bed described on p. 57 (E10433) [SH 296 878]. It lies between two zones of true fine-bedded rock, and cuts up into the overlying one, so that, here, the inference would be that there is inversion. A few yards to- the south the cutting is once more downwards, from which it is evident that this cannot he relied on to prove the true order of succession. Beyond the natural arch is beautifully even bedding (E10431) [SH 296 878] cut by small step-faults (a specimen of which is preserved in the museum) at intervals of an eighth of an inch. There are also true mylonising crushes, and curious faulted folds as shown in (Figure 129).

The Tuff is now becoming schistose, and, in the deep cove north of Careg-fran, there is a plane of movement above which it is for some yards hardly to be distinguished from the more ashy parts of the Gwna Green-schist. This degree of schistosity is not maintained, but the rock remains more or less deformed all the way_. to Trefadog. In other respects it is normal, and must have been quite massive and remarkably uniform.

On the foreshore below Castell drumlin, Trefa-dog, is the coarse tuff (E1543) [SH 288 858], (E10497) [SH 289 859], (E11078). It is considerably deformed. Then follows the junction section quoted on page 157, on whose north side is the 50-foot band of green-mica-schist (E11189) [SH 290 859] into which the tuffs graduate.

In the foregoing description, spots where bedding is to be seen have been, of necessity, dwelt upon, giving, perhaps, the impression of a good deal of bedded tuff. In reality theyarenarrow zones, the great body of the tuff being quite unstratified.

At Brwynog the true porcellanous epidosite is undoubtedly present, but is not massive. On the north side of the boundary-hollow the tuff alternates with and graduates into siliceous Gwna sediment; on the south, it alternates with purple phyllite of the New Harbour (Soldier's Point) Beds, so that both zones of passage are unruptured. The Tuffs are thin, but this is almost certainly due to their central massive part being cut out by the Bodfardden thrust-plane.

Boundaries

The nature of the northern boundaries has just been described. At the south end of Llanfaethlu village, north-west of the 'L.B.', schistose tuff is seen on a long low escarpment within a few yards of Gwna Green-schist, in the condition usual at the zone of passage. Except at this place, the western boundary of the Gwna tract is little better than conjectural from Crug-mor to Bodfardden, for it is almost entirely buried under great drumlins. There are, however, isolated exposures to guide it, and as it must in any case cross the foliation-strike it cannot fail to be serrated. The northern boundary of the New Harbour Beds, against which Gwna. Beds are brought from Bodfardden to Brwynog alluvium, must be a rupture, and has been called the Bodfardden thrust-plane. For about a mile east of Bodfardden-ddu the line is taken along a strong feature, but near Gronant it is suddenly shifted southward, probably by a fault along the alluvium, and thence for some two miles is guided through rather obscure country by small outcrops of quartzite and limestone. At Brwynog (where there are good exposures) the line is taken along a hollow on whose southern side is a sudden rise of metamorphism. But metamorphism- has already risen somewhat in the Church Bay Tuffs themselves on the north side of this line, decreasing where they graduate northwards into the Gwna Beds, and their material (much more crystalline) is recognisable among the New Harbour (Soldier's Point) Beds (see p. 158) on the south side of the hollow. As the thrust is still strong opposite Llanddeusant Windmill, the probable explanation is that at Brwynog it is cutting out all their central massive part, leaving both zones of passage intact. The thrust-plane is probably, like the Valley minor thrust-planes, a foliation-plane itself.

The Northern Inliers

The Garn Inlier

The Garn Inlier, which rides upon the Garn thrust-plane (Folding-Plate 9), and is elsewhere bounded by the Garn fault and the Arenig conglomerate, is of great stratigraphical importance, for on its rugged, steep escarpments three members of the Complex, with good zones of passage, are admirably exposed. The Green-mica-schist of the southern belt is of Holyhead, not Amlwch, facies, for though grit-bands exist they are feebly developed, hardly more so than in the country to the east of Valley, but it is easily identified as the Soldier's Point group. Though less crystalline than at Holyhead (E9684) [SH 320 902], (E11056) [SH 319 908] it is saccharoid, with fairly developed micas, resembling the condition prevalent about Llanddeusant. Clastic quartz is easily detected with the hand-lens. East of Bonw, just where a footpath crosses the Garn thrust-plane, jaspery phyllite and some bedded jasper may be seen; and close to the northern margin of the belt, at the tail of a dip-arrow, is a 10-foot band of soft basic schist, resembling the deformed New Harbour spilites, adjacent to which are seams of pale bedded jasper, so that there is no doubt as to the horizon. In the southern part of the belt is a sharp isoclinal folding (though never so rapid as at Holyhead), the fold-axes dipping north at about 40° (with rather steep easterly pitch), but the angle increases to 70° on the higher escarpments, and along the northern margin of the belt folding dies out, giving place to a steeply-dipping platy foliation.

At the same time the metamorphism falls off, and about 170 yards east of the spilite is a band with the characters of the Church Bay Tuff. Then comes the zone of passage, which has already been described (p. 158). Along it is a narrow grassy platform, beyond which is platy crypto-crystalline sea-green rock with seams of fine mica-schist, and this type passes in a yard or two into typical Church Bay Tuff. For the tuffs of the inlier are typical, and not always even schistose. In places they are quite massive, and are the same dull green, unstratified material, full of wriggling delessite veinlets, as on the shores of Church Bay. Near their southern margin they show some bedding, with seams of pale jasper like that of the New Harbour Beds.

Towards the summit, thin courses of a fine siliceous grit appear in them; these then break up, and in a few yards the rock (see p. 159) becomes the Gwna mélange. In this are several phacoids of white quartzite, one of which, just below the cairn, is not less than nine feet long. The matrix of the mélange (E10356) [SH 317 906] retains an ashy aspect, and there is but little anamorphism, the micas of the foliation being very small.

The Mynachdy Inlier

About half-a-mile west of Mynachdy a small strip of green Gwna mélange rises from beneath sheared Ordovician conglomerate (with which it is confusable near the junction), along the south side of the same slide as that which bounds the Gader gneiss at Porth Ogo'r-geifr (Folding-Plate 13).

The Fydlyn Inlier

The deeply indented coast of this inlier is almost continuous cliff, often 100 feet or more in height, and inland also the country is very bare and rocky, so that the inlier is very freely exposed. The whole of the north-western corner is extremely complicated (see (Folding-Plate 13), which includes the Fydlyn, Gader, and Mynachdy Inliers. The place-names used are taken from that map).

Trwlln-y-crewyn to Fydlyn Beach

Along the southern margin, rising on the steep Crewyn thrust-plane, is a broad band of Gwna mélange, running westwards from Hendy to form the great rugged headland of Trewyn-y-crewyn. In this mélange (E10373) [SH 291 914] the hard gritty augen are very fine, weathering like porcellanite; and are often closely crowded, lapping on to one another, leaving thus but little room for the schistose matrix. A coarse felspathic grit near the Trewyn is regarded as a nip of the Ordovician base. Near it are one or two small basic schists. The northern margin of this tract, for about half a mile west of Hendy, is a good deal confused by late silicification.

On the western cliffs, 133 yards north of the Trewyn, is a chasm, in which (Folding-Plate 10) a powerful-looking thrust is well exposed. It cuts out an unknown thickness of the Gwna mélange, for above it the green colour begins to fade in a yard or two, white seams of felsitic dust-schist appear, and in 16 yards there is a complete passage (see p. 161) into the Fydlyn rocks. Inland, the nature of the junction is not clear, it seems to zig-zag across the strike, and the thrust may be changing its horizon. Returning to the coast, a few thin grits remain in the Fydlyn Beds on the north side of the chasm, but they soon disappear, and for 180 yards the white felsitic schists (E9682) [SH 290 917]–(E9683) [SH 290 917] are finely exposed, this being the best section in the most purely igneous portion of the group. Shearing is powerful, but anamorphism feeble, and fissile matter winds about innumerable hard cores. Many of these are crowded with quite large phenocrysts of quartz, and here are the rounded cores that suggest original nodularity. Other hard cores are siliceous, and evidently due to ancient silicification at the close of the volcanic episode, long before the time of the great earth-movements. Except these' knots, all the Fydlyn rocks are considerably decomposed. The impression left upon the mind is that of a series of several thin lavas, with intervening tuffs, all now sheared together. Both lavas and tuffs must be highly acid and wonderfully free from iron, for in spite of the decomposition the cliff is in many parts nearly as white as chalk. Further petrological study is much to be desired. Just north of the thrust, the cliff can be descended, and at low water a good view can be obtained from beneath, as a broad rocky shelf is laid bare. About 100 yards along, a rude isoclinal anticline of Gwna mélange rises (Folding-Plate 10), but does not reach the cliff-brow. It is of structural importance, for it shows that the Gwna Beds really underlie the Fydlyn Group, and that the succession in the inlier is inverted. Viewing the cliff from Fydlyn outer island, this dark anticline can be made out, with indications of a syncline on its northern side. Then, at Fydlyn beach, Gwna mélange with a basic schist and a little jasper comes up and is thrust on to the Fydlyn rocks.

Fydlyn Beach to Porth-yr-hwch

The beach and alluvium doubtless conceal a shattered anticline, followed by another infold. For at the Fydlyn island and cliff the felsitic rocks appear again, the junction probably being close to the island, for at the cliff's foot, of the inner isle a little Gwna Green-schist is to be seen among the white rocks at low water. But on Fydlyn islands fine grit and even grey phyllite (E10353) [SH 292 917]–(E10354) [SH 291 917] (which have been searched for fossils) are present, and it is probable that most of the acid volcanic matter is pyroclastic here, but the bedding has been excessively broken up at the beach, though from a distance it seems tolerably regular on the outer isle. In a chasm at the seaward end some lumps that weather hollow have a very nodular aspect. Just where the inner isle faces the mainland, this thin bedding graduates once more into massive matter; and Fydlyn high cliff (pierced by the great caverns, Chapter 34) is composed of a very massive white rock (E11333) [SH 290 917], (E11334) [SH 290 917], (E11335) [SH 290 917], (E11336) [SH 290 917] like that of the southern cliffs. There is a vertical foliation, and some zones of silicification, but from the island there can be seen planes that appear to be bedding dipping at 40°, in which case the thickness of the mass must be about 130 feet. Massive though it is, the texture is far from homogeneous, for while some parts are little else than crypto-crystalline felsitic matter, now considerably sericitised, in others this matter functions merely as matrix to a crowd of sub-angular fragment's, chiefly quartz, with a few sericitised felspars. The rock therefore appears to be a pure tuff, with showers of broken phenocrysts detached by explosion and embedded in felsitic dust. But the upper parts of the great bed need further investigation.

Inland, both tracts of these acid schists, though well exposed, are difficult to separate from the sheared felspathic Ordovician grits, and the lines drawn are but provisional. The deep hollow of Llyn-y-Fydlyn does not seem to be determined by any rupture of importance. It may be worth remarking that, the succession being inverted, infolds of the Gneisses might have been expected to occur as outliers; and these should be sought for inland, as amid the general confusion due to the deformation of several acid felspathic rocks by earth-movements of at least two periods, they might easily escape notice.

Returning to the coast, the Fydlyn rocks are seen to graduate into the Gwna Beds, the latter passing quickly into mélange (Figure 32), the sections being on the high brows just above the cliff edge. In spite of the general inversion, this graduation is upwards, the natural order being locally restored by the overdrive on the major secondary syncline. The Fydlyn rocks come to an end at a vertical zone of shearing that is exposed in an interesting chasm which can be reached either by getting down a spur of cliff, or by coming through the great Fydlyn caverns at dead low water.

The zone of shearing behaves as a fault, but produces autoclastic mélange in the Gwna Beds, which are driven up against the Fydlyn rocks. Planes parallel to what seems bedding cross the zone, but they are merely jointing. Then come several yards of an ellipsoidal spilite with some jasper. This lava (E11337) [SH 291 917], though its felspars are badly decomposed, retains in places the subradiate structure of the variolites. On it rests a massive grit that seems to fill cavities in its surface, and then follows a large phacoidal limestone, part of which remains as a natural arch that bridges the next chasm. North of this is a grand rugged sea-section in dull green Gwna mélange (E11074) [SH 292 919] with large lenticular quartzites and limestones, and at its north end are some more ellipsoidal spilites with jasper. Planes like bedding dipping north are perceptible, yet the large phacoidal masses are standing almost vertically. This is one of the finest Gwna sections in the Island. Eastward from the coast a rugged tract of mélange extends as far as the Carmel Head thrust-plane, where it is on the point of wedging out. The fissile matter is chlorite and sericite, and the anamorphism low, perhaps rather lower than at Bodorgan. The hard lumps, rather short and subangular, instead of good curving phacoids, are chiefly of a very fine siliceous grit (E10375) [SH 301 920] now largely crypto-crystalline, like those on Trewy-n-y-crewyn. Besides a few quartzites there are some 23 nips of basic rock. Most of them are now chloritic schist, but about 266 yards east of Pant-yr-eglwys (E10381) [SH 299 920], although the pyroxene is chloritised and the felspar much crushed, they retain traces of ophitic structure, and are leucoxenic albite-diabases, evidently of the spilitic suite.

The passage (see p. 159) to the Church Bay Tuffs now comes on. At the cliff it is to be seen just on the north side of a chasm in which is a small shearing crush, but the boundary is unfaulted. Inland, the passage is perceptible all along the line, and the other good section across it is in the ravine west of Pant-yr-eglwys alluvium, below the Fydlyn path, where a wall runs along the crag's brow. The Church Bay Tuffs are good and typical both on the coast and inland, in spite of shearing.

Porth-yr-hwch

In Porth-yr-hwch (Plate 25) they end at a plane that probably cuts them off rather steeply against a wedge of Gwna Green-schist with limestone, that forms part of the cliff wall at the back of the beach. At its north end this Gwna Green-schist graduates into a yard or two of rock with white seams, evidently the passage from the Fydlyn Beds.

But, before the passage is effected, comes a powerful thrust-plane, which may be called the Hwch Lower Thrust-plane (Figure 130), very clearly exposed and well seen in (Plate 25), which drives massive white Fydlyn rock, at a lower angle than the dip, completely over the Gwna rocks, and, apparently, on to the Church Bay Tuffs, so that this wedge of Gwna Green-schist only runs a few yards inland. This thrust is probably an ancient one of the Mona Complex, for some carding of the rocks into each other is superimposed upon it.

The Fydlyn rock now forms the remainder of the coast of the inlier, and runs out into a precipitous high spur between Porth-yr-hwch and Porth-yr-hwch-fach, on whose ridge it simulates the Ordovician grit, and has been searched for fossils. But there is no doubt of its identity, for on the seaward nose of the spur the white felsitic matter appears in abundance, and the old Mona shearing is very powerful. The simulation is due to late silicification (which may be the cause of the survival of the spur) such as has affected the Ordovician rocks a little to the east. Some green zones of Gwna type, with grit, also occur in association with it, completing the identification. The massive Fydlyn rock (E11338) [SH 292 921], (E11339) [SH 292 921], (E11340) [SH 292 921] of Porth-yr-hwch is largely felsitic matter, sericitised as usual, with broken phenocrysts of quartz, and may safely be taken to represent the thick felsitic tuff of Fydlyn cliff, though it is less crowded with phenocrysts.

At the back of the cove is a small Post-Ordovician cross-fault, beyond which rise rocky heights composed of a yellowish type of Gwna mélange with small quartzites, confused by silicification (which renders the lines to the east somewhat uncertain). Appearing from behind the Fydlyn rock, it is the most northerly member of the inlier: and, the succession being inverted, must be rising on the north limb of the major syncline that takes in the Fydlyn rock. But the southern limb is in great part abolished by the thrust (Folding-Plate 10), which, cutting the major infold at a lower angle than its axis, drives the Fydlyn rock of the core completely over the Gwna Beds of the southern limb; and then abolished altogether by a higher thrust inland, which drives the Gwna Beds of the northern limb right across the core and on to the Church Bay Tuffs of the southern limb. These two old thrust-planes are then truncated by the Post-Ordovician slide that lets in the wedge of shale, over which the Gneiss of the Gader Inlier is in its turn driven on the Post-Ordovician Hwch thrust-plane.

The Gader Inlier

The lofty ground of Pen-bryn-yr-eglwys (Folding-Plate 13) (always known locally as 'The Gader') is bare, and there is good exposure, especially on the southern escarpments; but the great feature of the inlier is the grand sea-section, which rises to a height of 200 feet. The whole is gneiss, except a narrow Gwna band along the sea cliff. All the types of gneiss are present, but the flaky 'C' type is dominant (E8448) [SH 292 925], (E9672) [SH 289 927], (E9673) [SH 289 927], (E9674) [SH 289 923], (E9675) [SH 289 925], (E9676) [SH 289 925], (E9677) [SH 289 925], (E9678) [SH 289 925], (E9679) [SH 293 905], (E9680) [SH 289 926], (E10363) [SH 290 928], (E10364) [SH 290 928], (E10365) [SH 290 928], (E11001) [SH 292 925], (E11002) [SH 292 925], (E11003) [SH 292 925], (E11054) [SH 292 925]. Eleven strips of hornblende-gneiss are known, but none exceed 100 yards in length.

Traversing the sea-section southwards, the first 170 yards from the north point consist chiefly of grey flaky gneiss with lenticular granite sills (Figure 22), (Figure 23), and thin pegmatite seams and phacoids, while the smaller granites are full of films of gneiss (Figure 26). The felspar of the gneiss is albite. with some oligoclase, the biotite is largely bleached or else chloritised, and is often crowded with needles of rutile. Sillimanite is present., but is not abundant. It is interesting to note how the granite sills of the high land, thinning seawards, turn round so as to pass down the cliff along the foliation-dip (Figure 131). Their felspars are albite and albite-oligoclase, with some oligoclase, which is abundant in parts of the large sill that runs up to the north edge near Porth y Dyfn. Granitic permeation is now setting in, and at Porth-ffau'r-llwynog ('the fox's den') by getting down a great oblique buttress or spur to the sea, the section shown in (Plate 14) can be seen. There is hardly any dip or foliation as a whole, but the rock is a rude complex or 'plutonic breccia' of knots of albite-granite and dark gneiss, each included in each. The aspect strongly recalls the old parts of the Lewisian of Scotland, the dark knots here, however, not being amphibolite but fragments of the biotitic gneiss itself, which also runs in threads throughout the granite. Granitisation then wanes, and on the cliff to the south are excellent sections of flaky gneiss with lenticular interfelting of innumerable thin granitic seams. Although the foliation continually undulates, folding is rare, but here and there are micaceous partings that are sharply corrugated, in some of which are decomposing garnets as large as peas, and a few green crystals that may be idocrase. Permeation then again sets in, and waxes with such rapidity that all the four higher grades are to be seen in only about 40 yards. The great ' sill' that runs down the cliff overlooking Ogo'r Arian is really a locus of maximum granitisation, gneiss and granite being woven together within it by what may be called internal anastomosis. The felspar is albite-oligoclase.

The buttress between the two dark caverns at Ogo'r Arian (see p. 162) is composed of gritty Gwna mélange with trachytic fragments (E10368) [SH 289 924], very slightly altered, brought against the gneiss by an undulating fault. The band runs on southwards for another 110 yards along the lower part of the crags of the great headland, the fault then bending as if to pass into the chasm between the land and the rocky islets. This fault is not quite simple, some inosculation taking place between the two rocks. It has a clear bade eastward, so that unless it be reversed (of which there is no sign, but rather the contrary) the Gneiss is brought down against the Gwna Beds, which is a confirmation of the view here taken that the succession is inverted.

The highest point of the sea-section, called Trwyn-cerig-yr-eryr ('Eagle's crag headland'), is crested at its conspicuous point by a complex granitoid gneiss. Among the flaky gneiss in which this lies is a good deal of the siliceous 'A' type. The great section is highly complex, and would well repay detailed petrological study.

High types of granitisation again set in, and along the southern sides of the great headland, cakes of the large granite of Pen-bryn-yr-eglwys adhere to the cliff, dipping at high angles. The white quartz rocks here are late silicifications (see Chapter 19). The main sill is a good deal decomposed, but from a suite of specimens taken on the escarpments south of the summit of the hill, its felspars have been found to be albite and albite-oligoclase, with a very little oligoclase. It appears therefore that all the granites of this inlier are to be referred, not to the Coedana, but to the gneissic suite. The great granite is then driven southward upon the Hwch thrust-plane ((Figure 130) and (Folding-Plate 10)

The Corwas Inlier

This inlier is bounded entirely by thrusts and slides. The western part is composed- of platy green-mica-schist of New Harbour type with a few clastic grains. It is thin-seamed without any definite beds of grit, and is not of the Amlwch facies which lies close by across the Carmel Head thrust-plane, but of undoubted Holyhead (and Soldier's Point) type, a fact of great structural importance. In it lie some rattler massive basic schists, largely composed of epidote with needles of a pale hornblende, which may represent the spilitic lavas of the group, for, though no unfoliated cores have been seen, thin seams of pale jasper are associated with them. The best exposures of both rocks are on the south side of the pool, and thence at intervals to Corwas. The eastern part is composed of Gwna Green-schist; so that the boundary must either be a back-slip or a thrust at a lower angle than the true dip, but it is not exposed, and in the obscure marshy hollow there is room for at any rate part of the Church Bay Tuffs. Dull tuff-like matter, like the Church Bay Tuffs, occurs among the Gwna Beds near the line, and if the Tuffs be locally thin, the thrust need not be of great magnitude. The Gwna type is pronounced, and were there any doubt of the horizon, it would be sufficiently indicated by the single quartzite, which has thorough Gwna characters. The rocks are abundantly exposed on the rugged flank of Mynydd Eilian, east of the Pengorphwysfa road. On the higher knobs of the eastern margin is a good deal of thin-seamed chloritic phyllite (E10141) [SH 213 796], but the prevalent rock is very siliceous, the harder bands weathering to a salmon-pink. Often these bands are very fine, and have a foliation that appears to have been sealed-up ' by silicification and epidotisation at a late stage in the metamorphism. They are impersistent, but there is an absence of the usual phacoidal mélange, for the grits which give rise to such a mélange are absent. Indeed, even the coarser beds are quite fine, and lamination is general. The folding is a nearly a-clinal wrinkling on steeply dipping planes.

The basic sills at Pengorphwysfa are sheared, but have a chilled edge, and a tongue that seems to belong to them truncates the foliation of the schist, so that they are probably intrusions of a late stage in the Mona movements, as are those of Caerau (pp. 103, 320), which they resemble externally.

The Northern Region

The Llanfairynghornwy Belt

The rock which rides immediately upon the Carmel Head thrust-plane at Carmel Head (Folding-Plate 10), (Folding-Plate 13) is a green-mica-schist with clastic grains (E9512) [SH 294 930], of New Harbour type, like that prevalent in the Lynas division of the Amlwch Beds, more crystalline than the group that lies behind it, and isoclinally folded at high angles from the north (Figure 270)<ref>This folding is to be seen in an unpublished photograph (No. 1446).</ref>, the folding being older than the Carmel Head thrust-plane, by which it is cut. The Wig thrust-plane, which is not seen in section, may be of Pre-Cambrian age, and runs out in 100 yards upon the Carmel Head thrust-plane.

The rocks of Carmel Head itself, which ride first upon this and then upon the Carmel Head thrust-plane, are typical green Gwna mélange (E10374) [SH 298 931], in which is an extremely complicated zone full of great lenticles of quartzite and limestone, with seams of graphite-schist. On the low eastern cliffs, about 200 yards from the Point, are masses of spilitic lava with deformed ellipsoids (E10382) between which are nests of scarlet jasper. Variolitic felspar-brushes still survive in them. Thus, at this extreme point of Anglesey, the Gwna Group is curiously complete, and that in the space of only 200 yards. Reconstruction is very slight, and bedding survives at the Point, but south of that everything is torn to pieces, jaspers being ripped out of the spilitic lavas. Rocks of the same kind continue above the Cannel Head thrust-plane to Porth-yr-Ebol (as the cove a quarter of a mile south-east of Cannel Head is called on the .0004 and six-inch maps), where the great thrust is shifted by a fault. Dull ashy material then appears in it, and confused types, greatly crushed (E10992) [SH 373 945], continue to the Porth Padrig slide, and in Porth Gron, the cove between Ebol and Padrig, is a massive grit, extremely decomposed. These rocks, west of Mynachdy, have such an ashy aspect (see p. 159) that they were at first assigned in part to the Church Bay Tuffs, but they contain lenticular grits and quartzites, and must be placed in the Gwna Beds. The line has therefore not been coloured, but is retained upon the one-inch map, as the sections are most perplexing.

Gwna Beds (very slightly anamorphic—see pp. 168, 238) reappear beyond the Mynachdy Gneiss, and continue between the Carmel Head and Caerau thrust-planes to Llanfflewyn. On Mynachdy drive they contain a large quartzite, and much purple phyllite. This differs from the jaspery phyllites of the Amlwch Beds in being lumpy, lenticular, and associated with purple grit as it is in the Aethwy Region, and at the foot of the bold escarpment that looks down upon Mynachdy marsh is a thin strip of deformed spilite with nodular jasper. On the heights north of Caerau (see p. 159) matter of Church Bay type again appears, but the body of the rock is Gwna Green-schist, with small quartzites. The eastern boundary is here obscured by tile baking of the Amlwch Beds around the basic intrusions. Gwna Green-schist with large quartzites is well seen on the slopes of the Garn (E11064) [SH 319 913], (E11065) [SH 319 911], (E11066) [SH 326 907], (E11067) [SH 318 914], (E11068) [SH 319 911] about Hendre-fawr, and the old level driven close to the village was reported to Dr. Matley to have passed through a 12-yard limestone that has no outcrop.

Exposures are scanty for a mile, but Gwna Green-schist with small quartzites is again well exposed about the road-fork east of Bod-hedd and at the intakes of Llyn Llygeirian. On its northern margin it becomes (see p. 159) full of ashy matter, and typical Church Bay Tuff, a good deal sheared, is exposed by the outflow from the lake, so that the wedge is wide, and its outcrop must run as far west as the Maen-hir. The Caerau thrust-plane (see p. 217) is now thrown down by the Geirian fault, and, where it reappears, has almost overlapped on to the Carmel Head thrust-plane, which has also been thrown down. A platform only 50 yards wide, of Gwna Beds with a little limestone in them, winds round beneath the rugged escarpment of the Coeden beds for a quarter of a mile, and then the thrust-planes come together. Both must be at quite low angles. A thin plate of Gwna Beds reappears between them at Gwaen-ydog bay' (Chapter 18, (Plate 32)) and is completely shattered.

The Mynachdy Gneiss

This is wedged into the Gwna Beds in four lenticular tracts (Figure 94), the two larger being parted by the Ordovician tract, and the two smaller lying along the Caerau thrust-plane, The Gneiss differs in no way from that of other parts of Anglesey, save that sillimanite and garnet have not been seen, and is as coarsely crystalline as any. Many small granites have been picked out upon the maps, the gneissoid element is full of granitoid bands, and the granites are sometimes foliated. Both micas are present, much of the white mica being a bleached biotite. A granite (E10643) [SH 310 925] labelled by Blake 'Behind Mynachdy ' contains orthoclase, but the felspar of six specimens of the granite and gneiss examined by the present writer is albitc-oligoclase. Some of the rocks are highly quartzose and may have been silicified (see Chapter 19). The section (see pp. 162, 168) where granitoid gneiss is seen within nine inches of scarcely altered Gwna Beds is below the drive, a quarter of a mile from Mynachdy. In the tract west of the Ordovician rocks are good sections with every grade from micaceous gneiss to granite in lenticles two or three feet thick in which are wisps of gneiss, and an obscure amphibolite appears in the farmyard. In the tract east of the Ordovician the best exposures are on the escarpments north-east of the garden, and on some ice-worn bosses just north-east of the wall on the top, granite is seen to graduate into gneiss. The mylonising crushes are probably later movements of the Complex, the rusty partings Post-Ordovician. The Mynachdy gneiss is unfortunately extremely decomposed, and it is difficult to obtain a specimen with a fresh fracture.

The Coeden Beds

The outcrop of these beds is one of the most rugged tracts in Anglesey, so that good sections are innumerable; but as the rocks themselves do not present much variety, most of the local interest is in the structures. Coarser grits than usual, yet finer than many of the Mona grits, are to be seen in the valley due east of Tyddyn-y-pandy, 530 yards west of the Windmill; and on an escarpment 600 yards south of the 'c' of 'Mechell', overlooking a cottage. The last locality should be searched for composite fragments. Epidotic seams are present everywhere, but the roadside about 140 yards north-east of the Smithy may be mentioned<ref>The slides of these beds are (E8512) [SH 36 89], (E8513) [SH 36 89], (E8514) [SH 36 89], (E8515) [SH 36 89], (E8516) [SH 393 895], (E9312) [SH 396 898], (E10284) [SH 32 88], (E10401) [SH 377 901], (E10402) [SH 360 899], (E10403) [SH 360 899], (E10404) [SH 358 892], (E10405) [SH 357 901], (E10406) [SH 356 904], (E10407) [SH 359 902], (E10408) [SH 362 888].</ref>.

Sweeping steep isoclinal folds are displayed about 233 yards north of Llanffiewyn Church on the crags that look down upon the lake ((Figure 132), e), also about the rock-encircled little tarn east of the church ((Figure 132), a), about 300 yards south-west of Creigiau-mawr, and on the north side of Mynydd Mechell are the sections given in (Figure 132), b, c, d, f. Symmetrical folds occur on the mountain-side west of the Smithy. Some of the minute folds are shown in (Plate 21), Fig. 2. But there is no lack of folding-sections. Where it is not perceptible the limbs are simply longer, their dip often broken by minor folds, as at the old mill-tower north of Gwaen-ydog. At the double pitch-arrow north of Coeden stretching-lines cross the overdriven anticlines obliquely, indicating a spiral movement. Crumpling of the fine partings within the larger isoclines is well seen on the north of the large dyke near the 'c' of 'Creigiau-mawr'. Strain-slip in the same fine beds is to be seen south of 'L. B'., Llanffiewyn, but best of all on the south side of the road between the Windmill and Llanddygfael-hir, at the glacial arrow, where it is finely developed. The dip, determined by hard beds with lamination, is low, and between them the fissile schists, highly crystalline, show two series of divisional planes, neither of them parallel to those of the hard beds (Figure 70).

This is close to the junction with the Amlwch Beds, along which line there is a lowering of the dip of the isoclinal axes. Here also thinner-bedded rocks come on, well seen between the Windmill and Maen Arthur; and the passage by alternation (see p. 161) into the Bodelwyn division of the Amlwch Beds may be studied about Clegyrog-blâs.

At their west end, the Coeden beds are cut off by the Geirian fault, where they are pitching westward; and as the Amlwch Beds there are of the Lynas type, there may also be a rise in the angle of the Caerau thrust-plane, or a lowering of the bedding-dip. At their east end, they strike at the Amlwch Beds by the railway, but a strip (or strips) referred to them reappears beyond Gwredog, so that there must be an undulation of the pitch, a phenomenon of which there are cases all along the tract.

The Amlwch Beds

As this series has a great length of rocky coast-line, and also gives rise to very rugged tracts in the neighbourhood of Amlwch, it is one of the best exposed in Anglesey. Even the .0004 maps admit only of a mere abstract of its wealth of detail, so that the following account is but a condensed abstract of an abstract. The coast will be described first, from west to east; and then the interior, in a similar direction.

The Coast

Where the series first appears, driven up along the Mynachdy thrust-plane (Figure 222), the alternating type, with strong, hard epidotic grits, is pronounced; but these are succeeded in about 50 yards by phyllites, locally contorted, though rapid folding is not common in this part of the region. Beyond the serpentine, flaggy phyllite, probably of the Bodelwyn beds, forms most of the shore for some 600 yards, interrupted by a massive grit that gives rise to a cliff buttress. Alternating beds then come on and extend as far as Henborth. On two stacks of the foreshore ((E10385) [SH 325 938], and analysis, p. 52) these grits are curiously epidotised, sometimes in concentric shells, sometimes in the manner shown in (Figure 133), where the bedding is horizontal, the foliation of the fine phyllite at a moderate inclination, while thin epi-dotic bands in the grits dip at a still higher angle. On the foreshore of Henborth is the first considerable development of purple phyllite, beyond which are green alternating Lynas beds, and upon them rest (see pp. 159, 177) the Skerries Grits which here interrupt the Amlwch Series. Over them the alternating beds are thrust, but in the bay by Craig-yr-iwrch is much disturbance, with a strong line of brecciation running west-north-west, outside which a small stack is an outlier of Skerries Grit. Parallel with this, and opposite the south end of the reefs of Craig-yr-iwrch, is another zone of brecciation, giving rise to a gap in the foreshore no less than 100 feet in width.

The pleasant low promontory of Trwyn Cemlyn consists of the alternating group, with bands of purple schist (cut by thrusts) at the point. But some of the grits are thicker (to six inches), darker, and coarser than usual, and contain little fragments of pink felsite and scarlet jasper, as do the Skerries Grits, thus binding the groups together. On the eastern coast of Cemlyn Bay the same alternating group reappears, with occasional thicker and coarser grits, and a great many bands of jaspery phyllite and bedded jasper (E10415) [SH 339 936], (E10416) [SH 340 937], thirteen being mapped in 200 yards west of the reefs of Cerig Brith. Just at the east end of the great pebble ridge are some 20 feet of a compact, pale, flinty rock (E10412) [SH 336 932]. As it contains phyllitic partings and becomes gritty at the top, it would appear to be a cherty band in the alternating group. Folding is still subordinate to thrusting, but in the northern part of the cliff there is a bend in the strike, and a pitch to the north-east appears for about a quarter of a mile. Opposite the reefs of Cerig-brith is a massive grit, about whose eastern end wrap some 20 feet of breccia, like those of the crushes west of Cemlyn, and probably of Post-Ordovician date. It wedges out rapidly between the grit and a plane of sliding. Alternating beds then come on again.

On the moor above the cliffs are many more bands of jaspery phyllite, and eight small stout nips of sheared spilitic lava (E10419) [SH 340 936] which retain traces of the variolitic structure. On the. coast at Cestyll is another zone (E10420) [SH 346 934] of the pale cherty rock. The alternating group (E10410) [SH 330 928]–(E10411) [SH 330 928], (E10413) [SH 345 934]–(E10414) [SH 345 934] (interrupted only by the wedge of Gwna Beds that crosses the neck of the headland west of Porth Wnol) forms the coast all the way to that point, on which there is a great development of the jaspery phyllites, finely exposed. In Porth-y-gwartheg is a spilitic lava, truncated in a confusing manner by a Paleozoic basic sill. On a stack in a creek immediately to the south are some coarse grits (E10418) [SH 347 939] with three-quarter-inch pebbles of quartz-felsite, albite-trachyte, grit, and Gwna quartzite. They seem (see p. 158) to be involved with the Amlwch Beds, but may possibly be a broken infold of inverted Skerries Grits. Small fragments of a felsite also occur in the alternating beds about 150 yards east of Porth-y-gwartheg, on the north side of the spilite. There is now more folding, but thrusting along planes nearly parallel to the old bedding is still dominant. (Figure 65), (Figure 66).

The Amlwch Beds as a whole now turn inland below the Wylfa thrust-plane, which is exposed in the cliff at Porth Wnol (Figure 96), but there is a wedge of them on the western cliffs of Mynydd y Wylfa. They reappear for a few yards in the semi-circular cove at Wylfa; and again on Cemaes beach, where, among bedded jaspers, they contain a good spilitic lava (E10507) [SH 372 937] with ellipsoidal structure. It is full of epidote and actinolite, but the variolitic radiating texture is well preserved.

Where the group once more emerges on the coast in Bull Bay, the alternating type is pronounced, and continues all the way to the Carmel Head thrust-plane beyond Point Lynas. The junction of the Lynas beds with the Skerries Grits is perfectly exposed on the headland north-west of Cave'. It has been described on p. 158, and is a complete passage by change of material, the two types alternating for some little way.

South of that chasm begins an excellent section, below the grassy bluffs, in the alternating group, which is much more gritty than in the Cemlyn country, for much of the schistose element between the hard bands is really grit. Folding increases, but thrusting nearly parallel to the bedding, never permitting any one bed to range for more than a few yards, is still the dominating structure. Jaspery beds appear, increasing eastwards, and there are some small nips of basic schist. The metasomatic limestone (see Chapter 19) lies a few yards west of the southernmost inlet of the bay. Beyond that, the jaspery phyllites and bedded jaspers become very numerous (Figure 134), 40 having been laid down upon the .0004 maps along 400 yards of coast, which, measured across the strike, is a proportion of one in every four yards. Nowhere in Anglesey are the nature and relations of these beds better displayed than (E10535) [SH 437 936] and analysis] upon the cliffs at the chasm about 60 yards north of the path which comes down from the Coastguard Station. A few yards north of this chasm (opposite the dip-arrow in (Figure 134)), an ellipsoidal spilitic lava some 10 feet thick, lies in the cliff between the jaspers. It is the one that was analysed (p. 55), (E10259) [SH 461 906], and although less well-preserved internally than those of Llanfwrog, is of the first importance, this being the only section in the New Harbour Group where the whole of a spilitic flow is laid bare from top to bottom, together with its relations to the embedding jaspers. It is partly schistose, lout ellipsoids, with epidotised varioles, are well seen at the seaward end. About 120 yards further east are the structural sections given in (Figure 67), showing the processes of lenticular disruption of the grits and carding of them in with the softer schists. They are best seen by getting down the main cliff on to a platform a little further out. Another 120 yards eastwards again, on an escarp mental ridge across a chasm, are the conglomerates (E10540) [SH 441 937] described on pp. 51. There is not the slightest doubt of their being well within the Ainlwch Beds, in spite of the identity of the pebbles with those of the Skerries Group. Some of the pebbles, especially those of the old sediments, retain their smooth rolled form, and are about an inch and a half long, but those of the igneous rocks are often considerably sheared. On Graig Ddu are sills of venous quartz containing a felspar, partly albite, partly anorthoclase. Gritty matter continues to increase, and as we turn round the headland to Porth Offeiriad the Amlwch Port facies comes on, hard white-weathering bands lying between fissile green beds (E10559) [SH 448 938] almost as hard and gritty, and indeed rather coarser. On the ice-moulded cliffs as we approach the Port there is good folding, and the beds are cut across by the foliation.

On Llam-carw the rocks are driven southward on powerful thrust-planes. East of the chasm comes a space in which the hard bands are much thinner than usual, and the jaspers do not reach the coast. At the wild ravine of Aber-cawell, folding (crossed by the foliation) again becomes conspicuous, and east of it is a strong development of the schistose green grit between the hard bands (E10560) [SH 461 936]–(E10561) [SH 461 936] which are finely developed at the cove a quarter of a mile further on. At the rugged gap of Ffynnon Eilian is again schistose green grit, folded on steep isoclines, and here the jaspery phyllites (E10534) [SH 466 934] once more emerge upon the coast. Bending a little to the south, numbers of them appear in Porth yr Ychain, with a sheared spilite, confused by the presence of a metasomatic limestone, and very complex. The whole cliff becomes extremely gritty as it bends round; and just south of the y' is a zone of brecciation that is evidently a late movement of the Mona Complex, for the matrix of the breccia has itself become schistose, though very slightly reconstructed. On the west cliffs of Porth yr Ysgaw are spilitic lavas (E10443) [SH 476 930] and jaspery phyllites; and on the west cliff of the buttress which divides the cove, is about a foot of white saccharoid limestone, associated with a thin basic schist, folded over in a crumpled anticline. It seems to be unique, and is probably an ancient reconstructed calcite-vein. On the western cliffs of Point Lynas, near the 72-foot level, in alternating beds (E10448) [SH 478 933], (E10449) [SH 478 933], (E10450) [SH 478 933], is one of the finest folding-sections in Anglesey, with the long straight limbs shown in (Figure 52). On the eastern side of the Point, nearly opposite this place, bedding -dipping at 30° to 40° is crossed by foliation at a lower angle. Just before Porth y Corwgl is reached are several bands of sheared spilitic lava (E10442) [SH 481 936]. Then, on the north cliff of the cove, are typical bedded jaspers (E10444) [SH 479 937], and at the cliff's foot the Amlwch Beds are driven southwards upon the Carmel Head thrust-plane.

The Interior

Inland, the Amlwch Beds are very unequally exposed, the centre of the region being pbscured by great drumlins (see Chapter 31), while the eastern parts about Amlwch Port are among the most rugged in the Island. There seems more fine material than on the coast. The wide phyllite zone of the shore west of Henborth has been traced to Nanner, and the lines for it (which are only-approximate) are engraved on the published one-inch map, but have been left uncoloured. Uncoloured lines for similar phyllites will be found south and north of Llanfechell, and a train of them runs through Rhosbeirio to Hafod-llin. The best exposures may be found by means of the drift-lines, but these fine beds do not furnish rocky land, and it is likely that more of them are hidden by the drumlins. The largest tract is that which lies along the Carmel Head thrust-Diane het:wenn Gwredna and Bryn-Eilian.

The beautiful folding shown in (Figure 135) is at a spot 320 yards north by east from Llanfairynghornwy Church, in phyllites with fine thin hard seams, and the folds are sharper than is shown. The tract among the basic intrusions east of Caerau is interesting because of the preservation of the ancient bedding in the indurated aureole.

The Bodelwyn beds are well exposed from Maen Arthur to Clegyrog-blils, and again at Bodewryd (though more gritty there). Like the Celyn beds of Hely Isle, they are full of sheets of venous quartz, and are more crystalline than the fine seams in the Lynas beds to the north. There are some nips of a thin basic schist near their junction with the Coeden beds, the same horizon, in fact, as that of the thin basic schist of Holy Isle. The whole thickness of Lynas beds, with la vas and jaspers, does not appear between them and the Skerries Grits of Bwlch, and much of it must be cut out by thrusting. The large phyllites near here, and the largest of all, the Llyn Llaethdy phyllite, are probably in reality the Bodelwyn beds, which had not been recognised as an horizon until after the completion of the map. Flaggy but not hard beds, with jaspery phyllites, prevail about Rhosbeirio (E8502) [SH 393 914], (E8503) [SH 393 914], (E8504) [SH 393 914], (E8505) [SH 393 914], (E8506) [SH 393 914], (E8507) [SH 393 914] and there are some obscure basic rocks that may be spilitic tuffs. The dynamical structures that simulate Oldhamia are by the barns east of Ysgellog, near the dyke. There is little doubt that the phyllites that range from Tre-gele to Rhyd-y-groes are also on the Bodelwyn horizon.

Near Gadlys and Neuadd, Cenmes, the Amlwch Beds are exposed close to the shattered Gwna Beds, as on the farm lane east-northeast of Neuadd. Yet their isoclinal folding is not shattered, showing that they are brought up from a lower tectonic zone. At the 200-foot contour, north-north-east of Bettws gate, they contain a 2-foot lenticle of Gwna quartzite. To account for its position by earth-movement is almost impossible. It is more likely to have been a floated block, like those mentioned on pp. 51.

Alternating Lynas beds, with one or two small basic schists, are well exposed all along the north side of the road from Cemaes to Amlwch, the bedded jaspers developing finely on the south side from Bryn-llywd eastwards. At the west end of Amlwch town are several masses of spilitic lava, some of them containing fine saccharoidal calcite mixed with epidote. There is a still better group of them to the east of Amlwch Port (E10531) [SH 461 933]–(E10532) [SH 461 933], very fully exposed. They retain traces of ellipsoidal structure, but are all more or less deformed, and the southern margin of the largest one (south of Aber-cawell), with which jaspers are interbedded, is converted into a chloritic schist with granular ternary albite. The alternating facies is pronounced in this district, except in a large crescent-shaped zone to the north of the word Llaneilian', which is a tolerably homogeneous grit-schist. There is fine isoclinal folding, with slightly oblique foliation, about the 'B' of 'Bryn-Eilian' (Figure 49).

The rugged moor east of Amlwch Port (Plate 55) is perhaps even a better locality than the coast for the study of the Amlwch Beds with bedded jasper, as they are here intensely folded. Beautiful steep isoclines are to be seen everywhere, especially just outside the town, a few yards north of the end of the street called Ednyfed Hill (where, as shown in (Figure 40), they become locally symmetrical), and by the pool east of that. The repetition of the jaspers (E10536) [SH 456 932], (E10537) [SH 456 932], (E10538) [SH 456 934], (Plate 2), Fig. 3; (Plate 21), Fig. 1 to the east of this pool on the Port Moor is wonderful (Figure 136), 40 nips having been laid down on the .0004 maps in 1/64 of a square mile. The pitch undulates, and it is probably this which keeps the jaspers away from the coast at Aber-cawell. Close to the 100-foot contour east of the 'l' of Brickpool is a four-inch oval boulder of granite, and a small one of Gwna quartzite. They are not in coarse, but in fine siliceous flaggy beds, and were doubtless floated to this place (pp. 51, 303).

The Gwna Beds of Gynfor, Pant-Y-Gaseg, and Wylfa <ref>Pronounced 'Gunvor', and 'Wylva'.</ref>

Along the rugged and often lofty sea-board the Gwna Beds are finely exposed, and at Hell's Mouth are cut back into sea-cliffs 200 feet in height. The special features of the district are: the waning of their vulcanism; the fine development of the quartzite, limestone, and black beds; the low grade of anamorphism, and the feeble schistosity of the Autoclastic Mélange. The western wing, from Porth Wnol to Cemaes, may be called 'Wylfa'; the narrow eastern wing between Porth Wen and Bull Bay, 'Pant-y-gaseg'; and the central and principal portion between Cemaes and Porth Wen, by the ancient name of 'Gynfor'.

General characters

1 — Vulcanism — The Gwna vulcanism is here at a minimum, especially in Gynfor and Pant-y-gaseg, where only one spilite is known, and that a very small one. Concurrently with this, the alternating beds (and the resulting mélange) lose the green tint which they possess everywhere else in Anglesey, and become ordinary grey grits and phyllites without any sign of pyroclastic dust. That these grey beds are but a facies is certain, for not only do the quartzites and limestones appear among them in the same relations as usual, but the green colour does not wholly disappear, green zones remaining (see below, pp. 308, 311) at Penrhyn and Porth Wen. There are a few rather coarse diabases, decomposed and ferrified, and as one of them cuts the limestone, it is probable that they are intrusive, and were albite-diabases. of the spilitic suite.

2 — Survivals of original structures — In spite of the extraordinary break-down of the group, the original bedding of the Gwna sediments is more often to be seen here than anywhere else. From Hell's Mouth to Graig Wen the continuity of the great quartzite (E10953) [SH 398 948], (E10954) [SH 399 947], (E10955) [SH 393 949], though its width is modified, is unbroken, and low down on the escarpment a thinner quartzite, that seems unlikely to be the same repeated, also ranges for nearly half a mile. Between them, the bedding of the Alternating group is often well preserved, and the rocks are seen to have been for the most part exceedingly even-bedded fine grits (E10520) [SH 399 947] with partings of grey shale. The grits are usually an inch or two in thickness; but there are many quarter- or even one-eighth-inch seams, and they often show good lamination internally as well. Near the quartzite they are whiter, and we have rapid alternations of shale and thin quartzite. The shales themselves are finely laminated, but may be gritty.

Bedding is well. preserved in thin quartzites with fissile partings at the foot of the great cliff at Ogo Gynfor, just where the Ordovician conglomerate rests unconforinably upon them, as will be seen by turning to (Plate 29). Thin-bedded quartzites again survive near the end of the headland west of Llanbadrig Church, and also about 400 yards to the south of the church. On the low cliffs of Penrhyn, Cemaes, grey grits and phyllites can be seen in rapid alternation.

The limestone is for the most part wonderfully massive, especially in the large old quarry west-north-west of Gadlys (Penrhyn-mawr of the six-inch and .0004 maps), but is finely bedded and even laminated along the south wall (E10518) [SH 368 937]–(E10519) [SH 368 937], where it alternates with the graphitic phyllites. The same is the case on the sea-cliff buttress to the south of Penrhyn-mawr, and in the quarry at Llanlleiana farm-lane gate (E10946) [SH 387 950].

3 — Grade of anamorphism — This is lower than in any other part of the Mona Complex in the Island, lower even than in a few of the Palaeozoic intrusions (see Chapter 18)<ref>A certain amount of difficulty arises from the fact that, in Gynfor and Pant-y-gaseg, powerful Post-Ordovician movements are superimposed upon those proper to the Mona Complex. The Gwna Beds, however, are vastly more broken than the Ordovician, which without doubt rest (see p. 243, and Chapters 13, 14) unconformably upon them; so that the effects of the two great movements can, in most cases at any rate, be disentangled.</ref>;connected with which fact is the rarity of folding, a phenomenon hardly to be seen save in the graphitic phyllites and laminated limestones. Even in the autoclastic mélange, it 'is remarkable how much the quartz of the grits has escaped from optical strain. In Gynfor and Pant-y-gaseg, the finer material hardly passes beyond the phyllitic stage, except at the places mentioned above, where there was a little spilitic dust, which evidently lent itself to re-crystallisation, the product (E10521) [SH 370 939] being then an ill-developed Gwna Green-schist. In Wylfa, this condition is much more prevalent. It is noteworthy that when the matrix is green, it is invariably more fissile, the volcanic dust having yielded chlorite more easily than the pelitic matter yielded sericite.

4 — Characters of the Autoclastic Mélange — The mechanics of this differ from those of other Gwna districts, the fragments, especially in Gynfor, being rarely lenticular, but comparatively short, blunt, sub-angular, or sometimes rounded. At the fine sections on Cemaes Bay, west of Gadlys, they are ellipsoidal or even rudely spherical, and the mélange is a characteristic pseudo-conglomerate, whose phyllitic matrix displays relatively feeble, though always some, trace of shearing. Even here, however, there are (E10923) [SH 373 942]–(E10924) [SH 373 942] lenticular cores, with phyllite sweeping past them, so that the rock is but a variety of the usual mélange produced under a less compressive stress. The compression towards the west was greater, but all over the district, even in Wylfa, there is a tendency to the blunt and the sub-angular.

On the escarpment below Graig Wen, where bedding is well preserved, a stage in the production of the mélange is revealed. There is a cleavage at a narrow angle to the dip, yet when the beds break up, it is chiefly along a series of later thrusts at a lower angle (see p. 193), and this may happen so quickly that mélange is developed in a few yards. In the massive quartzite at Graig Wen the process begins by the development (Figure 137) of divaricating schistose films, from which others branch off into it. In rare cases (Figure 138) the -old bedding of the thin grits holds together in a fragment. No good evidence has yet been found of a schistosity later than the brecciation.

In the extended sense indicated on p. 66, the autoclastic general mélange includes not merely the alternating grit and phyllite, but every other member of the group. At many sections, especially on the headland west of Llanbadrig Church and on the heights above Porth Wnol, quartzite, limestone, diabase, and jasper, ripped away from the beds between which they originally lay, and torn and rubbed into phacoids or ellipsoids, have been driven in among the wreckage of the alternating grits and phyllites; producing a many-coloured mélange that is really indescribable, and must be seen in the field to be envisaged.

Wylfa

The first appearance of the Gwna Beds is on the western side of Porth Wnol, where a belt of green mélange, with a little quartzite and -limestone, runs across the neck of the headland between two ruptures, the Amlwch Beds being clearly thrust over them from the north. In Porth Wnol there is probably a fault with a downthrow to the east, against which the Wylfa thrust-plane runs out at a very narrow angle. The thrust (Figure 96) is clearly exposed in the cliff, and the two green series are well contrasted.

On a great projecting buttress (Plate 22) about 70 feet in height, which is cut by a late basic dyke, is an amazing lenticular mélange, giving a vivid impression of the disrupting forces. The resistant quartzite is seen in the act of breaking down into great rudely phacoidal masses, between which the wreckage of the thin-bedded grits and phyllites is being driven along the lines of shearing. On the heights above is a mélange hardly less extraordinary (shown in an unpublished photograph, No. 1721), large ellipsoidal cores of quartzite, limestone, and jasper, lying in a dull Gwna Green-schist with torn strips of jaspery phyllite. Similar phenomena may be seen all alon_g the western cliffs of Mynydd Wylfa, save where a wedge of Amlwch Beds 60 yards in width is driven up through the Gwna Beds. A zone of quartzites and limestones, many- of which are large enough to show upon the six-inch maps, is traceable across the headland, and there is much purple phyllite. On its high northern cliffs the phyllite-and-grit mélange is developed upon a great scale, and is intermediate in character between that of Gynfor and of the other parts of Anglesey.

In the southern of the two chasms at the 'M' of 'Mynydd' (Figure 139) are 10 feet of a crushed spilitic lava, with two-foot ellipsoids well preserved, and plenty of scarlet jasper in the interstices. The texture in the slide cut (E10504) [SH 358 945] is not thoroughly spilitic, and it is possible that an albite-diabase and a soilite have been crushed together. In the northern chasm<ref>The sheared Paleozoic dyke (see (Figure 139)) must be distinguished from these rocks.</ref> is a similar rock (E10505) [SH 358 945], and near the north-east point of the headland are masses of dull chloritic schist with jasper.

An unexpected feature of the Wylfa mélange is the presence of three small granites. One is on the heights above Porth Wnol, the others lying to the south and north of the southern band of spilite on the western cliffs (Figure 139); and they are 51, 90, and three feet thick respectively. They are (E10422) [SH 352 943] albite-granites with a good deal of white mica, and are greatly crushed. The original junctions are destroyed, and no thermal alteration has been seen. Being albite-rocks, it is probable that they Wong to the Gneisses, and are shattered nips like those of lilanrhyddlad and Mynachdy (pp. 284, 296).

Some clue to the extraordinary complex near the north-western point will be afforded by (Figure 139), but even the .0004 map is but an abstract.

On the beach to the south of the Lifeboat Station are some light-coloured and decomposed felspathic beds with grey films, which closely resemble the Fydlyn rocks; and there seems no serious rupture between them and the green mélange. Eastwards by Wylfa house, the same green mélange contains dull basic schists, with a little jasper, and some jasper) phyllites.

Gyufor

The coast west of the Hell's Mouth Fault — At the back of the beautiful semi-circular cove the Amlwch Beds touch the sea, and on its eastern side the Wylfa thrust-plane is exposed, the two green groups being well contrasted as before. Just above the thrust is a small nip of quartzite and limestone; and the Wylfa type of sedimentation continues as far as a point north of the rh ' of Penrhyn', where an uncoloured line runs out at a little chasm in which is a small thrust. Here the grey Gynfor type of sedimentation conies on, and the Penrhyn section is one of the best and most accessible for a study of that type. The grey grits and phyllites are largely in the state of mélange, a range of lenticular limestones and quartzites being driven into them. Yet the bedding is not utterly destroyed, and opposite the house on the cliff-edge are some over-folds from the north (figured by Maley: Quart. Journ. Geol. Soc., 1899, p. 662) which only just survive disruption.

Towards the east end of the Penrhyn section, the grey beds become rapidly dull-green, at the same time shearing more, until, among the dykes, the fissile partings (E10521) [SH 370 939] are a true Gwna Green-schist like that of other parts of Anglesey. After we round the point into Cemaes Bay, the Wylfa thrust-plane must again run out, but it is probable that the visible rupture is a Palaeozoic slip cutting it, for a dyke which is injected is in its turn sheared and brecciated.

Beyond the Amlwch Beds of Cemaes Beach, the first object of special interest is the great buttress on the eastern cliff, which is composed of limestone with much black phyllite, folded, and cut by minor thrusts; the grits and phyllites of the cliff being, apparently, driven on to it. Hence, for 300 yards, is a very fine section (best approached from the north-west headland) in (E10293) [SH 299 754]–(E10294) [SH 285 769] the most conglomerate-like phyllite-and-grit mélange in Anglesey; a crowd of ellipsoidal and sub-spheroidal cores of dark grey grit in a rudely schistose matrix of grey phyllite.

At the headland, between this mélange and the great limestone, is a deformed spilitic lava, some six feet thick. It seems to pass below the limestone at a low angle, soon, however, wedging out. Ellipsoidal structure still survives, and in the inter-ellipsoidal skins (now schistose) are small aggregates of scarlet jasper. The spilite (E10929) [SH 371 940] is slightly porphyritic and vesicular, and somewhat variolitic, the fine felspars of the matrix having a tendency to radial grouping. It is the only unequivocal spilite known in Gynfor.

The great limestone of Penrhyn-mawr (six-inch map name), the widest limestone-outcrop in the Mona Complex, is cut off eastwards by a fault with a downthrow to the south-west. It is finely exposed both on the coast, and in the large old quarry below Penrhyn-mawr, in which (except along the south wall) it is wonderfully massive (E10518) [SH 368 937], and analysis]. On the coast it shows bedding in several places, and about 100 yards north of the quarry's mouth some of this bedded rock is beautifully oolitic. Mélange emerges in two or three places on the floor of the quarry, and again in the cove to the north, from below the limestone. The south wall of this quarry is the best exposure of the graphitic phyllite with laminated limestones.

The little bay beyond, on either side of the wedge of Glenkiln shale, is chiefly occupied by straw-coloured phyllites and quartzites, often well-bedded, but a good deal decomposed.

The quartzite at the end of Llanbadrig Point shows good bedding, cut by curving shears; and there are some thin bedded quartzites a few yards to the north of it. The extraordinary variegated mélange on this headland has been remarked upon above (p. 306), and the general complexity is added to by a number of small Palaeozoic dykes, and by late silicification, pyritisation, and ferrification. All the best sections are on the north crags. Besides the alternating beds, quartzite, and limestone, there are phacoids of typical scarlet jasper (E10506) [SH 373 945] (to be distinguished from some Terrified grits which are there) some of which are three feet in length, doubtless torn out of a spilite that is not now seen. Diabase of the intrusive type is also involved in the mélange. Large limestones then come on, the disruption is mitigated for a while, and between them, some 200 yards west of Llanbadrig Church, are the best exposures of the peculiar black quartzite (E10508) [SH 373 945] (p. 80), in beds about a foot thick.

By the old limekiln, black phyllites appear, and great lenticular masses of limestone sweep down the cliff along the foliation-dip. On their eastern margin, at the Wishing Well, is the best development of oolite (E9176) [SH 376 947], 10816] (Figure 8), a lenticle 30 feet thick, and very massive.

The cliffs thence to the Ordovician are chiefly phyllite-and-grit mélange, with a faint greenish tinge. In it, north of the church, is a large, dull, basic mass, rudely ellipsoidal and probably a spilite, though no jaspers have been seen in it. Just above it a three-foot quartzite shows folding, an unusual circumstance in Gynfor. Further on, a great quartzite undulates horizontally for some 200 feet in an inaccessible position.

The Gwna rocks that underlie the Ordovician at the great Ogo Gynfor section are grey mélange with quartzites. They are illustrated in (Figure 220), (Figure 221), and (Plate 29). But where the Complex rises once more en masse upon the major thrust-plane it is represented by the long limestone that runs inland to Llanlleiana farm. On the cliff, the massive parts are cut into great phacoids by innumerable curving minor thrusts at rather high angles from the north, but the thinner beds are isoclinally folded more rapidly than any rocks in Gynfor, the folds pitching westward and being traversed by a cleavage. It is locally dolomitised. One of the small but coarse diabases appears within it, the junction being transgressive and probably intrusive. On its northern margin the limestone graduates into phyllite-and-grit mélange, some of whose grits are pebbly and should be searched for composite fragments, though it is not always easy to distinguish between true and pseudo-pebbles. In the deep nook a powerful-looking thrust (Figure 140) brings, by a curious accident of the tectonics, quartzite on to quartzite, both being contained in mélange. The end of the long quartzite on which the Ordovician conglomerate rests wedges out against the fault before it gets down to the sea.

Between the Ordovician rocks and Porth Llanlleiana, the cliff is a steep dip-slope, and the circular limestone seems to be a cake resting upon grit and phyllite. Then comes the great headland of Dinas Cynfor with its large quartzite, which widens from 30 to 220 yards as it crosses the headland. At the western end it contains a limestone, but at the eastern end (where its bedding is perceptible) no less than eight appear in it. This is probably due to repetition, for at Hell's Mouth beach a large wedge of mélange is driven up into it. The eastern section in the quartzite is nearly 200 feet in height, and is very grand, but is visible only from a boat. The Gwna grits of the escarpment below thi; quartzite are in places pebbly, and contain (E11038) [SH 387 952] fragments of ancient schists.

Inland sections west of the Hell's Mouth Fault — These are both numerous and excellent, and a few only, of special importance, can be mentioned. The lines are often difficult to draw, and are also obscured by Ordovician infolds. Grey mélange is finely seen to the north of Neuadd, near the boundary, and to the west-north-west of Llanlleiana farm. The larger bands of limestone are all well exposed, being for the most part massive, but in the quarry by Llanlleiana farm-lane gate there is laminated limestone with black phyllite. The quartzites are lenticular, except the long band on which the Ordovician rests west-north-west of Llanlleiana, and even this is interrupted. About 200 yards to the south of that farm one of them contains two little limestones. On a boss at the 'd' of 'Llanbadrig ' (Pen-terfyn of the six-inch map) is the best example of the coarse diabases, but even this one is poorly preserved.

Gynfor, east of the Hell's Mouth Fault — The Hell's Mouth fault, which divides the district, and is marked by alluvium and a long boggy hollow, must be a downthrow on the west, as Ordovician infolds occur nearly a mile further to the south-west on that side. But its behaviour is anomalous, for it hardly shifts the Ordovician purple conglomerate. It is discussed on p. 219. At Hell's Mouth beach, its breccia, several feet wide, is exposed in a deserted mine-level.

The junctions of the one large and two small Gwna wedges that are let in among the Church Bay Tuffs and Amlwch Beds on its eastern side inland are not exposed, but must be ruptures. The large one, being among Amlwch Beds, must be either very deep, or be thrust southward as well as ruptured. The characters of the Gwna Beds are the same as on the west.

At Hell's Mouth itself there is a junction with the Church Bay Tuffs, which is discussed on p. 160. The section in the mélange and quartzite is a magnificent cliff 200 feet in height, but, though visible obliquely from Dynas Cynfor, can be. seen in its true proportions only from a boat. The quartzite, about 90 feet in thickness, runs up the cliff at a very high angle. Its junction with the grit and phyllite needs further study. Thence it is continuous all along the heights, but at Graig Wen its width is rapidly increased to 200 feet, and some green phyllites of the Gwna Beds, as well as purple ones, of the Ordovician purple conglomerate, are driven into it, and its southern side (which is flaggy) is traversed by the divaricating shears shown in (Figure 137). Just beyond Graig Wen it is cut off altogether, but reappears at the natural arch on Porth Wen, where it is well bedded, and contains thin phyllites. Evidently, therefore, the widening is due to movement, which must be in part of Post-Ordovician, but is probably also in part of Pre-Cambrian date. The grits and phyllites of the escarpment, though often in the state of mélange, display bedding at intervals all the way from Hell's Mouth eastwards, as mentioned on p. 305.

At Porth Wen works all the rocks are decomposed, and, but for the purple conglomerate, it would be well-nigh impossible to separate the cleaved and sheared Ordovician rocks from the Mona Complex.

The cliffs along the south side of Porth Wen are dip-slopes, so that the rocks of the beach may rise inland. At a cave 160 yards west of Castell one may walk at low water through the quartzite, and find the grit and phyllite beyond it in an open chasm where the roof has collapsed. The eastern cliffs of the bay are sections in mélange with thick bands of limestone. One of the green bands alluded to on p. 304 in connexion with vulcanise (E10523) [SH 406 944] is seen in the cove south of the limestone headland near Castell. East of this headland the cliff is, for about 100 yards, a dip-slope with cakes' of limestone and mélange. The second green band (E10524) [SH 407 945]–(E10525) [SH 407 945] is well seen in a cave a few yards to the north of this cliff. Following it are flaggy brown schists, and variable mélange full of lenticular fragments of nearly all the Gwna rocks, as far as the base of the Ordovician. On a boss about 100 yards south of Castell, adjoining a large limestone, is another of the diabases, but in very poor preservation.

That the Gwna rocks of Porth Wen are bounded by a fault on their eastern side is certain, but its course and nature are uncertain. A fault that runs out in the nook east of the little limestone headland cannot bound them north of Castell; and the boundary might be drawn along features as a winding line a little further east, and regarded as the outcrop of a nearly horizontal thrust-plane, save that the course of such a thrust in the Church Bay Tuffs to the south-east would be difficult to understand.

Pant-y-gaseg

The Gwna Beds driven over the Glenkiln shales in the cove south of Trwyn Bychan (Figure 95) are chiefly mélange in the cliff, but quartzite comes on at once above, and there is a black quartzite (E10949) [SH 408 946] at the cliff's brow. A few yards to the north, the Church Bay Tuffs are driven over them on the Trwyn Bychan lower thrust-plane.

The long narrow ridge of Pant-y-gaseg Hill is composed of grey mélange with many small nips of quartzite and limestone: This Gwna band fails to reach Bull Bay, and seems to be cut off by a small thrust which is exposed in a little cove.

Another compound zone like it lies about 160 yards to the south, but wedges of schistose tuff are driven into it; and between the two zones lie a number of detached strips of quartzite and mélange, with purple schist, that are involved with the schistose Church Bay Tuffs. At the farm south-east of Pant-y-gaseg is a curious rock that recalls the purple conglomerate of time Ordovician, but it seems to belong to the Mona Complex. These broken Gwna bands also fail to reach Bull Bay. They are undoubtedly nips of the Gwna Beds let into Church Bay Tuffs, but it is doubtful whether the lines are drawn at the correct horizons, for some of the green rocks are bedded and may be Gwnas. They need further study, especially as light might be obtained on the local relations of the groups.

Almost on their strike, a white felsitic-looking schist is let in between thrust- and shear-planes in the Bull Bay cliff. It recalls the Fydlyn rocks (as do the white beds of the Lifeboat Station at Wylfa), but ruptures of sufficient magnitude to bring in that horizon are difficult to postulate here, and this rock also calls for further study.

The Skerries Group in the Trwyn Bychan Tract

Hell's Mouth to Porth Wen — Where they first appear at the Hell's Mouth fault, these rocks do not present their characteristic appearance, having passed into pale green schists, which, however, have the usual homogeneous character of the tuffs, and resemble those of the southern part of the Church Bay section towards Trefadog. At Hell's Mouth itself they rise from below the Gwna mélange. The nature of the junction, which is accessible at low water, has been discussed on p. 160. The change is rapid on the (rather high) reefs, and slower in the rocky chasm of the cliff a few yards to the east of them. A well-rolled pebble of quartzite of Gwna type (E11251) [SH 393 949] about three-quarters of an inch long, was obtained from the tuff on the beach. Near Bryn Llewelyn a few bedded jaspers and thin basic bands appear as we approach the Amlwch Beds.

From Hell's Mouth to Porth Wen they are well exposed on a high bossy tract; and, as Bryn Llewelyn is approached, the foliation diminishes, and the rock rapidly assumes its massive aspect, with characteristic epidosite clots of irregular shape: At the gate between Bryn Llewelyn and the chapel, also by the lane just east of the 'n', overlooking Porth Wen (E10513) [SH 408 940], scraps of pink felsite can be found in the green tuff, often with re-entering curves, as of lapilli. By the roadside north of the 'a' of 'Gardwr' it contains a block of Gwna quartzite about six inches across which cannot be an autoclast, being 240 yards from the Gwna mélange. Here and there along the junction, especially at the back of Porth Wen where the formations are seen close together, the tuffs take on somewhat of the aspect of the Gwna Green-schist, as if there were a passage by increase of pyroclastic dust. Beyond the Castell fault (or thrust—if such it be—see p. 312) the tract is split by the two narrow bands of Gwna rocks, around which there are again signs of passage (see p. 312), especially on either side of the Burwen road. Between and south of these Gwna bands the tuffs vary a good deal, being sometimes a good epidosite, sometimes a green schist, sometimes rather siliceous, details of which are noted on the six-inch and .0004 maps. Whether these be true variations in composition and states of reconstruction, or whether they be isoclinal nips and rolls bringing in the passages to the beds above and below is not yet known. All along the southern margin from the Hell's Mouth fault eastwards intermediate types are found, and the line is arbitrary within limits of some 50 yards, as might be expected from the nature of the zone of passage exposed on the cliffs of Bull Bay (see pp. 158, 315). These intermediate types are seen about Bryn Llewelyn and Bod-hunod. The tuffs are well exposed in bossy ground. The Skerries Grits have not been recognised with certainty west of Liechog-isaf. Whether they be cut out by thrusting, replaced by tuff, or overlapped by the Amlwch Beds, is uncertain.

Trwyn Bychan to Bull Bay — Between Trwyn Bychan and the junction in Bull Bay, two miles of rugged sea-cliff afford an almost uninterrupted section of these rocks, and their great structural features are strikingly displayed upon the lofty northern crags. The dominant type is a dark-green schist with a platy foliation dipping northward at high angles (E9316) [SH 410 948], (E10527) [SH 409 948], (E11039) [SH 409 948], but it is often feeble and the epidosite scarcely sheared.

The Tuffs first appear (Figure 95) upon the major thrust-plane which drives them southwards over grey Gwna mélange with lenticular quartzites. By descending a northward-pointing spur of cliff this thrust-plane can be finely seen in a cave, whose floor, inclined at 30° to 40°, is its bare sole, the thrust also forming a small outlier on the north end of the spur. A few yards to the north is a much greater cave, beyond which another buttress juts out, accessible by climbing down obliquely (with careful regard to foothold) along some narrow ledges above the cave's roof, and from this buttress a fine view can be obtained of the Trwyn Bychan thrust-plane, on a cliff about 100 feet in height. It is here inclined at about 8°, and is cut by a minor thrust at a higher angle, while another seems to displace it south of the cave. But these thrusts can be seen in their true proportions only from a boat (Figure 95). On the eastern shoulder of the headland the cliff is cut back a good deal, and by descending there and turning to the left the great thrust-plane itself is accessible just where its outcrop turns round the extreme point. It is here horizontal, and the sole has a thin floor of quartz, on which are slickensides indicating that the movement was from north-north-west. In some parts there is about an inch of mylonite. For 80 yards eastwards the cliff is cut back, and the gently sloping floor of the recess, rudely triangular in plan, is the sole of the thrust with a few small outliers upon it. Then the cliff suddenly turns northward, ending the recess, and the Trwyn Bychan thrust-plane passes out of sight along its foot. Beneath it, instead of mylonite, is a zone, six to 12 inches deep, of minor thrusting (Figure 141).

Thrust after thrust now appears in the northern cliffs, and in the space of half a mile from Trwyn Bychan four more great recesses with triangular floors and vertical eastern cliffs expose their soles in the same way as at the headland. Movement is persistently from north-west or north-north-west. The outcrop of the last one turns round inland and re-appears in a chasm 147 yards further east. Between these major thrust-planes,the great sigmoidal curves into which they bend the foliation sweep grandly up the cliffs, and minor thrusts produce the same structure in miniature. On some eastward-facing crags 30 yards beyond the disappearance of the Trwyn Bychan thrust-plane, the little thrusts between the sigmoids are only a few feet or inches apart (Figure 64), and the sigmoidal cores are traversed by still smaller ones at intervals of a quarter or even an eighth of an inch. Similar sections may be seen at many places, a good one being in the lane at Bull Bay village, south of 'L.B'. Sheets of venous quartz that lie along the foliation have been bent siginoidally and thrust in the same way, and a two-foot sheet a little above the fifth of the great thrust-planes has been converted into a granoblastic schist (E10528) [SH 417 946]. This, and the fact that the thrust-planes- are 'healed up' so as to become in their turn planes of foliation, show that all these are true ancient structures of the Mona Complex; though the mylonite at Trwyn Bychan points to Post-Silurian movement having taken place along a pre-existing rupture. Towards Bull Bay little is to be seen but the old foliation dipping at high angles, except certain zones of silicification, also of ancient date.

Of lithological variety there is little to record, indeed the accumulation must have been extraordinarily homogeneous, and but for the foliation it would appear almost as one solid mass, merely a little coarser here or finer there, with sonic short epidositic grits. Bedding is hardly to be seen, but can be traced, at a crag just east of Trwyn Bychan, for a few feet, like the local bedding of the tuffs in Church Bay. As in those tuffs, however, there are a few short purple seams, best seen at, and half a mile west of, Porth Llechog. They are to be distinguished from the zones of recent oxidation to which these tuffs are liable. The clots of epidosite, which first suggested that these rocks were a facies of the Church Bay Tuffs, are to be seen everywhere (sheared out into thin short green bands in the more foliated parts), but specially good localities are the crags a little east of Trwyn Bychan, 100 feet or so above the thrust-plane; and some bosses about 70 yards south-east of a deep coast-chasm, north of the 'll' of 'Windmill'.

Little scraps of pink felsite are generally to be found if searched for; but larger fragments, all of Skerries types, have been seen at the following places. A well-rounded pebble of granite (preserved in the Museum) about an inch in length, was obtained from schistose tuff (E10526) [SH 422 946]: matrix only] 60 yards south-west of the eastern cave'. Similar fragments, but sheared, may be seen 270 yards to the west, around the great stone chair. Fragments of pink felsite occur above the most easterly of the triangular soles'. Felsite and Gwna quartzite pebbles are not rare at the cliff's brow over the great cave of Trwyn Bychan, one pebble being bisected (Figure 142) and thrust up a third of an inch along the foliation.

Towards Porth Llechog the tuffs (E1545) [SH 428 944]–(E1546) [SH 429 941], (E10990) [SH 427 944] are more schistose, and there are some short pale purple phyllites, but green grits of Skerries type appear on the northern crags of the point. The western cliffs of Bull Bay afford a continuous and easily accessible section. At Porth Llechog are highly sheared tuffs that simulate phyllites, but thin grits of Skerries type soon begin to appear, and after we pass the narrow chasm about 100 yards to the south, massive Skerries Grits come on in force, with here and there a few thin fine ribs that seem to herald the Amlwch Beds, while the schistose tuff keeps on recurring in bands. On the south side of a chasm some 30 yards north of the felsite dyke a white felsitic schist (p. 312) is let in between shear-planes. It is on the strike of the wedge of Gwna Beds. South of the dyke, the Skerries Grits are again well developed, and are very massive. Then follows the fine section displaying the passage to the Arnlwch Lynas Beds (pp. 158, 300), which closely resembles the passage at Llanrhwydrys cliff (pp. 158, 317).

The Skerries Grits between Llanfechell and Llanrhwydrys

Their first appearance inland is at a knob at the south end of the Rectory garden of Llanfechell, where there is a little flagginess at the top, as if the Amlwch Beds were near. Then, just beyond the bridge at the west end of the village, is a range of good exposures, in which are (E11005) [SH 366 912] fragments of micro-pegmatite like those of The Skerries. That these are not on the main infold is shown by the appearance of Amlwch Beds at one or two places between them and the Bwlch road. The tract at Cromlech seems also to be isolated, but its relations are obscure.

Where the main infold is nearly wedging out, there is a section on the south side of the road at the Ll ' of Llanfechell, which is of stratigraphical importance (cf. p. 158), for on the southern escarpment is a clear passage by alternations into the Amlwch (Lynas) Beds. Some of the best places for a study of these grits are about Bwlch (E8442) [SH 37 90], (E9309) [SH 354 914], (E10409) [SH 353 913]. There is a little bedding, crossed by the foliation at a higher angle; but most of the grit is massive, and the fine bands are impersistent, some, indeed, being merely sheared mud-galls. At the roadside quarry the unrolled rhyolite-quartz may be obtained in quantity, there are three-quarter-inch fragments of salmon-coloured felsite, and it was from this quarry that the pebbles of old schistose grit (Figure 2) were obtained. On the little moorland west of Groes-fechan the grits are extensively exposed (E10933) [SH 350 917], and there are again some impersistent fine beds. There is a southerly roll-over in them near the southern end, and as this reaches the margin of the grits for some 200 yards, it is probable that a thrust-plane runs in obliquely there. As these dips are quite local, normal northerly ones being resumed at once in both directions along the same junction, they point merely to disturbance, and are no evidence for the grits rising en masse on an anticline from below the Amlwch Beds. At the interesting ancient farmhouse of Mynydd-ithel are good exposures, and across the river to the north-west Amlwch Beds appear in their turn on a southerly dip as if rising on an anticline within the grits. Southerly dips occur again to the north of Pen-yr-orsedd, but the usual northerly direction is quickly resumed. These dips to the south may be due to a refusal of the stubborn, unstratified mass to conform to the general isoclinal folding of the region. But there are south-easterly bedding-dips at Caerau, so they are more likely to be due, as is the case in Holy Isle (see pp. 208, 262, 277) to deflection about large unseen basic intrusions, of whose existence here (see p. 321) there can be no doubt. Between Pen-yr-orsedd and Phis-cemlyn are pebbles, usually deformed, attaining a length of three-quarters of an inch, most of which are salmon-tinted felsites, but with hypabyssal acid fragments like the great boulders of the Skerries, and fine quartzites of Gwna type, as well as films of grey schist which appear to be reconstructed mud-galls. Some of the grit is like the epidositic Trwyn Bychan tuff. There are good exposures also at the east end of the lagoon (E10937) [SH 333 940]. On the stack in the bay near Craig-yr-iwrch the grits are very pebbly, containing (E10384) [SH 306 929] Gwna quartzite and jasper as well as the acid igneous fragments, and also old mica-schists with a foliation oblique to that of the enclosing rock.

The finest section through the Skerries Grits upon the main Island is where they run out to sea below the drumlin on Llan-rhwydrys cliff. They are not so coarse here as they are inland, but the usual fragments can be recognised. The section is of the first importance, and has been briefly described on p. 159, on account of the conclusive evidence to be seen at its southern end of a passage from the Skerries Grits into the Amlwch Beds. The position is in the north cliff of a creek, just beyond a dyke and a small fault which drops the Skerries beds a little. Along the cliff's foot emerge the Lynas beds, flaggy grey-green phyllites with alternations of thin hard grits. Then the coarse massive Skerries Grits come on, with flaggy phyllites in them at intervals. Further up, at the turning of the cliff, there are still some fine beds, but they are now impersistent, ending irregularly by contemporaneous erosion. Towards the east end of the section the phyllites reappear, but the coarse grits are thrust on to them again. Finally, at the end of a beach, there is (beyond an obscure dyke) a crush at a cliff's foot, and the Amlwch Beds come on en masse. But the thick phyllites of Henborth are not present, and there is no sign of passage, so it is evident that the Amlwch Beds are driven over on a thrust-plane. How the grits have narrowed north-westward from Pen-yr-orsedd can be seen upon the map. Now, at the south end of the sea-section the dips turn round upon the foreshore, and become south-easterly, so it follows (see pp. 177, 218) that the great compound isocline of the main infold is rudely boat-shaped, and that its base is rising seawards.

The Three Mouse Islets

The Middle Mouse — As this differs from the other two, it will be considered first. Most of it is composed of pale green phyllite (E9318) [SH 383 959], (E10509) [SH 382 959], (E10510) [SH 383 959] with bands of hard fine green grit, and a few seams of jaspery phyllite; undoubtedly the Lynas division of the Amlwch Beds. The foliation crosses the bedding, and as it is rather feeble, the original nature of the sedimentation of the series can be studied better on this islet than at any other place. As well as the fine grits, there are some rather thicker and coarser purple-green ones. At the east end a very massive bed of the same type (mentioned on p. 158) rises, but it is pebbly, and very ashy-looking, with mud-galls and fragments an inch or two in length. The matrix is chloritic, but the galls and many of the pebbles are purple-stained; and the rock recalls the Tyfry Grits, as well as those of the Baron Hill outlier, or even the Arenig conglomerates of the Tywyn Trewan, which often resemble them. The fragments include spilite, albite-trachyte and keratophyre, rhyolitic albite-felsite, bedded gritty tuff with fragments of keratophyre, micro-pegmatite with porphyritic albite, Gwna quartzite, and quartzose schists with an old foliation (E10511) [SH 382 959], (E9319) [SH 383 959], (E9320) [SH 383 959], (E9321) [SH 383 959], (E9322) [SH 383 959], (E9323) [SH 383 959], (E9324) [SH 383 959], This grit must be regarded as an exceptional development of the Skerries Group.

The West Mouse consists of a massive, hard, grey-green grit, of much the same type as that of Llanrhwydrys. There are patches with small pebbles of the kinds always found in the Skerries Group, but most of the rock is comparatively fine. North of the Beacon good even bedding is to be seen, while a rude foliation dips in the opposite direction, and there are a few purple streaks. The grit belongs without doubt to the Skerries Group (E10572) [SH 303 942]–(E10573) [SH 303 942]. Two lines of late crushing cross the isle from east to west.

The East Mouse is composed of a schistose but massive epidositic ashy grit of the type found at the junction of the Church Bay Tuffs and Skerries Grits, showing that the zone strikes eastwards along the sea-floor towards Point Lynas. There are a few purple seams, like those about Porth Llechog. The greater part is rather fine, but on the southern escarpment the greenish mudstone clots run together into rude anastomosing bands (E10539) [SH 445 942]. The foliation dip is high, but there are southward thrust-planes at lower angles, producing sigmoidal curves like those of the Trwyn Bychan tract, though less powerfully. Here and there the fine tuff contains curiously isolated pebbles, but on the southern escarpment is almost a conglomerate, with many pebbles (E11253) [SH 445 942], (E11254) [SH 445 942], (E11255) [SH 445 942], (E11256), (E11257) [SH 445 942], (E11258) [SH 445 942], (E11259) [SH 445 942], (E11260) [SH 445 942], (E11261) [SH 445 942], (E11262) [SH 445 942], (E11263) [SH 445 942], (E11264) [SH 445 942], (E11265) [SH 445 942] an inch or two across, and ranging up to a foot or more, among which are quartz-felsites, micro-pegmatites, hypabyssal granitoid rocks of the kind so abundant on The Skerries, old greenish grits, and Gwna quartzites. The grits and quartzite, one of which latter (E11253) [SH 445 942] is six inches in diameter, are much more numerous than in the Skerries conglomerates. The igneous pebbles are deformed, but the sedimentary ones often retain their original rounded water-worrr form. There are bands of coarse grit with fragments of quartz-felsite about as big as peas, and many that are merely felsite-quartz with a little matrix adhering. A very little more disintegration would furnish the unrolled felsite-quartz (p. 59) so abundant in the Skerries Grits.

The Skerries

This remote and rugged little archipelago ((Plate 26) and (Folding-Plate 14), from which the place-names here used are taken) comprises no less than 17 islets, all but two of which are accessible at low water (some, however, for a short interval only) from the lighthouse island, and that is connected with the other large island by a footbridge. No one who knows the rocks of Anglesey can have a moment's doubt that the Skerries rocks belong to the Mona Complex, or that they, the grits of the Llanrhwydrys infold, and those of the Trwyn Bychan tract belong to one and the same member of the series. All three have the same petrological peculiarities both on the large scale and the small, and the same peculiar pebble-contents. But as The Skerries are by far the most interesting of the three, and have furbished much more decisive evidence as to the chronological order of succession, their name is the best that, can be given to the group.

The rocks are intermediate in character between those of Bwlch and Trwyn Bychan.<ref>A suite of 19 slides (E10579) [SH 268 948], (E10580) [SH 268 948], (E10581) [SH 268 948], (E10582) [SH 269 951], (E10583) [SH 270 951], (E10584) [SH 266 947], (E10585) [SH 266 947], (E10586) [SH 266 947], (E10587) [SH 265 948], (E10588) [SH 266 947], (E10589) [SH 265 948], (E10590) [SH 266 947], (E10591) [SH 269 950], (E10592) [SH 268 948], (E10593) [SH 268 948], (E10594) [SH 268 947], (E10595) [SH 268 948], (E10596) [SH 268 948], (E10597) [SH 266 947], (E10598) [SH 268 948] (Plate 2), Fig. 4, Fig. 5, has been cut from the grits and boulders of The Skerries.</ref> Those of Bwlch are ashy grits, those of Trwyn Bychan gritty tuffs. In these, the gritty and the epidositic-ashy matter are more nearly balanced. The typical grit of The Skerries is extremely massive and tough, with tints varying from grey to dull olive-green, but is nearer to the Bwlch than to the Trwyn Bychan type. The greater part of it is perhaps not quite so coarse as that which is prevalent at Bwlch, but both coarser and finer varieties are frequent. Sub-quadrangular rhyolite-quartz may often be observed, and volcanic fragments abound in all the slides. The matrix is full of epidosite and white mica, with crystalline grains of epidote as well. The massiveness of the formation all over the islets is remarkable; but local bedding has been observed in some five places, where short bands of mudstone or the disposition of pebbles render it perceptible, especially on the little northern peninsula and on Ynys Arw, dipping at high angles to the north. By the deep cleft in Ynys Arw these mudstones are undoubtedly true beds, and in place. But elsewhere (and they are seen in many places), especially in the conglomerates, they are broken up, rolled, and cut off in such a way as to show that contemporaneous erosion is the cause, not cataclastic deformation. The best locality is on the south cliffs of Ynys Arw.

Four series of dynamical divisional planes can be distinguished, viz., foliation, thrusting, jointing, and faulting. The foliation is, except in certain zones, rather less intense than it is at Bwlch, but is accompanied by good mineral reconstruction in epidote and mica, dips being at high angles to the north. The most powerful deformation is at the Lighthouse garden, where boulders of granite have been rolled out into long lenticular strips as in (Figure 3), p. 61, and internally sheared as well. On the western cliffs north of the footbridge a one-third-of-an-inch quartz-vein that cuts the foliation at right angles is itself shifted many times along it, thus revealing a second movement along the same planes. Thrusting, also from the north, but at lower angles, crosses this foliation. Two thrust-planes traverse Ynys Arw, and others may be seen on the Lighthouse crag (Figure 143), but the most powerful thrust-plane is that along which a little sound has been eroded, isolating the Toucan, the peninsula, and Berchan. The grits are mylonised upon it, their epidosite flowing into streams in which the deformed quartz-grains float.

The most conspicuous series is a system of jointing with southeasterly and north-easterly dips, that simulates bedding. It is not a pure jointing, for at the north end of the large ' footbridge ' island and on the northern peninsula some feeble crushing has been observed, and it is older than the Paleozoic sills, which are injected along it. Finally, there is a system of late faults trending approximately north and south, which have initiated several of the little sounds.

Isolated pebbles may be found anywhere, and beds of conglomerate occur all over the islands. The nature of the pebbles is much the same everywhere, but it may be mentioned that the two-inch pebble of Gwna quartzite was found on the south, slope of the Lighthouse crag, and the best one of Gwna jasper under the wall on the eastern side of the garden. Ten beds of conglomerate have been laid down upon the 25-inch' (.0004) maps; a broad one on the southern cliffs of Ynys Arw and on Flood Point islet, five thinner ones at the Lighthouse and garden, one on the south point of the footbridge island, one on Cave Point, one on the little northern peninsula, and one on the Toucan. The broad zone on Ynys Arw is the great boulder-bed alluded to, and is the most remarkable zone in the islands. On the west cliffs of the deep cleft internal bedding can be seen in it, but the great granitoid boulders are upon the brows of the southern cliffs.

Basic intrusions of the north

These include serpentines, ophicalcites, dolerites, and atnphibolites, and are known at 13 places in the Northern Region. The intrusion of Mynachdy is exposed on the coast, at a pit north-east of the house, and by the stream that comes out of the marsh. It is really a small complex, composed of true serpentine, ophicalcite, amphibolite and dolerite. On the coast amphibolite and serpentine cling to a cliff of baked Amlwch Beds, and in the floor of the nook serpentine and variegated ophicalcite rudely alternate in five- or six-foot bands, often schistose. It is here that the garnet-serpentine (E10996) [SH 308 930] with stars of antigorite is found. In the pit, green serpentine again alternates with ophicalcite, and there are obscure dolerites. The pit was worked for asbestos, which may still be seen there and in blocks in the farmyard walls. Three specimens examined had the refractive index of chrysotile, but the serpentine may contain tremolite as well. By the stream the Amlwch Beds are powerfully baked, and as contact-spots in the baked rocks on the coast (E10939) [SH 308 930] are slightly frayed out and cut by strain-slips, the intrusions must be ascribed, as in Holy Isle (pp. 109, 208–11), to an interval in the great movements.

The four intrusions of Caerau (E10948) [SH 325 919] are dykes, with chilled selvages. They were coarse dark ophitic dolerites, but their augite survives only as cores within green hornblende. Their felspar, now very decomposed, was an andesine-labradorite. The two intrusions west of Cefn-coch resemble them, but seem still more amphibolised. These intrusions also are surrounded by induration-halos, well seen at Caerau. The indurated sediments (E11343) [SH 322 918] are highly altered. Clastic quartz survives, but the matrix is almost wholly transformed into white and brown micas, in well-formed plates as much as 0.25 millimetres in diameter, which lie in all directions, accompanied by cubic iron-ores and a little tourmaline. Some beds are crowded with pseudomorphs that resemble andalusite, but they are indistinct in thin section, and are now chiefly composed of white mica. This alteration is much too intense and too extensive to have been produced by the visible dykes, and must be ascribed to a much larger (and probably gabbroid) intrusion of which they are apophyses, now either buried or completely removed by erosion. The original bedding of the Amlwch Beds is well preserved, the indurated rocks are but faintly schistose, and small shifts that cut the junctions are healed-up', so these dolerites must also belong to an interval in the great movements, are probably connected with the serpentines, and may be equivalents of the gabbros of the Strait of Holy Isle. The preservation of their augite is doubtless due to the protection afforded by the wide indurated mass in which they enclosed themselves. To an unseen member of the same suite of intrusions is to be ascribed a zone of schist at the foot of the north cliff of Porth-newydd (six-inch map), the beach at the north boundary of the Ordovician of Mynachdy (E10383) [SH 298 931], full of pseudomorphs about one-eighth of an inch in length which lie in all directions, but are sometimes frayed out along the foliation. They are now composed of mica, chlorite, and delessite, with some quartz, but show good cross-sections as of an orthorhombic mineral with combinations of pinacoid and prism, and are zonal, so that they may be referred with confidence to andalusite.

At Cefn-coch Mill there is a good tremolite-schist, on the strike of which an ophicalcite has been quarried. About half a mile south-west of Llanfechell is a green serpentine intimately associated with a purple-red ophicalcite (E10938) [SH 364 909], (E11018) [SH 369 912], (E11019) [SH 369 912] (E11020) [SH 369 912], (E11025) [SH 369 912], (E11034) [SH 369 912], which comes on in mass along its northern side and contains antigorite-stars among the carbonates. Two more ophicalcites lie to the south of it and are exposed in the lanes. All these rocks are schistose in parts. At Tregele there is a massive limestone (E515) [SH 356 926], purple-red but variable, with serpentinous patches and a bright green antigorite, best exposed in the garden of the chapel-house. At Hafod-onen is a similar limestone full of grains of serpentine, with a little talc. Some of it is bright red and rather hard, simulating jasper. Parts of the carbonate-mosaic (E10388) [SH 423 914] show very clearly the mesh-work structure of serpentinised olivine, picked out in hematite, thus revealing that the limestone is a second pseudomorph of a peridotite. At Tyddyn-dai is a larger mass of the same kind, in which is a band of tremolitic serpentine. The carbonate is (see analysis) dolomite, so probably all these hsematite-ophicalcites are dolomitic. The intrusions of this suite are on or near the horizon of the Bodelwyn beds.

The Middle Region and its inliers

Gneiss of the Middle Region

The Southern End

The only place where this is known to the south of the Holyhead Road is in the ravine at Bryngwran, and the exposures are poor. At the Treban fault by the main road north-east of Fach is a strong feature, at whose foot the gneisses are crushed. Between Treban and Caerglaw, opposite the alluvium, a feature again overlooks the road, but on the southern side, and the crag is composed of mica-hornfels, considerably crushed. This locality is important (see p. 162), for, only 65 yards away on the north side of the road, gneiss appears, with abundance of granitoid matter. On the south side granite is sharply delimited from hornfels, but on the north graduates into gneiss. This gneiss (E9189) [SH 369 768] is thoroughly foliated, whereas the structure of the hornfels is crystallisation along bedding. Finally, the gneiss is an albite-oligoclase-biotite rock, rich in sphene and apatite, thus differing completely from the composition of the hornfels. Just east of this, a wedge of acid gneiss runs up into the basic. It is quartzose, with sillimanite (E9807) [SH 372 768], (E11393) [SH 371 767] 'faserkiesel' being visible on the rough weathered surfaces. A high boss 350 yards north-north-west of the 170-foot level is the place where sillimanite was first found in Anglesey, but much richer localities are now known.

Allor

Between Treban and Clegir-mawr (which may be called the Allor tract, from the conspicuous Craig-yr-allor or 'Altar Crag' a little north of the alluvium) is the finest development in the Island of the hornblende-gneiss, all whose phenomena can be studied with perfect ease upon its crowd of rugged bosses. Some sections have been already described on pp. 131–3; but it may be convenient to add here a list of the best localities for study. The old unfoliated mottled rock (E9905) [SH 379 777] (Plate 12), Fig. 4 is best seen 500 yards north by east from the road-fork at Clegir-mawr and on the top of a large boss at the south end of Werthyr alluvium; and its drawing-out, with tracts of white albite (Figure 12), at the T' of Pandy Treban', near the north end of a long crag, facing the alluvium. Typical banded gneiss is everywhere, but the summit of Craig-yr-allor (E8479) [SH 372 774], (E8480) [SH 372 774], (E8481) [SH 372 774], (E8482) [SH 372 774], (E9803) [SH 371 774], (E10729) [SH 372 773] (Plate 12), Fig. 1 shows it grandly; folded, on a north-west strike, with epidote, knots of coarse albite, and a little microscopic pyroxene (E10729) [SH 372 773]. Another excellent section is at a quarry 320 yards north by west from Clegir-mawr. The strips of biotite-gneiss (E11187) [SH 376 776]–(E11188) [SH 376 776] are frequent, and the largest are shown upon the one-inch map. On the eastern side of the south bay of Werthyr alluvium a gneiss with pale biotite (E11378) [SH 374 778] contains abundant sillimanite in its quartz, and there is a similar band west of the road-fork of Clegir-mawr (E6101) [SH 378 775],] which points to these inclusions not being products of the basic magma, but xenoliths of the gneiss-proper. Perhaps the best exposure of an albite-pegmatite is at a small quarry (Plate 13) close to the fork in the lanes west-south-west of Clegir-mawr; and the borders of coarse hornblende are well-developed on the boss west of this one, as well as at the quarry 320 yards north by west from Clegir-mawr. For others see (Figure 13), (Figure 14), (Figure 15). The great 'plutonic breccias' of (Figure 16), (Figure 17), (Figure 18), are on the southeastern side of a lofty boss about 320 yards west of Clegir-mawr. Though the basic mass as a whole has a north-east to south-west trend, the strikes within it are extremely variable. The terminating fault on the north-east is conjectural, and an alternative line is added on the six-inch maps. Late mylonitic films may be seen anywhere.

Llandrygarn and Gwyndy to Lleclwynfarwydd

At Llandrygarn, the primitive black basic nodules with shells of pegmatite are at a boss 160 yards south-south-west of the Church, and the rude hornblende-gneiss produced by their drawing-out (E6102) [SH 382 795] is seen at an escarpment near the south end of the farmhouse just across the little alluvium to the north-west.

The only place where potassium-felspar has been found in the gneisses is at a large quarried boss 100 yards east by north from the church, in a biotite-gneiss of most unusual character, containing porphyroblasts an inch or more in length of clear white orthoclase, which are Carlsbad twins. The felspar of the body is albite-oligoclase. This rock has therefore affinities to the Coedana granite, and, as that is exposed on the strike to the south-west with only half a mile of covered ground between, is likely to be due to permeation from an apophysis of it. Turning for a while westward: in the long train of knobs to the south of Clegir-gwynion almost every variety can be seen, with amphibolitic lenticles as well. Whether the granites in it belong to the Coedana group or to the gneissic series is not yet certainly known, but that of the great boss at Clegir-gwynion is an albite-rock. Another granite sill is exposed at Treferwydd. A quarry by the roadside at Pen-yr-orsedd, though not very good as a structural section, owing to late crushes, is perhaps the best collecting-spot in the region for the normal quartz-albite-biotite-muscovite gneiss (E9951) [SH 384 800], which is very fresh (though the biotite is chloritised). There are white-mica pegma-tites, and a little sillimanite. North and east of this are good exposures, with 19 lenticular hornblendic masses, the larger of which are shown upon the one-inch map, but they often simulate the biotitic rock, and only a drastic search will reveal the true proportions of the two, especially as there is a light-grey type containing both minerals (E1637) [SH 389 801]–(E1638) [SH 390 800]. South of the road, between the two farms, are hornblende-pegmatites with albite-oligoclase; and the most southerly bosses near the dyke afford very clear sections with irregular inclusions (Figure 19), (Figure 20), in a coarse oligoclase-granite. The lines about here were Often perplexing to trace, on account of the variable dip and strike, together with the uncertainty of the types themselves. North of Gwyndy, between the roads by the School, is heavy crushing, both mylonitic and brecciating, and some hornfels is involved with the gneiss in this, so that the granite wedges probably belong to the Coedana intrusions. But most of the rock seems to be crushed gneiss.

The large bosses to the south of Gwyndy are of gneiss, coarse and granitoid, with a few siliceous seams. The granitoid matter frequently graduates into the gneiss, among which it may make its appearance along the strike by decrease of quartz and micas, and increase of albite. The strike changes rapidly. The best sections in the whole Island for a study of the structures of the gneiss are the escarpments that overlook the Llynfaes road immediately to the south-east of Gwyndy. They have been described and figured on pp. 141–2, and display perfectly the four highest stages, i.e. lenticular interfelting, permeation, granitoid gneiss, and gneissoid granite. It is hoped, however, that collecting will not be attempted here. Clear though the exposures are, the rock is decayed, so that good specimens will not be obtained, and the attempt will merely injure one of the finest sections in Anglesey.

On the slopes below Llechcynfarwydd Church the gneiss is but moderately exposed; but the large boss at the yd, 617 yards east-south-east of the church, in the angle between the road and the farm-lane, from which came the specimen analysed (E9939) [SH 386 808], and also (E1635) [SH 386 807], is one of the most interesting in the Middle Region. Lenticular interfelting is well seen, with albite-oligoclase-pegmatite. But the body of the gneiss is richer in sillimanite than any in the Island except those of the Nebo coast. It is chiefly in 'faserkiesel', and large parts of the rocks—are much richer than the specimen that was analysed. Garnet also is more plentiful here than anywhere else, the crystals weathering brown, and into hollows.

Henblâs to Mynydd-mwyn-mawr

The picturesque rocky tract about Henblâs is notable as the great locality for the banded gneisses. The material (E9666) [SH 401 810], (E9667) [SH 398 808], (E9668) [SH 400 811], (E9669) [SH 400 810], (E9670) [SH 401 810], (E9671) [SH 400 811] into which the granite-bands (E9458) [SH 399 808] have come, is chiefly 'B. C'., i.e. the granular and flaky biotite-gneisses, but some 150 to 200 yards west of the house a good deal of the siliceous 'A' type (E9950) [SH 397 807] is present also. On a steep escarpment about 150 yards north-west of the house, overlooking a deep drain, is a fine section in lenticular interfelting with an approach to permeation (E6106) [SH 398 808]. Though banding is seen to the west, the great banding sections are on escarpmental bosses looking southwards (Plate 15), which will be found between 100 and 250 yards north of the bridge where the lane from Sherry crosses the brook. All the banding phenomena of pp. 139–140 are here clearly seen, but the rock is not as fresh internally as it looks, and the banding sections should not be injured by attempts to collect from them. The albite-granites are very simple (those laid down upon the map are exceptionally thick, and are not the bands of the 'lit-par-lit' injection), but the intervening gneiss varies, being locally garnetiferous and containing abundant apatite. Much of it also is a pale hornblende-gneiss, in which are some of the large crystals of sphene. About 320 yards north of the bridge the regular, even, 'lit-par-lit' injection disappears, and the granites anastomose, running together into knots, in which they are apt to become coarser. In these there are inclusions of the gneiss (both biotitic and hornblendic), but the granite-margins are here somewhat nebulous, and permeation is beginning to set in. Lewisian and Eastern Sutherland types are both present. There is a little folding, but not on any ascertained plan. The dips about Henblâs are for the most part northerly and low, but the direction varies a good deal in places. The basic mass west of the farmhouse contains (E9945) [SH 399 807] a zeolite which Dr. H. H. Thomas refers to natrolite. It is a true anamorphic product, and may be almost original.

The small basic masses west of Mynydd-mwyn-bach are pale foliated diorites (E9986) [SH 401 817], (E11383) [SH 410 824] with the large spheres. Good exposures occur again at Mynydd-mwyn-mawr, the types being largely granular with some flaky, and many small hornblendic lenticular masses. Garnet, unusually fresh (E6107) [SH 408 823]–(E6108) [SH 408 823], (E11384) [SH 410 824], with well-preserved brown mica, is abundant in some albite-gneisses (E9991) [SH 406 823]–(E9992) [SH 409 823] that contain also oligoclase. Lenticular interfeltin_g is the chief mode of granitoid injection. The dips are high and the strike changeable.

Gneissic inliers of the Middle Region

All of these are essentially of the same nature as the main tract itself.

Cae-howel — Lenticular interfelting and incipient permeation prevail here, the granitoid element being rather coarse; and there are a few small amphibolites. In several places there is an epi-clastic aspect, as though remains of the Ordovician conglomerates were still adhering to the surface, especially between Cae-howel and the alluvium. But the fragments may be cataclastic. Penrhiw — This is a micaceous permeation-gneiss, granitoid in parts. A coarse vein south-east of the house contains large crystals of pink orthoclase, as if the Coedana granite were present. Ty-hên — There is a good deal of the fine siliceous type, and also some very micaceous bands. Tai-uchaf — The inlier to the north-east is of coarse gneiss. Llyn Traffwll — The perplexing little complex' on the western shore, near the church, contains two inliers of granitoid gneiss, with a sill of gneissoid granite in the northern, and knots of amphibolite in the southern one. As the difficulties of the sections arise from the relations of these to the Arenig conglomerates and the diabase sill, the complex' is discussed under those heads (see Chapter 14). Llanerchymedd— The large inlier of Bryn-gwallen (E1632) [SH 407 838], (E1633) [SH 407 842], (E1634) [SH 407 842], (E9800) [SH 404 836], (E10722) [SH 402 836], (E9808) [SH 404 836] is composed largely of the lenticular flaky type, with both micas, but the granular type is also present, and there are granite sills. Near the south end there is banding. East of the north end of Ty-croes lane a hornblendic gneiss contains very fresh brown biotite. Where the lane joins the Llanerchymedd road abundant sillimanite is present, and in a quarry by the roadside at the bend a granitoid gneiss contains green pseudomorphs probably after garnet. The two small inliers east of this one are obscure. The three to north and west resemble it, there being good banding at the south end of the north-westerly one. The one north of Chwaen-bach is granitoid, as is the small one in Chwaen-goch farmyard. The last one of all, west of this, is a gneissoid granite.

Basic Gneisses near Llangwyllog

The long train that follows the margin of the Coedana granite from the railway to Llanfihangel-tre'r Beirdd (E6099) [SH 454 831], (E6103) [SH 440 815], (E6104) [SH 442 816], (E6105) [SH 428 808], (E10735) [SH 455 832]–(E10736) [SH 456 832] is not well exposed, the best exposures being by the railway, by Ynys-goed, north-west of Bryn-goleu, and south of Phis Llanfihangel. By the railway strikes are locally north-west. The rest of the way the dips are for the most part at high angles off the granite, bit the relations with the adjacent rocks are not seen, save that south of the Plâs a quartzose gneiss appears on its southern ridge, which may be a re-action rock with the Penmynydd schists. For the most part the basic band is fairly foliated, and should be called a hornblende-gneiss, but is a massive diorite at some places near the north-east- end. Near Plâs Llanfihangel it is a hornblende-albite rock occasionally rich in sphene, rutile, and apatite, and with large decomposing garnets. It is thus allied in composition to the basic gneisses of the Middle Region and the Nebo Inlier. But epidote and zoisite abound, and the albites are partly saussuritised, so that its condition resembles that of the basic gneiss of Holland Arms.

Gneiss of the Nebo Inlier

In this inlier, conveniently named from the great smooth hill of Nebo, the gneiss attains its greatest elevation (550 feet), but the hill itself does not furnish good exposures, decomposition being strangely advanced for a mass that has been able to survive as a monadnock (see Chapter 34); and there are far better crags on the 300-foot platform between it and the sea. The coast, however, is an excellent though not lofty -section, interrupted only by the wedge of shale at Porth Helygen. All the gneissose types are well represented, the flaky ('C') type being dominant as usual.

The Interior

The best section -in basic gneiss is by the alluvium, south-west of Pensarn,—a foliated biotite-diorite, passing into hornblende-gneiss with pegmatitic seams. A foliated granite lies beside it, and the small one to the north of that shows transgressive junctions. The great sphenes described on p. 130 are in a hornblende-albite-biotite-gneiss (E10850) [SH 446 903] (Plate 12), Fig. 2 rich in ilmenite and in large broad apatites, collected by Blake, who remarks on the sphenes. The locality is given simply as 'East, of Parys Mountain', and has not been identified. None of the large granite sills reach the coast,- but can be studied easily, a good locality being by the Nebo thrust-plane at Plâs-uchaf. They, and also the thin sills on the coast, are albite-granites with much oligoclase, and are therefore, like those of the Gader Inlier, to be referred not to the Coedana but to the gneissic suite. The Gneisses are well seen between this and the Rhos-mynach farms, unusually fresh rose-coloured garnet (E10267) [SH 482 910] being abundant on a small boss 300 yards south-east of Rhos-mynach-fawr. Graphite can he found at the farmyards of Rhos-mynach-isaf. The forsterite-limestones (E204), (E10264) [SH 483 901], (E10265) [SH 480 902], (E10266) [SH 488 911] and analysis, (E10568) [SH 489 911], (E10842) [SH 487 909] are exposed at some barns south of the 'B' of 'Bryn-fuches', also at a 'quarry<ref>It is nearly certain that (E204) came from this quarry, as it is labelled 'Bryn-fuches'. It was collected many years ago, probably during the one-inch surveying.</ref> 300 yards west-north-west of that house, and at three bosses on the limestone band that runs from Rhos-mynach-isaf to the northern boundary of the inner.

The Coast

The Coast (which should be visited at low water) will be described best by beginning first at the south end and then at the north end, proceeding in each case as far as the wedge of Ordovician shale at Porth Helygen beach. It is unfortunate that, except the siliceous bands, the rocks are deeply decomposed all along, for they are the most beautifully crystallised and mineralogically the most interesting in Anglesey.

So soft are they as to be easily crumbled by the fingers, and hammered fractures are absolutely useleffs, but the structures and many of the minerals can be clearly seer and studied on the sea-washed face. In addition to this decomposition they are, for nearly a quarter of a mile on either side of the Old Red Sandstone, stained to a deep red colour (see end of Chapter 21).<ref>Nevertheless, the slides (E9527) [SH 491 909], (E9528) [SH 491 909], (E9529) [SH 490 901], (E9530) [SH 491 909], (E9531) [SH 491 910], (E9532) [SH 492 911], (E9533) [SH 491 909], (E9534) [SH 491 910], (E9535) [SH 491 907], (E9536) [SH 491 908], (E9537) [SH 491 903], (E9538) [SH 491 908], (E9539) [SH 491 909], (E9540) [SH 491 908], (E9541) [SH 491 909], (E9542) [SH 491 909], (E9543) [SH 491 909], (E9544) [SH 491 910], (E9545) [SH 492 904], (E9546) [SH 492 902], (E9547) [SH 490 913], (E9548) [SH 492 904], (E9549) [SH 490 906], (E9550) [SH 491 910], (E9551) [SH 491 910], (E9552) [SH 491 909] have been made with such care and skill by, Mr. Rhodes that the characters come out with unexpected lucidity.</ref>

Southern portion — The rock that rises on the Nebo thrust-plane is a straw-coloured flaky gneiss with lenticular pegmatites, among which thin siliceous seams quickly appear. Then, banded granular ('B') gneiss develops, and keeps on alternating with the flaky (' C') type as far as the stream's mouth north of the Old Red beds; some sharp folding repeating the bands. There is not space here for the details, but all can be clearly seen at low water, and are of the nature indicated. Sillimanite can generally be found in the flaky gneiss, usually in the micas rather than in quartz, but good, localities are: 270 yards from the Nebo thrust-plane, in a straw-coloured micaceous gneiss (E10269) [SH 489 898] (nearly all whose white micas are, as all along this coast, bleached biotite); and a reddened gneiss (E9529) [SH 490 901] (Plate 12), Fig. 3 at the end of the fir-tree fence that comes from Bryn-fuches, just a quarter of a mile from the thrust-plane. The latter is perhaps the richest sillimanite rock in Anglesey, and the mineral occurs in sea-green aggregates quite unaffected by the staining and decomposition. Beyond the stream's mouth aforesaid, the rocks are rather less interesting, save that the pegmatoid lenticles are well seen. There are some that seem to have been diorite, but utterly decomposed; and the section is traversed by a zone of brecciation, as well as by many mylonising thrusts. The graphitic gneiss alluded to on p. 136 is close to an old copper mine, which raises a suspicion that the graphite may be meta somatic and of Pala3ozoic date. At the south end of Helygen beach is an ill-foliated rock, with green pseudomorphs, probably after idocrase.

Northern portion — Beginning again at the north end of the inlier, the rock that rises from the great Gwichiaid slide is a rough granular ('B') gneiss, which, beyond a heavy rock-fall, is driven sea-wards on a very steep thrust over a flaky one with sillimanite. The two types (with lenticular pegmatites) keep on being brought against one another by thrusts and slides as far as the headland, where the zone of calcification (see Chapter 19) is about 100 yards in width. South of this, beautiful sillimanite-gneiss comes on, in which a zone of hard siliceous ('A') type (some of which is dark with hornblende) occurs. Granitoid matter now appears, and we pass into a permeation-zone, in which (as on the coast of the Gader Wier) all the four higher grades are developed in the course of a few yards (Figure 144). Bust here the sills of gneiss ose granite are never more than a few feet thick.

South of this, is the most interesting part of the whole coast-section. The positions may be obtained by reference to the mouth of a streamlet, not shown on the one-inch map, which is nearly opposite the top of the 'P' of 'Porth'. On the north side of this little stream is the first of the beautiful sillimanite-biotite-idocrase gneisses (E11391) [SH 491 910], and at its mouth is another zone of granitisation. A yard or two southwards is the most beautiful crystalline rock in Anglesey, a coarse flaky gneiss with large micas, and full of sillimanite, rose-garnet, and green idocrase. Some seams are almost half composed of silky sillimanite, but through much of the rock it is in radial groups, whose crystals are almost columnar. The outcrop is not convenient of access, being chiefly on the floor of a little nook. Similar gneiss, in which are pegmatoid augen with biotite and sillimanite, but without the rose-garnet, follows; and about 70 yards from Porth Helygen beach a group of thin siliceous bands of the 'A' type rises, some of which have a very clastic aspect, though no clastic grains are to be seen in thin section. They lie between seams of the highly crystalline idocrase-gneiss.

In these remarkable crystalline zones<ref>(E9533) [SH 491 909], (E9534) [SH 491 910]. (E9527) [SH 491 909], (E9528) [SH 491 909], (E10835) [SH 491 908], are some of the best slides, with sillimanite.</ref> (Plate 12), Fig. 5 are one or two gneissoid granites two or three feet thick, and many thin ones, which constitute a yard or so of banded gneiss at one place. The section ends, at the north end of Helygen beach, with a reef of most beautiful sillimanite-idocrase-gneiss, brilliant with large bleached biotites, and full of thin lenticular pegmatites. The sillimanite is in stellate groups.

The Coedana Granite

The only places where this granite is visible on the western coast are the north side of Careg-lydan, a reef accessible only at dead low water; and the little crags to the south of the 'Ff' of 'Fferam', where a ten-foot sill (E9904) [SH 329 715] (with some isolated phacoids) runs through the hornfels. It is heavily crushed. At Pensieri (E9907) [SH 336 722], and again by Drudwy mill, are two of the marginal white-mica-granites. Near the north end of Llyn Maelog are inclusions of dull-green hornfels. By the lake-end the granite is porphyritic, and this is a good place to study the peculiar nature of the phenocrysts, but in the railway cutting close by the same type (E9460) [SH 335 728] is in a fresher condition—fresher, indeed, than at any other known exposure. Porphyritic structure is also frequent about Plâs Maelog and Neuadd. Thence there are frequent exposures, the type being for the most part normal (E9943) [SH 350 752]–(E9944) [SH 350 752] (though a fine pink white-mica-granite with orthoclase, microcline, and albite occurs 540 yards north by west of Dyfria), as far as Ceirchiog and Tre-ddolphin, at which place are some obscure dioritic rocks that may be xenoliths.

The xenolith west of the Ceirchiog Smithy is cut by countless veins and sills of granite, ten being laid down upon the six-inch maps, and the relation of several exposed in the quarry east of the farm-house. On the bosses between it and the Smithy thin fragments are enclosed in the granite. On the heights that overlook the south side of the Holyhead road about half a mile west of Caer-glaw, the great xenolith of mica-hornfels is traversed by some 14 sills, and on the westward slopes the relations of the Coedana granite are to be seen in admirable sections (Figure 145), (Figure 146), (Figure 147), (Figure 148), the best that are known in the Island. The sill-relation is dominant, but veins also cut the bedding of the hornfels at high angles. The granite (E9804) [SH 368 768] is an albite-white-mica type, and is not porphyritic, but it is here that the felspar of a micro-pegmatite has been pseudomorphed in white mica, suggesting that orthoclase has been present. By the riverside east-north-east of Y Werthyr a sub-basic diorite appears in the granite, which again traverses and includes mica-hornfels (Figure 149). At and north of Caer-glaw are many inclusions, and on a crag-face 250 yards north of the cross-roads by Gwalchmai Inn the junction shown in (Figure 150) is seen.

The finest exposed tract of this granite is the rugged moor of Gwalchmai, traversed by the footpath which leads to Clegir-mawr. Here all its characters may be studied: the normal rock, the porphyritic, with the peculiar character and capricious distribution of the red phenocrysts of orthoclase, occasionally the old foliation, and everywhere the late planes of mylonisation. A little south-west of the place where the path enters the granite-moor, by a forked drain, is one of the rare aplitic veins, and a quarter of a mile north-west of Bryn-ala (E9910) [SH 389 776] is a white-mica variety with original foliation. All along the western flanks from the Holyhead road to Gwyndy, apophyses penetrate the mica-hornfels, excellent sections being seen south-east both of Bryn-crogi and of Gwyndy. Thence to the base of the Ordovician the Coedana granite comes against the Gneisses without any intervening zone of hornfels being visible, but the nature of the junction is unknown.

Along the lane at Y-foel are good intrusive junctions in folded mica-hornfels, and there is a faint foliation (Figure 151). Albite and orthoclase are both present, with albite-micro-pegmatite, and white mica (E9975) [SH 402 797]. In the rocky tract about Eirianell there is extensive exposure, second only to that of Gwalchmai moor. The granite is often coarse, and in the northern part of the tract is porphyritic, with occasional old foliation (as at the glacial arrow and by the road south of the large felsite), and a few small xenoliths of hornfels at the farm due north of the same felsite. and elsewhere. At Plâss Coedana granite rises over an anticline of mica-hornfels. From Neuadd to beyond Coedana Church is a long rocky tract, and the granite shows all its leading characteristics. Among the Palaeozoic dykes towards Neuadd the porphyritic orthoclase is admirably seen, with the late mylonisation of the Complex; and west-south-west of the church are some of the best examples of the ancient foliation (Figure 9). In the Coedana district is also a system of vertical planes coated with silvery films that seem to be minute white mica, but the original micas of the rock are not pulled out along them. About the farm at the 'oed' (Maengwyn of the six-inch map) there are sills with slight marginal foliation traversing the hornblende-hornfels; and in the farmyard (when tolerably clean) both sills and veins can be seen (Figure 152), (Figure 153), the granite in its turn containing xenoliths of mica-hornfels. It is here that the margin of the intrusion (E10721) [SH 432 824], which is of the usual texture, contains a little tourmaline.

From Coedana to its final disappearance in the north-east the granite is tolerably normal, but by the lane side south of the 'Ll' of 'Plâs Llanfihangel' are nests of greenish white mica. The shift at Cwyrt is, on account of its suddenness and of the trend of the features, considered to be due to faulting. The termination at Maen-addwyn is drawn from features. At its last exposure (E9996) about 500 yards south of Hebron, the granite, which is red (probably from Old Red staining), contains hypo-porphyritic orthoclase in rather small crystals.

Hornfels

The Coast, Llantaelog, and the Marginal Aureole — The best section in the most prevalent type of the crypto-crystalline hornfels is on the western coast opposite Careg-lydan, a cliff about 18 feet in height, easy of access except at high water. The hard lenticular cores and schistose seams that sweep past them are very clear, some cores being internally banded, the banding in one case turning round and being truncated by the schistose seam. The hornfels, as a whole, shows no bedding. At the next section, about 140 yards to the south-east,-the rock (E6118) [SH 328 717] is more micaceous and schistose. At the last one, south of the 'F' of Fferam', the hornfels, which is but slightly micaceous, is distinctly bedded -in places. This is the rock that was analysed (E10188) [SH 329 716], p. 95]. A yard or two further on a granite sill about 10 feet wide runs out south-westward, and some phacoids of sheared granite occur near the south side of the rocky knoll close to the glacial drift. The hornfels here bears a marked resemblance to the Church Bay Tuff, and is full of small clastic grains of quartz (E11190) [SH 329 715], (E6117) [SH 329 715]. In a gap south of this crag is a strong mylonising crush, just before reaching which the change into fine mica-schist of the Penmynydd Zone comes on, unbrokenly though rapidly (pp. 127, 342).

Along the lake side crypto-crystalline hornfels is fairly exposed, and again around Llanfaelog Church, 170 yards east-north-east of which is a quarry in the analysed variety (E9888) [SH 338 730] with spots of chloritised xanthophyllite. This higher grade of alteration is found almost at the end of a great wedge that runs for more than a mile into the granite. At the seaward end of the long marginal tract schistose crypto-crystalline hornfels is well seen by Pensieri marsh and at Bryn-du, after which exposures are somewhat scanty (though the type remains the same) (E6116) [SH 361 743] as far as Penrhyn, Llanbeulan.

The rock then becomes very bare, and so continues all across the moors of Gwalchmai. Just beyond the Holyhead road the band ceases to be marginal, though the granite is very narrow on its eastern side. In the tract between Penrhyn and the Holyhead road good banding is visible, and at a spot 275 yards south-south-east of the tenth milestone are the thin quartzites (E9185) [SH 378 762], (E9186) [SH 378 762], (E9187) [SH 377 761], (E9188) [SH 378 760], (E9856) [SH 379 761]. They are two to twelve inches thick, and graduate by quarter-inch seams into the hornfels. About 190 yards to the east, a purple phyllite occurs within the zone. The aspect of the hornfels in this tract continually recalls that of the Church Bay Tuffs. A clear junction of crypto-crystalline hornfels with granite is to be. seen at the large boss at the south end of Caer-glaw marsh, in the angle between the two lanes, on a little crag facing south, where the granite contains inclusions. Both rocks are typical, and there is no chilled selvage. North of the Holyhead road is a good deal of the siliceous adinolic type (E9857) [SH 386 763], and where a lane crosses the high moor towards the Clegir-mawr footpath-this is beautifully bedded for many yards at a time. Remains of clastic texture can be detected here and there (E6113) [SH 393 763]. There is no better locality in the Island for a study of the original nature of the hornfels. Good exposures are seen again between Bryn-ala and Bodfeillion, where, 400 yards north of Bryn-ala, is the banded hornfels with encarsioblastic spots of chloritised xanthophyllite (E9972) [SH 392 777] and analysis], which is rich in small tourmalines and leucoxene. It is noteworthy that where this long tract ceases to be marginal, its grade of alteration distinctly rises, as in the Llanfaelog wedge, though never attaining the condition of the inner xenoliths. The tract at Fach may, be marginal, as gneiss appears in the ravine beyond. Much of it is siliceous, with fine micaceous seams, but it is rather obscure.

Passing to-the northern end, the hornfels that wraps round the granite as a great aureole to the east of Llanerchymedd is for the most part crypto-crystalline and of the usual grey-green type. It is well exposed from the Old Windmill to Bachau (E6119) [SH 427 839], at Cwyrt, Ynys-fawr (E10003) [SH 444 832] and analysis], and Bodafon-glyn. Much of its chlorite is probably after xanthophyllite. Epidote-hornblende-hornfels occurs at the farm south of the 'h' of 'Bachau' (E10001) [SH 429 832], and in the south-east angle of Maen-addwyn crossroads (E6112) [SH 461 839], and a calcareous epidote-hornfels north-west of Clorach bridge. The important section at the south end of Mynydd Bodafon (E10341) [SH 463 847], (E10342) [SH 463 847], (E10343) [SH 463 847], (E10344) [SH 463 847], (E10345) [SH 463 847], (E10346) [SH 463 847], (E10347) [SH 463 847], and analysis] (Plate 10)  Fig. 1 (pp. 96, 160, 336) where bedded hornfels with original structures dips under the quartzite, is just above the road, 300 yards east by north from the 329-foot level north of Maen-addwyn. As would be expected from this section, the line drawn to separate hornfels from Bodafon moor-schist is merely provisional, though necessary.

The Xenoliths

Turning now to the great xenoliths within the granite, that of Ceirchiog is siliceous, rather like the Fach tract; that north-west of the Smithy is crystalline biotite-hornfels, with unusually large idiomorphic sphenes in the boss north-east of the farm (E11191) [SH 361 765]. The xenolith on the south side of the Holyhead road west of Caer-glaw is of great importance, being the finest exposure of the mica-hornfels, traversed by some 14 granite sills (see (Figure 145), (Figure 146), (Figure 147), (Figure 148). The specimen analysed (E9806) [SH 370 767] came from here. The xenolith, like the Fach hornfels, is brought against the gneiss by the Treban fault, which must be Pre-Ordovician; and it is evidently faulted on the south also. The hornfels is most beautifully- crystalline (E10361) [SH 32 75], (Plate 10),  Fig. 2], with micas one-eighth of an' inch across, and tourmalines 11395, 6115] easily visible to the unaided eye, while the original bedding, picked out by fine granoblastic seams, is often perfectly clear. This is the only place where the large oval groups of orthoclase have been seen (E9901).

Mica-hornfels, again well bedded, and with thicker siliceous beds, occurs among the small basic sills by the river east-north-east of Y Werthyr (Figure 149). It also borders the Coedana granite from Clegir-mawr to Gwyndy, and runs past the Glan-yr-afon road for 300 yards, between two granite sills. It can be seen also on the western side of the western sill, and must therefore adjoin the Gwyndy gneiss. The junction is concealed, but as each rook (see p. 162) maintains all its own characters in full, and as the gneiss is brecciated in the hollow, there is evidently a fault. Mica-hornfels occurs at Gwyndy itself, and runs a few yards north-east of the cross-roads, but at the quarry 200 yards from thence (E9948) [SH 394 795] the hornfels is crypto-crystalline, and so are most of the xenoliths from there to Cwm. At Y-foel there are once more fine exposures of mica-hornfels (E9974) [SH 402 797] dipping under granite, which cuts an old pre-intrusion folding.

The epidote-hornfels of Cwm (E9949) [SH 404 800] and analysis] (Plate 10),  Fig. 4 is the finest development of that type in the Island. The spot where bedding in it, and the passage into it from bedded mica-hornfels are best seen is- on the side of the crag about half-way from the farmyard to the pool. Some veins of a red granite whose felspar is albite-oligoclase cut this hornfels. At the 'e' of 'Coedana' the strip shown is mica-hornfels, which appears also (E6120) [SH 424 826] at the boss 240 yards north of the 'o' (near Maen-gwyn of the six-inch map). But it is there quite subordinate to, and passes into, the much more exceptional hornblende-hornfels [(E10002) [SH 425 825] and analysis] (Plate 10),  Fig. 3, (Figure 152), (Figure 153), of which this is the only considerable mass known in the Island. It is well bedded, and there are seams with unorientated mica, so that the union of the types is complete. Granite sills, with slight foliation, occur in this boss, and veins at the farmyard.

The Deri Inlier

The rocks of this inlier are well exposed on Pen-y-graig-wen, at other places indicated by the drift-lines, and in the two stream-sections. Its outer boundaries are obscure except at Pen-y-graig-wen. By far the most conspicuous rock is the granite, but the section in the Deri water leads one to suspect that the mica-schists, being less durable, may occupy considerable drift-filled hollows within the granite area. The granite, which is unusually white, and of medium grain, is an albite-white-mica-rock, no orthoclase having been seen. But, as it produces hornfels-alteration, it would seem to be a portion 'of the Coedana granite. To the south of Deri-isaf, and to the east of Llaneuddog, it is locally foliated, and at the 'y' of 'City' this foliation is cut by an albite-white-mica-pegmatite with micas an inch in diameter. There are some fine white parts that simulate quartzite, and appear to be silicified. The included strips at Pen-y-graig-wen are composed of a chlorite-leucoxene-hornfels, but the inclusions west of Deri-isaf are crystalline mica-schist of Penmynydd types, and in the Deri water east of the house there is almost as much mica-schist as granite, but the bands cannot be traced on. Mica-schist appears to occupy most of the inlier towards the river, but is not well exposed, except in the channel by the mill (in dry weather) and at a roadside quarry west of 'P'. It is fissile (E6098) [SH 464 873], (E10272) [SH 464 872] with well-developed white micas, and some alkali-felspar as well as quartz in long grantiles, the foliation being sometimes beautifully corrugated. In it are bands of rather fine quartz-schist or quartzite, now granoblastic, with some grains of albite that seem blasto-psammitic. The group is evidently that of Bodafon, somewhat more foliated. At the roadside quarry the quartzite is split by lenticular seams of mica-schist near the junction, which is visible, and in the bed of the river at the mill the mica-schist contains long phacoids of quartzite. Both rocks are purple with haematite, doubtless a staining derived, as on Mynydd Bodafon, from the Old Red Sandstone.

The Foel Inlier

This inlier emerges on a compound isocline in the hill east of Llanerchymedd. It is composed of mica-schist with some bands of quartzite. The mica-schist (E10679) [SH 4258 8442] (unlike the schistose hornfels across the valley to the south) is finely fissile, with micas occasionally well-developed, and is of a grey tint. The quartzites (E10170) [SH 425 843] are also fine, though massive, and are granoblastic, with zircon and garnet. Their micas have been replaced by deles-site, which forms also stars in the quartz-veins. Both rocks (though here unstained) are of the same character as those of the Deri Inlier. Towards that they strike, and are evidently on the same zone of metamorphism; suggesting that the Penmynydd Zone has wrapped round the hornfels of Bachau, to come between that and the great zone of gneiss.

Mynydd Bodafon

The Quartzite, Hornfels, and Moor Flags

Mynydd Bodafon rises in a promontory of the Middle Region, to which Graig-fryn and four smaller inliers among the Old Red Sandstone also belong. A curving line runs from Bodafon-glyn to the Old Red Cornstone east of Maen-addwyn, coinciding for some way with a fault that passes through the quartzite, but for the rest of its Course merely indicating a zone of metamorphic change. West of this line is crypto-crystalline hornfels, already described on p. 333. East of it, the rocks that emerge from below the quartzite are but partly hornfels, and east of the Bodafon-glyn fault two strips only of it have been separated out, near the farm. The rocks of the tract east of this fault vary a good deal, but being fine flaggy mica-schists, and best seen on the wide moor north of the mountain, may be conveniently called the Bodafon Moor Flags'. Their flagginess is imparted by films of lepidoblastic mica, but between these films the minute micas that crowd the rock (E10042) [SH 476 857] lie in all directions, as in the hornfels, to which they are therefore closely related. Indeed, the rock with pseudomorphs of andalusite, referred to as a hornfels (p. 93), is a pale band among the moor flags (E10043) [SH 473 858] about 100 yards north of the 'n' of 'Bodafon'. And in the junction series (pp. 96, 160) at the 'base' of the quartzite, though the slides (E10341) [SH 463 847], (E10342) [SH 463 847], (E10343) [SH 463 847], (E10344) [SH 463 847], (E10345) [SH 463 847], (E10346) [SH 463 847], (E10347) [SH 463 847], and analysis] (Plate 10),  Fig. 1 were selected to bring out the hornfels characters, the alternating schistose beds have the characters of the moor flags. One exceptional rock (E10048) [SH 465 859] about 200 yards north-east of Bodafon-glyn, is much more highly crystalline — a corrugated schist with large muscovite, chloritised biotite, idiomorphic pseudomorphs after garnet, tourmaline, zircon, and other minute minerals of high refractive index. The moor flags often have the same purple staining as the quartzite. About 400 yards west by south from Fedw-isaf they contain small curving lenticles of purplish grit, in which clastic grains just survive; so there can be no doubt that they are in great part sedimentary, though the horny bands with a felsitic aspect may be pyroclastic dust. For a yard or two above its apparent base the quartzite becomes a little flaggy, and graduates into them by alternations. Now at the south end of the mountain the beds on which it rests are largely hornfels, at the north end they are the moor flags. It is evident that hornfels and flags are one and the same formation, stratigraphy" thus confirming petrology as to the clastic (if in part pyroclastic) origin of both. The flags are to be seen here and there all over the moor, but the best exposures are between the little moorland village and the north end of Plâs Bodafon woods.

In the quartzite, on dip-sections and along the north-west escarpment, divisional planes can be made out which are undoubtedly stratification, for they dip always at the same angle as the base' wherever that is well exposed, and to the basal' flagginess of the quartzite near that. The beds were thick, and the rock uniform on the whole. Clastic grains are best seen near the east end of the southern escarpments, about 90 yards from the gap where some cottages are; also on the north-east parts of the Graig-fryn Inlier, which seems on the whole a little less reconstructed than Bodafon.<ref>The Bodafon rocks can be studied in the following slides: (E10038) [SH 465 847], (E10039) [SH 467 846], (E10040) [SH 467 846], (E10041) [SH 471 838], (E10042) [SH 476 857], (E10043) [SH 473 858], (E10044) [SH 463 846], (E10045) [SH 462 856], (E10048) [SH 465 859], (E10706) [SH 462 851], (E10707) [SH 470 857], (E10708) [SH 475 871], (E10709) [SH 474 871], (E10710) [SH 473 840], (E10341) [SH 463 847], (E10342) [SH 463 847], (E10343) [SH 463 847], (E10344) [SH 463 847], (E10345) [SH 463 847], (E10346) [SH 463 847], (E10347) [SH 463 847], (E6132) [SH 463 849]–(E6133) [SH 478 859] (Plate 10),  Fig. 1.</ref>

Tectonics

Reference was made on pp. 184, 221 to this chapter for detailed field-evidence in support of the views there set forth as to the structures, expressed also in the sections (Figure 156), (Figure 157), (Figure 158). The quartzite mountain, 583 feet in height, rises from the moor whose levels are about 300 to 400 feet, and which in its turn ends off in a steep feature overlooking the Old Red and Ordovician country. To east and north this feature is enforced by a rampart of quartzite, forming a circlet of bold crags. Bedding is clearly visible in this quartzite, dipping always away from the flags of the moor at angles varying from 20° to 30°; while the moor flags themselves, whenever exposed at all near (and along the northern rim the junction is almost visible), dip under it at corresponding angles, the direction changing with the curve of the quartzite rampart, which presents low sinuous escarpments to the moor. Further, in its course north of Plâs Bodafon woods there are three gaps in this rampart (the third of which, greatly exaggerated on the one-inch map, is only seven to 17 yards wide), in which the moor flags are seen to pass down upon the dip under the quartzite (Figure 154), (Figure 276), (Figure 278). About a quarter of a mile west of Fedw-isaf, some, small faults, running south-west, break the rampart, and on the upthrow sides of two of them little inliers of moor flags appear, one of which is very small. At the north end of the moor the rampart suddenly ends, and a rugged escarpment of quartzite, fanng due west, runs right down the steep feature.- The moor flags, as at the gaps, pass under the quartzite, but the junction is often obscured by a tangle of thorns and gorse. About 200 yards further on a small outlier of quartzite clings to the face of the feature, dipping now northwest. There is therefore clear evidence that Bodafon Moor is a broad symmetrical anticline with a pitch to the north-east. The quartzite on its western limb has been removed by denudation, that on its eastern limb strikes not for Mynydd Bodafon but for the Graig-fryn Inliers. But this great anticline is far from simple. The moor flags are locally corrugated, and their dips (for foliation is evidently not far from bedding) indicate many undulations. About the little village these appear to be symmetrical, but further south to be isoclinal, with three narrow nips of quartzite.

Now the great quartzite of Mynydd Bodafon itself is a complicated group of synclinal outliers upon the eastern limb of this broad crumpled anticline (Figure 155). That of the northern summit seems to be a simple isocline, but for a sharp uprise of the flaggy base just before the eastern side is reached (Figure 156). The western faces of the mountain are escarpmental all the way along, with the flags dipping under thick-bedded quartzite. In the middle of the rugged hills, east of the tarns, the moor flags rise on a broad subsidiary anticline (Figure 157). West of this the main ridge, though elsewhere an isoclinal, is for a while a symmetrical syncline, with a small nip of quartzite on each side. The broad anticline of moor flags carries on it five small nipped outliers of quartzite among the cottages (Figure 155), (Figure 157), so it must be corrugated in its turn, as well as cut by some small faults. Quartzite comes on again to the east, but the form of its northward termination, just before it is overstepped by the Old Red, suggests that the moor flags are about to rise again and that this ridge is a sharp compressed isocline. On the broad southern summits all the dips observed are easterly, at an average angle of not less than 24°; in spite of which the 'base' of the quartzite on the southern face only drops 100 feet in a distance of 1,950 feet. It must therefore be folded, and the folding must be isoclinal. One of the principal folds must be on the strike of the central anticline of the mountain, by the little tarn. The others are inferred from the rises and falls of the 'base' along the 400-foot contour. There must also be a syncline of pitch, taking in this tract of quartzite between the tract of moor flags in the middle of the mountain and the southern margin. The depth to which the isoclines descend in it can only be conjectured, but from the solidity of the mass it is probably considerable, as indicated in the section (Figure 158).

The amplitude of the main Bodafon anticline cannot be less than 1,000 feet, measured upon the apparent base of the quartzite in the visible part of the fold, and, with the rise of the pitch and the plunge to the adjacent complementary synclines, must be several times as great. The whole of this ancient folded mass is driven over the Ordovician rocks ((Figure 156), north end. See also (Figure 154), (Figure 276), (Figure 278) on the Bodafon thrust-plane.

The Penmynydd Zone of the Middle Region

The coast section will be considered first, then the western margin, then the rest of the inland tract, and then the eastern inliers.

The Coast

The main junction of Braich-llwyd, which has been described on p. 124, is at the north-west brow of a chasm. Though an unbroken passage from Gwna to Penmynydd schist, it is very rapid, and is complete in only a few yards from the chasm's brow. Moreover, the Penmynydd type that is developed is one of the most highly crystalline of the Middle Region, a flaser, undulose mica-schist comparable with that of the Aethwy Region, and with quartz-augen; the band being 400 yards in width, and the widest one of that type known in the region. At Porth Cwyfan the flaggy type comes on, and in it are a number of bands of the pale green variety of hornblende-schist (E10179) [SH 340 682], which are highly epidotic but contain true zoisite as well. Thin acid seams adjoin them; but there is folding, and too much evidence of movement for the original relations to be ascertained. At the church islet reefs the flaser type appears again, and also on" both flanks of the rugged Porth China headland at the bay's end. Near the outer end of the summit, however, it takes in a nip of-the oft-repeated quartz-schist, limestone, and graphite-schist group (E10178) [SH 333 683], (E10182) [SH 348 677], (E10183) [SH 333 683], (E10184) [SH 333 683], There is powerful folding at high angles, overdriven somewhat from the south, and doubtless thrusting, for the limestone is a curved and split lenticle some 20 feet wide, and the graphitic schist, only an inch or two in thickness, frequently nipped out. The limestone is calcitic, light-grey with dark bands, foliated and saccharoid, though fine; and besides the true graphite-schist is much dark mica-schist, full of seams of quartz. Limestone appears again in Porth China, where the flaser mica-schist is highly crystalline. But along the cliff westward it becomes finer and more flaggy.

On Ynys Meibion are many destructive thrust-planes (Figure 159), probably the latest movements of the Complex. Caethle begins with micaceous flaser schist, but most of the rocky bight is occupied by a somewhat unusual type, siliceous, but massive, and greenish, as if epidotic. Then flaser schist comes on again, succeeded, where the dyke skirts the coast between two chasms, by a large mass of flaggy schist, almost a quartz-schist in parts. The high headland called Trwyn Euphrates consists of highly crystalline flaser schist with quartzose auger. About 150 yards further on, a chasm runs in, and on the north brow is the rock with surviving felsitic structures (E9184) [SH 328 698], (p. 123). It has decomposed along the joints, is banded, locally foliated, and stripped into thin seams,_ which lie in a decomposed basic schist, and are sharply folded. About 100 yards to the north, on the low cliff-top, among hard-banded mica-schist, are the beds (E11086) [SH 329 699] (p. 123) of albite-grit. Their clastic quartz can be seen in the field with the hand-lens. That there should be two survivals of original structures here may be due to old thermal effects of the basic intrusions making the rocks more resistant. On and about Ynysoedd Duon 16 small basic masses have been laid down upon the six-inch maps, party unfoliated, partly hornblende-schist (E10174) [SH 330 698], (E10175) [SH 331 700], (E10176) [SH 329 699]. The best junction-sections are on the islets (accessible at ebb), the best zoisite-amphibolite is 200 yards to the east. The mica-schist in which they lie (E6137) [SH 336 703] is variable, but mainly flaggy, with bands of quartz-schist. On the south cliffs of the large cove west of Gate House is the most quartzite-like of all the quartz-schists of the Penmynydd Zone (E9183) [SH 331 704] but it is completely granoblastic.

About the Telegraph Cable Hut are the best exposures Of the triple group — quartz-schist; graphite-schist, and limestone, which are here considered in order from north to south. The Hut is built upon a ridge of flaggy quartz-schist, and on the south cliff of the little cove, composed of highly crystalline flaser mica-schist, a very thin slip of graphite-schist appears just at the corner of the beach. On the seaward strike of this, in the same cliff, is the limestone, a range of overlapping lenticular nips, one of which is about three feet thick and about 15 feet long. It is a white calcite-marble (slightly dolomitised in places) with plates of mica, like that of Bodwrog, but finer in grain, graduating into the adjacent rock through thin zones of calc-mica-schist very poor in felspar (E10689) [SH 331 706], 8518–22]. The 60-yard quartz-schist that follows must be broadened out by repetition, for the preceding rocks are driven into it; besides which there is a fine section in true inter-bedding, bands of quartz-schist two to six inches thick lying in mica-schist at intervals of a foot or so, forcibly recalling the sections on the northern coast (pp. 305, 309) where thin quartzites alternate with Gwna phyllite, save that the rocks here are highly crystalline. Then, mica-schist rises on a corrugated anticline, cut off by a nearly vertical thrust at a little chasm, so that locally the limestone group seems to lie below the quartzite. In the chasm is the best as well as the most petrologically interesting exposure of the graphite-schist (E10030) [SH 331 704], ((Plate 10), Fig, 6), rich in rutile and xanthophyllite, a corrugated group of black seams whose total thickness may be about three feet, lying in mica-schist. On the south cliff of the chasm are two small nips of limestone. Quartz-schist then appears once more above, and comes down, but with heavy crushing, in the large cove west of Gate House already mentioned.

Returning to the north of the Cable Hut, the marginal fine flaggy type appears on the Barclodiad headland, and is strongly developed. A thin limestone is driven into it in the chasm where the sheared dyke outcrops (Chapters 16, 17).

But on the southern foreshore of Porth Nobla comes a local rise in crystallisation with reversion to the flaser type (E6136) [SH 329 712], (E9900) [SH 329 711], and analysis, (p. 112). Biotite as well as muscovite is plentiful and in good preservation, partly intergrown with, partly penetrated by the muscovite. Garnet and pyrite are present. The felspar is an untwinned albite, ill-preserved. There is probably a rupture in the cove, as indicated by the features, for the schist of the reefs beyond is fine and flaggy, falling in crystalline grade up to the junction with the hornfels (p. 127). This is at the crag south of the 'Ff ' of 'Fferam', where there is a glacial arrow. A few yards from the south-east side of it is a gap, in which is a strong mylonising crush, and at first sight this crush (a steep thrust from the north-west) appears to be the boundary. But inspection of the north-west side shows that, for a yard or two, the rock is still Penmynydd mica-schist, though fine; and that there is an unbroken passage into it from hornfels. On the south-east side the Penmynydd type is evident. The passage is therefore a real one, and the thrust, which outcrops within the Penmynydd Zone, fictitiously accelerates it by bringing a marginal part of the Zone on to an inner part.

The Western Margin

No other section across the junction is known, but all along the boundary the two rocks exhibit the same changes where near together as they do at the coast. The best sections where granitoid matter can be seen in the mica-schist are as follows: a high boss 330 yards west-south-west of Fferam, Llanfaelog; the east side of Tre-ruffydd farm-lane; the quarry by the eleventh milepost at Gwalchmai; and the knobs just below the road at Bodwrog Church (E8484) [SH 412 786]; also at a farm at the south-west end of the granite sill that lies within the Zone at Gwalchmai. The breaking-down of the granite into granoblastic matter can be seen at the lane-side in the same sill, north of the Windmill; at a crag 140 yards north of Gwalchmai Inn; at Bodwrog Church; and at the figures '263' near the church. At the last place, granite and Penmynydd schist interdigitate (as shown on the six-inch maps), and are seen only four feet apart, the schist being almost a quartz-schist (see p. 127).

The flaggy marginal zone is about half a mile wide, and well-marked as far as Gwalchmai, but narrows at Bodwrog, and then becomes indistinct. The type is exposed on many isolated bosses, but the best sections are at Gwalchmai, especially in and about the little ravine of Penrhyn, where a speckled variety is occasionally seen (E9858) [SH 382 757]. The specks are biotite, the rock is rich in alkali-felspar, and minute garnets are plentiful.

The rest of the Inland Tract

is but moderately exposed except at the railway cutting of Ty-mawr, and along some ridges between Grugor-bach and Bwlch-y-fen; but is evidently composed of the same types as those cut by the sea-section. The railway cutting goes through the main junction with the Gwna Green-schist, but unfortunately a bridge wall and a large dyke come just at that place. There are, however, the usual indications of a passage. West of this is a continuous section for 450 yards, in which the type is mainly flaggy, so that the passage cannot be at quite the same horiion as at Braich Llwyd. There is also much of the flaggy type about Erddreiniog, but on the whole the micaceous flaser type appears to be the more prevalent. At Grugor-bach is a highly crystalline schist (E9940) [SH 380 735] with lavender-polarising chlorite after biotite, abundant epidote, leucoxenised ilmenite, and some tourmaline (a rare mineral in the Penmynydd Zone) which has a curious habit, being chiefly in bundles of needles that appear to replace biotite. The porphyro-blastic albite-schist (E10177) [SH 348 688] (Plate 10),  Fig. 5 is seen at a boss at the first 'r' of 'Aberffraw', and 250 yards from the outlet of Llyn Frogwy there is another (E10066) [SH 428 771] full of delessite and epidote. The garnet-schist (E9993) [SH 400 768] of the north-east end of Llyn Hendref, (partly analysed) contains chloritised xanthophyllite (crowded with rutile) which had a considerable axial angle.

The hornblende-schists are chiefly dark, and usually contain porphyroblastic albite and abundant zoisite, suggesting derivation from zoisite-amphibolites like the intrusions of Ynysoedd Duon. Feathery hornblende-porphyroblasts are well-developed at Pen-yr-allt, and the remarkable variety with foliated pyrite was found in a pit on the east side of the road 500 yards south-south-east of Gwalchmai-uchaf cross-streets (E9859) [SH 376 753], (E9994) [SH 393 753], (E9995) [SH 393 753]

The rutiliferous schist (E10058) [SH 418 778] is at 141 yards south by west from the 268-foot level at Bwlch-y-fen. Its iron-ores do not appear to be ilmenite, so the titanium seems to have all gone to the production of rutile. The relations of the granite at Bryn-goleu are not visible. About three-eighths of a mile east of Erddreiniog what seems like an acid gneiss is obscurely seen, and 140 yards to west-north-west a basic rock has a somewhat gneissose aspect. Folding is not conspicuous as a rule in this zone in the Middle Region; and though locally sharp folds are frequent, they hardly disturb the dip, and there is no good pitch, Local. bends in the strike will be noticed on the map, the greatest of which is at Bwlch-y-fen.

The Triple Group — Detail of the triple group of quartz-schist, limestone, and graphitic schist is of much more than local, it is of stratigraphical importance (see p. 125). But an outcrop of all three together will be looked for as but a rare occurrence by those who realise, the vast number of thrusts which traverse the Mona Complex, yet seven triple (and six double) outcrops are known, in the main tract, and in its inliers. Graphite schist is associated with the quartz-schist west-south-west of Llangwyfan New Church. The long bands of quartz-schist which range through Gwalchmai-uchaf and Bodwina to Bwlch-y-fen are generalised from, probably, ranges of shorter, overlapping outcrops. The black schist is not yet known, actually with them, but occurs 200 yards north-west of Tyddyn-gwyn, only 250 yards away. About Bwlch-y-fen 15 bands of quartz-schist have been laid down upon the six-inch maps; and with them, at the B', also west and south of it, are four nips of limestone (E6134) [SH 418 777] one of them 40 to 50 feet in width, partly light. fine. and dolomitic but partly darker. Dark schist lies close by, so probably the graphitic schist is present. The rocks are very siliceous west of Bodwrog Church, and ought perhaps to have been separated as quartz-schist, of which three bands have been mapped (on the six-inch scale) near the limestone, among highly crystalline Baser mica-schist. Of the beautiful Bodwrog marble (E10057) [SH 407 776], (E10690) [SH 400 776], (E8523) [SH 401 775], (E8524) [SH 401 775], (E8525) [SH 401 775], (E8445) (b), (E8557) [SH 401 773] (Figure 160), 12 bands are known, one of which is sharply folded round. Both groups are well, the south-westerly very well, exposed. Besides the snow-white marble analysed. (E10057) [SH 407 776] there are red varieties, in some of which are green silicates. In the large quarry at the north end of the southern group are several bands of graphite-schist, some of which yield a strong black streak; and on the south-east side the limestone is dark, thin-bedded, and interleaved with seams of graphite-schist, very much as in the large quarries at Cemaes (pp. 85, 305). The group may therefore be safely correlated with those of Cemaes and Llanfaethlu. The marble of Erddreiniog, which is so rich in accessories (E10080) [SH 464 811], (E10081) [SH 464 811], (E11396) [SH 464 811], and analysis, was, at the time of the surveying, only to be seen in an abandoned quarry about 17 yards long, south of the little square wood. About 10 feet of rather massive beds were visible, foliated, and with thin partings of mica-schist.

The Inliers

These are chiefly composed of the flaser type, and sharp folding, with a good pitch to north-east, is more conspicuous, but some of them have a close external resemblance to the adjacent Gwna Green-schist. About a score of lenticular inliers run in a. double train from the sea to the Old Red Sandstone, passing on both sides of the great Engan zone of spilite-schist. Many of the lines are conjectural, and adjacent inliers may be continuous in some cases. In and at the foot of the cliff north-east of Braich-llwyd, Aberffraw, an inlier of mica-schist is cut very obliquely by the sea. The zone of passage on the buttress has been described on p. 124, but that on the other side is to be seen on the cliff wall. Calcareous bands in the mica-schist graduate into a 20-foot bed of crystalline grey limestone, with which is a one-foot bed and many seams of good graphite-schist, and at the north-east end are thin bands of quartz-schist, so that the whole of the triple group is present. (E10181) [SH 343 676], (E10185) [SH 343 677], (E10191) [SH 343 677]–(E10192) [SH 343 677]. On the foreshore about halfway to Aberffraw Sands is another inlier of mica-schist. In the cliff on its west side, close to the little chasm, is a decomposing graphite-schist. There seems to be a small fault at the streamlet, but not enough to break off the wedging of the inlier into Gwna mélange, with a small foliated limestone (E10182) [SH 348 677], (E10186) [SH 348 677].

In the large inlier east of Hen-ysgubor, are thin fine dolomitic limestone (E6135) [SH 395 746] and quartz-schist, and as blocks of dark mica-schist lie near, the graphite-schist is probably present also. The limestone (E10056) [SH 396 744] just east of the dyke rises from under mica-schist on an anticline seven feet wide. In the Druid Farm inlier, 300 yards south-west of the house, grey crystalline limestone (E10691) [SH 414 753] is folded in with quartz-schist, and the basic rocks are hornblende-schist. In the mica-schist adjacent to the limestone there are thin graphitic seams. Just west of the farm the Gwna rocks contain clastic grains, and 366 yards south of the house there is a suspicion of clastic albite in the mica-schist (E10024) [SH 414 753]–(E10025) [SH 414 750]. North of Druid and east of Tros-y-rhos is a small inlier seen only on the east side of one large boss. But at that boss the types are seen only four feet apart, and there seems to be a passage (E10064) [SH 417 760]–(E10065) [SH 417 760]. The clean grey limestone of the large inlier southeast of Bodwrdin (E10079) [SH 392 717] is foliated, with films of graphite, as in the Cemaes limestones. In the same- inlier, between the dykes, are several signs of passage from the Gwna Green-schist, and at one of them, south-west of the 115-foot level, by a dip-arrow, a two-inch phacoid of jasper lies in a micaceous variety of this (E10069) [SH 400 720], (E10070) [SH 400 720], (E10071) [SH 400 721].<ref>At some such place as this, the Bodorgan thrust-plane may yet be picked out from among the other foliation-planes.</ref> The inlier that is crossed by the road from Llangefni to Bodffordd south-west of Ty-gwyn contains a grey dolomitic limestone with calcite veins, a thin quartz-schist, and some graphite-schist (E10204). Grey limestone lies just at the north-east end of the inlier east of Tregaian, and there are thin quartz-schists in the one south-east of this. Along the first one are exposures very near the boundaries. As limestone and graphitic schist occur several times close to the margins of these inliers, it is evident that the Zone develops at different horizons from those of the principal tract (pp. 220, 343).

The Gwna Beds and Tyfry Beds of the Eastern Middle Region

The coast will be considered first; then the south-eastern margin as far as Llangefni; then the inland lying between that margin and a south-west to north-east line drawn through Cerrigceinwen; then the Ceinwen volcanic zone; then the Engan spilites; then the western tracts.

The Coast

Aberfraw — On the south-east side of the chasm where the Penmynydd Zone comes on is typical Gwna Green-schist. At Braich-llwyd clastic grains are visible, and as we pass up north-east from the point, the rock is purplish, with little red fragments. Then follow the zones of change on the flanks of the Penmynydd inlier, whose rapidity is remarkable, so close to schists with surviving clastic textures. Beyond the inlier are typical Gwna Green-schists, in which clastic matter, though present, is inconspicuous. Then comes the other Pejunynydd inlier, and then the Gwna Green-schist of the river's mouth. This Aberffraw section would repay very careful study.

The Bodorgan Headlands — Where the section begins again beyond Aberffraw Sands we are just on the strike of the Ceinwen spilites, and these, converted into chloritic schists and accompanied by a limestone, appear among Gwna Green-schist, which is not normal, but full of sheared fragments torn from them. The rugged coast from here to the great headland of Twyn-y-parc, is the finest section across the Autoclastic Mélange in the Island (Plate 7). To describe it in detail is impossible, the wealth of structural evidence is practically inexhaustible, and it must be visited to be envisaged. So no more will be attempted here than to indicate points of special interest. The grade of metamorphism is low, the schistose matrix being dull-green; but there is relatively little matrix, the greater part of the rock being hard grit, often crowded in closely overlapping phacoids, and the larger lenticular masses are for the most part quartzite, limestone and spilite being rare. Small strips of partly bleached jasper, torn away from their native spilite, occasionally lie between the grits, with a few of jaspery phyllite. Some of the grits are coarse and should be examined for old composite fragments. Opposite the word 'Bay' are several large quartzites, one of which forms a conspicuous ridge, and then, folding, runs in on another parallel ridge. It shows bedding, is unusually rich in albite and tourmaline, and contains old compact fragments that may be felsite (E10169) [SH 359 667]. On the cliff 100 yards to the north-west of the high ridge the quartzites are partly split, and the base of one of them wedges in and out, the foliation of the mélange remaining at high angles (Figure 161). The structures recall those of Porth Wnol ((Plate 22), and p. 307) on the northern coast,- but the mélange here is far more strongly foliated. It can be seen unusually well close to these quartzites. Bedding is rarely visible, but can be seen, over-folded from south-east, on a boss just above the cliff on the west side of Porth Cadwaladr. A little inland is a small limestone, and -there are others in the cliff south of the cove, with a sheared spilitic lava. Beyond them are some rather massive grits. The low cliffs of the next sandy cove afford excellent sections of the mélange, which is also (E10277) [SH 361 659] magnificently displayed all over the crags of Dinas Trefriw. In the north-west corner of Porth Tywyn-mawr bedding has been preserved in the act of breaking down. The basic sills now begin to appear. They are described below.

Phyllite, both green and purple, increases as we approach the headland, and on the craggy brow South of Porth-ro the sections are of great importance, for clear passages of the Tyfry phyllites into mélange can be seen both along and across the strike. In the deep chasm beyond is a rose-limestone. The great headland of Twyn-y-parc itself, more than 100 feet in height, is a rugged mass of mélange, with nips of quartzite, limestone and diabase. Near the seaward end are some 20 feet of unusually coarse grit, with fragments that resemble jasper, but the rock should be further searched. Oligoclase as well as albite has been found in these grits (E10241) [SH 368 649], (E10242) [SH 368 649], (E10243) [SH 368 649], (E10244) [SH 490 666], (E10245) [SH 457 906].

The sections hence as far as Dinas-lwyd are highly complex, and cannot be followed on any but the .0004 or at any rate the six-inch maps; mélange with quartzites and limestone alternating with phyllites in which are beds of grit, and the whole traversed by sills of epidiorite partly schistose, in which are inclusions of grit and hornfels. The outer islets appear to be of massive diorite. These phyllites have been provisionally coloured with those that occur in the zone of passage to the Tyfry grits, but the whole group needs further study. On the eastern side of the semi-circular cove and at the old quarry, they show bedding, besides a venous banding. On the south-facing cliff before Dinas-lwyd is reached, schistose diorite, hornfels, phyllite, grit, and quartzite have all been sheared together, and the complexity of the rock is at a maximum. Dinas-lwyd is another fine crag of mélange, which then occupies the. coast all the way to the Millstone Grit, and reappears in the Bodorgan inlier, but contains more nips of limestone than on the western coast. At Bone Twni cove the grits, less torn up, contain the bodies referred to on p. 151 as castings of annelids.

The epidotic epidiorites (E10233) [SH 375 660], (E10234) [SH 369 653], (E10235) [SH 376 653],_; (E10279) [SH 383 662] (p. 75) are well exposed at many places both on the coast and on the slopes among the dunes, passing often into chlorite-schist upon their margins. Coarse diorite forms a high knob just above the 'd' of 'Bodowen', and occurs also in Porth-ro, where its pale augite survives. It is possible that some deformed spilitic lavas have been coloured with these intrusions; for some 300 yards north-east of Porth-ro is a curious chlorite-schist with what seem to be remains of ellipsoids and jasper; and studded with small white bodies (E10236) [SH 370 658] composed of interlacing colourless needles, apparently an amphibole, in which are veinlets of clear albite. The rock may be a variolitic lava that has been thermally altered. Inclusions, more or less thermally altered, are to be seen at Porth-ro, at the south-facing cliff near Dinas-lwyd, already mentioned, at a cove a quarter of a mile northeast of Dinas-lwyd, at Bone Twni Cove, and indeed at all the good sections. In Porth-ro, one of the sills passes down to north-west at moderate angles, in spite of the high foliation-dip. At the foot of the south-east cliff, its chilled selvage can be Seen, with inclusions of purple phyllite converted into pink epidotic hornfels. Both rocks have been in places torn up, sheared, and incorporated into the mélange; yet in parts of the hornfels old folded bedding is preserved, and an old cleavage, with small slips along it, healed up and welded by the thermal action (E10237) [SH 369 653], (E10240) [SH 368 652], (E10278) [SH 369 652]. At Bonc Twni Cove the original sill-relations are visible, the diorite slightly transgressing the bedding of grits and shales, which have been locally converted into true adinoles (E10238) [SH 383 662]–(E10239) [SH 384 662]. The bedding is overfolded from the north-west, and there is again an old cleavage, and yet a shearing. of the diorite. On the south-facing cliff southwest of Dinas-lwyd, tongues of the basic rock have been sheared out until as thin as paper, and it is noteworthy that, though the thicker tongues contain inclusions that are in the condition of hornfels, no thermal effect has been induced upon the Autoclastic Mélange as such. It is evident, therefore (see also p. 76) that these intrusions must be later than the earliest folding with its resultant cleavage, but older than the production of the Autoclastic Mélange. It would seem as if they belonged to the same interval as do the dolerites of Caerau (see pp. 103, 321), though those are labradorite rocks, which these do not seem to have been.

South-eastern Margin

Along this the chief interest is the train of infolds letting in the beds regarded (see pp. 63–4, 160) as a zone of passage from the Gwna Group to the Tyfry facies of the Skerries Group. Though coloured with the Skerries Group, they include not only true tuffs, but grits and phyllites; and the lines drawn are but provisional, coinciding probably rather with dynamical horizons' of escape from autoclastic break-down than with stratigraphical horizons. The whole zone calls for more minute petrological research. From the Bodorgan inlier to the railway, mélange comes to the margin, well exposed east of Hermon Church site, where some coarse grits contain albite (E10688) [SH 383 693] and blue quartz, beyond which are some three-eighths of a mile of obscure ground. Then: in the cutting between the tunnels, appears a clearly and rather thinly bedded group, fine flinty green grits (containing broken laths of albite) with green and purple phyllites. Similar beds are exposed in the eastern tunnel (which is not wholly bricked in), and can be seen plentifully in the debris taken out, now lying over the tunnel's roof (Figure 162).

In the cutting there is a vertical cleavage (probably that which is older than the diorite intrusions) and a folding on vertical axes (Figure 41). Similar beds appear again at the tunnel's eastern mouth, and it may be that the group should be given more space upon the map. In the next tract is a rock of great importance, quarried 170 yards north-north-east of Trefdraeth Church, and used for the walls along the road. It is an epidosite-tuff, 'with clots', and little scraps of pink felsite, of whose identity with the Trwyn Bychan and Church Bay Tuffs there can be no doubt. Mélange then forms the margin all the way to time Henblâs Water, finely exposed along the strong feature which bounds the Carboniferous Series (see Chapters 22, 23).

Green grits and mudstones form the tracts that have been separated out about Henblâs. They are not very well exposed, but it is significant that where the Henblâs Water cuts back the Carboniferous and lays bare its floor for 200 yards, this group is the only one revealed, as if it were about to come on in force beneath the coalfield. With two interruptions, the passage-beds then continue along the margin for the remaining two and a half miles to the Llangefni fault, beyond which they have not been recognised. They are well exposed about Llangristiolus, especially below Cefn-canol on the north-east side of the late dyke, where (see p. 160) there is a perfect passage from Tyfry to Gwna Beds, and as perfect a section showing the rapid breakdown of Gwna bedding into mélange. The section is about 60 yards from the Llan-fawr path. No better exposures of the passage-beds themselves exist than the quarried knobs (see p. 160) about Nant-newydd (E9688) [SH 450 745], (E9689) [SH 457 751], (E9690) [SH 457 751], (E10198) [SH 453 745], (E10199) [SH 453 745], (E10200) [SH 456 749], (E10697) [SH 45 75]–(E10698) [SH 45 75]. At the farmyard is a grit with a white felsitic matrix, and some purple ones; but those to south and east are green, with abundant fragmental albite and an epidositic matrix like that of the Skerries Grits, in beds a foot or so thick, between which are thin epidotic mudstones. A little further on, where the boundary crosses the 100-foot contour, is a good conglomerate with pebbles an inch in diameter of spilitic lavas (often licematised), albite-trachytes, and pinkish felsites, as well as large broken albites, while in (E10200) [SH 456 749] there is a fragment of granitoid quartz containing a long apatite, and another that resembles a true gneiss.

Between the margin and the line through Cerrigeeinwen

This great zone, more than 11 miles in length and a mile or more in width, is almost entirely composed of. the Autoclastic Mélange, but though abundantly exposed in several rocky tracts, few features are yet known in it that are not still better seen on the coast. Any planes but those of the steadily striking and almost vertical foliation are very rarely to be seen.

The first rocky tract extends from Dinas Trefriw to the Bodorgan woodlands. Some of the grits of the mélange are quite coarse, and one of them [A.P. 238] at the glacial arrow south-east of Trefriw contains orthoclase, a rare felspar in the Mona sediments. One zone is doubtfully separated as a Tyfry grit. In the railway cutting west of the tunnels the mélange is less full of grit than usual, and bedding is visible about 150 yards from the tunnel, dipping west at moderate angles. The quartzite of Bethel, though well known from having been worked, is of less petrological interest than many, being almost wholly quartz. The rocky tract from Trefeilir woods to Capel-mawr contains many nips of quartzite, some of limestone, that of Ty-calch (E10197) [SH 418 712] being white and calcitic, and its coarser grits may yield composite fragments. To the north of Henblâs the quartzite nips are unusually large, and the one south-east of the school contains little fragments that may possibly be jasper. The top of the plateau and the slopes from the 258-foot level to the ravine up which the late dyke runs, are very bare and display the mélange finely; in which, 200 yards south of Cefn-canol, is a schistose breccia of grit and fine siliceous matter. By Nant-newydd a rose dolomite with jasper shows that the Llanddwyn Group is just caught in. At Cefn-cwmmwd is a nip of reddish Tyfry grit (E10009) [SH 435 743] very rich in albite and spilite, and with many fragments of tourmaline-mica-schist in which is alkali-felspar. The plateau is again very bare between the branching roads to the north-east. Several quartzites which are involved in the mélange are not as white internally as usual.

Llangefni — About Llangefni there are ten quartzites. The white crags of the one cut through by the river are conspicuous among the fir trees from the station. . It contains (E10196) [SH 457 758] albite, spilitic lava, and a fragment of ancient mica-schist such as is found in the Tyfry Grits. The inlier in the Carboniferous at the Old Windmill contains (E9954) [SH 466 759] tourmaline, rutile, zircon, and fragments of ancient quartzose and micaceous schists. The mélange is well seen beyond the church (E10063) [SH 460 763], (E10193) [SH 459 760], (E1541) [SH 456 759], (E1542) [SH 456 760], and in the railway cutting at the station; while the Dingle, in which several fine sections are conspicuous, demonstrates its continuity all across the belt. Alongside the path, about 570 yards north-west of the station, was found (see p. 160) a one-and-a-half-inch pebble (E11249) [SH 455 762] of deformed hypabyssal albite-rock, identical with those of the boulders of the Skerries Conglomerates. Quarter of a mile north-west of Pen-lan a tributary ravine comes in from the east, a little to the south of which two zones of jasper, jaspery phyllite, and purple grit have been incorporated. Yet at its mouth is bedding, well preserved, in rocks that recall those below the Graig Wen quartzite of the north (p. 305). At the great curve between Pen-lan and the Dingle a broad band of compact rock is passed through, best seen just inside the wood and in the railway cutting opposite. Greenish internally, but weathering white, it has the aspect of a felsite, or of the silicified rocks of Part's Mountain. But it is (E10173) [SH 454 765] finely clastic, with little albite fragments, and the mosaic is largely composed of crypto-crystalline alkali-felspar, so it would appear to be an albite- or quartz-albite-dust.

In the cutting it graduates laterally into grits of the mélange, and is sheared along with them. That it should be an adinole is improbable, no intrusion being visible. Similar rocks, though more siliceous, are seen at a cottage 533 yards east of Tre-Hwfa, at the Cromlech crag by Henblâs, in the cutting at the Station, and in a Tyfry grit south-east of Plâs-bach (E10172) [SH 440 751], (E10194) [SH 426 718], (E10195) [SH 458 756], (E10073) [SH 405 719]. They recall the rocks of Tyfry and Newborough (pp. 355, 373). A narrow rocky tract of mélange (E6125) [SH 463 773], with some small quartzites (E6124) [SH 464 779], runs along the base of the Old Red Sandstone, from Llangefni by Rhos-meirch to a point east of Pen-y-cefn, beyond which this belt becomes obscure.

The Ceinwen Volcanic Zone

is identical with that of Newborough and Llanddwyn, the ellipsoidal spilites, ashy rose-limestones, jaspers, and jaspery phyllites, all being present, as well as the Tyfry grits. It is finely exposed at Cerrigceinwen . and Taldrwst, and the high steep knobs at Cerrigceinwen Church would be a complete reproduction of those of Llanddwyn but for being thickly draped in vegetation. The ellipsoids are of the smaller type, seldom a foot in diameter, and are well seen all along the tract, but there is a degeneration into chlorite-schist at the south-west end as well as in the outlying strips west of Trefeilir and at Aberffraw Bay. Indeed, even at the best, the rocks are not quite so free from deformation as in the type-area. Some degree of luematisation is usual, but the great mass is green. Jasper is common in the interspaces, with some limestone south-east of Bodrwyn. About 250 yards west of Cerrigceinwen Church there is a vesicular spilite (E10017) [SH 421 738] with signs of variolitic structure, in which are the jaspers that have been searched for pseudomorphs of vesicles; while in a rock three-eighths of a mile west of Parc-glas the structure is still stronger between a crowd of objects that look like amygdules (E10062) [SH 385 703]. West of Hendre-bach is a curious porphyritic variety. Haematised fragments of the lavas are common in the limestones, as in the one south of Taldrwst (E10075) [SH 412 725], often, when deformed, simulating jaspery phyllite. The felspar had been pseudomorphed in all the specimens cut. Albite-diabases (E10016) [SH 423 736], (E10076) [SH 410 726] are present, but have not been delimited on the one-inch map. The larger limestones (E10693) [SH 427 735]–(E10694) [SH 427 735] are well exposed, and often jaspery. The one 330 yards south-west of Hendre-bach is a mottled rose-dolomite. It contains the jaspery phyllite and spherulitic jasper (E10011) [SH 427 745], (E10012) [SH 428 744] that were analysed. There must be a small serpentine at Bodrwyn, probably resembling that of the Pentraeth eastern Inlier, for many years ago some asbestos was found there which (Mr. J. J. Ffoulkes, of that farm, informs me) came from about 200 yards below the house. A specimen preserved in the Museum of the University College of North Wales has the refractive index of tremolite. Further information on this volcanic zone will be found on the .0004 maps; and the whole suite of sections will repay study, for they are very complex.

Tyfry Beds — Tyfry Grits, rather fine but rich in albite (E10010) [SH 422 737], are seen on the high boss west of Cerrigceiniken Church; but the most interesting is a coarse one (E10074) [SH 405 719] by the end of the lane that goes north-west from Tan-lan. It is full of broken albites, fragments of keratophyre, spilitic lavas often haematised, old grits and quartzite, and is the one that contains a fragment of mica-schist with nine little tourmalines. Yet it is highly sheared, and has developed more new mica than many parts of the mélange, in which it is therefore incorporated.

That mélange is, on the south-east side, the country in which the Ceinwen zone lies; but on the north-west side is Gwna Green-schist, and the zone is brought within 170 yards of a Penmynydd inlier of western Gwna facies at Plâs-bach, and almost as near at many other places. The existence of a thrust-plane, therefore, need not be doubted; and it must be folded sharply. But it has been considered better to leave the line undrawn upon the maps, as it may not coincide with the outcrops of the inliers, and would in that case be conjectural in our present knowledge.

The Engan Spilites

The great Engan zone of spilite-schist, nine miles in length, is best studied along the five miles of steep and rugged bosses between Coron lake and Mona House. The rock is remarkably uniform in character, and the petrological descriptions given on pp. 77–8 (E10019) [SH 388 722]–(E10020) [SH 421 745] apply everywhere, with mere slight local variation, so that intrusive diabases are likely to be very rare, if not absent. Ellipsoidal structure is now barely discernible, for the deformation has been very powerful. Possibly it was never as marked as in the Llanddwyn spilites; but the presence of the jaspers and the chemical composition (p. 78) leave no doubt of the rock having been a spilite. The characteristic hmatisation also survives in many places (E10360) [SH 395 726]. The condition of the jaspers, now sheared-out into long lenticles and bleached (E10013) [SH 418 746], (E10014) [SH 418 746], (E10015) [SH 418 746], has been described on p. 88.

Long thin strips of a siliceous schist are also numerous, and, in places, large enough to be shown upon the maps. Here and there, as on the Ty'n-dryfol bosses, they contain cores with clastic grains, and are evidently caught-up inclusions of the Gwna sediments. But they are compact, and have a baked aspect, as if slightly altered thermally by the great lava. They need further investigation (see pp. 121, 368). It is very difficult to believe that the Engan spilites can be intrusive. The continuity of the basic schist is more seriously interrupted by many bands of Gwna Green-schist, often of considerable size. Some of these are due to folding, but the occasional alternating sections point to the existence also of several flows on slightly different horizons.

Unusually fine sections are afforded by the conspicuous great boss west of Tre-ddafydd-isaf, with shearing of the jaspers. A quarter-inch vein was noted, with epidote and transversely set fibres of pale actinolite. About Soar many strips of Gwna Green-schist lie within the spilite, but the broad tract at the 'B' of 'Bodwrdin' is poorly exposed, though sufficiently to show that the cross-faults must exist. The bleaching of the sheared jasper lenticles is easily seen on the Soar knobs. Haematisation survives on the steep lofty boss of Dinas, at whose south-eastern foot also some original igneous textures are visible. The siliceous seams are numerous all across the spilite at and south-east of Ty'n-dryfol, and are shown (much exaggerated in width) upon the one-inch map. Some of the clearest exposures are afforded by thin ones on the boss between the channels of alluvium. At the 'w' of 'Prys-iorwerth' the foliation passes obliquely through a jasper that has not been drawn out into a phacoid (Figure 163).

Very fine exposures occur again at Cerig-engan. On the escarpment below Craig-his spilite-schist alternates with Gwna Green-schist, and one such band rises (Figure 164) from below some of it on a little anticline. All the characters are again finely seen about Mona House, especially on the peninsula that runs into the alluvium, which is the best of all the localities for a study of the bleaching of the jaspers, here sometimes in phacoids a yard or more in length and six inches thick. The spilite-schist itself shows what seem to be survivals of ellipsoids, and a banding, with dark green seams that probably represent their dark outer skins. There are also about here hard salmon-pink seams, and the whole is cut by post-foliation movements. At the cottage among the crags north of the little alluvium beyond the road, Gwna Green-schist seems to both underlie and overlie the spilite, passing down on a north-east pitch which can be seen in the siliceous bands. Good exposures are found again near Tre-hwfa-bach, the usual characters appearing, as is also the case to the end of the line of strike near Trefollwyn, and at the extreme south-west end at Aberffraw.

The Western Tracts

The Gwna sediments have been converted into green-schist on both sides of the Engan spilites. On the south-east, original textures survive occasionally, as at a knob north of Prys-iorwerth close to the spilite (E10023) [SH 408 727], and other places along the strike to the south-west, the rock being Well exposed south-east of Dinas. North-west of the Engan spilites clastic textures are much rarer, and most of the green-schist is completely reconstructed, mica being often well-developed, and the aspect of the rock approaching that of the Penmynydd Zone. Quartz has separated out in great quantity, imparting a rugged appearance to the bosses, just asin the western parts of the Gwnas of the Aethwy Region, which these rocks vividly recall. There are good exposures north of the railway, and again at Soar (E10426) [SH 388 724], whence came the rock that was analysed. At Bryn-yr-odyn it is very typical (E10021) [SH 401 741], and quarter of a mile to the north is fine folding. But, consistently with the high foliation-dip, it is on vertical axes (Figure 42), (Figure 43), with nevertheless a good pitch to the north-east. From the map it will be seen that along this tract the pitch undulates considerably, being sometimes to the south-west. Near the 12–13 milestone on the main road the type approaches that of the Penmynydd inliers (E6126) [SH 411 755]. Another tract of good exposures is from Tre'r-gof to near Hen-eglwys, folding being visible, with pitch first to south-west then to north-east, and sometimes a distortion of the strike. Some of the quartz-aggregates in these schists contain felspar. At a little boss 300 yards south-east of the summit of Dinas (E10022) [SH 397 722], this is albite, but 233 yards west-south-west of the road at Tre-ddafydd-isaf it is orthoclase. About 400 yards west of Mona House a two-inch vein with pink orthoclase cuts the foliation. The extreme western parts of the tract, and most of the north-eastern, are but scantily exposed, except among the Penmynydd inliers east of Tregaian, where there are a few little limestones. But the character of the rocks remains much the same. A few outliers have been recognised among the Penmynydd schists. A little limestone of Gwna type occurs in one north-west of Aberffraw.

The Pentraeth Inliers

The Western Inlier

is but moderately exposed, though sufficiently to show that the strike of its masses is oblique to its trend; and there is a narrow rocky ridge between Ty-coch and Tan-y-graig. Gwna G-reei-schist without original structures, and rapidly folded with an inconspicuous north-easterly pitch, is well seen at Bryn-gwallen (E6140) [SH 488 758], much jasper and purple phyllite of local Tyfry type being incorporated in it. The little inlier of Penmynydd rocks is visible in three quarries on the western side of the road near the Smithy, but its relation to the Gwna Beds is not seen, though the changes of type in the middle quarry suggest a passage. It is crystalline, with well-developed mica, and is (E11437) [SH 504 699], (E11438) [SH 504 699], (E11439) [SH 504 699], a true porphyroblastic albite-schist rich in epidotes and other minerals like that of Aber-ffraw, though less regular in structure.

The broad basic band, visible in a scattering of small exposures, is chiefly chloritic schist, much of which appears to 'be deformed spilitic lava; but diabasic texture survives in some of the green schists, and a rock (E9851) [SH 522 793] 220 yards south-south-east of the 103-foot level is an ophitic albite-diabase with pale augite. It has not yet been possible to delimit spilite and diabase in this band. At Ty-coch is a beautiful ashy rose-limestone with spilite fragments. Spilitic lavas (into which some Tyfry Beds (E11250) [SH 503 767] are wedged) follow along the ridge.

Tan-y-graig high boss is composed of the remarkable glassy variolite (p. 72, (Plate 5), Fig. 3 (E11222) [SH 518 785]–(E11223) [SH 518 785]. Its ellipsoids are a foot or two in diameter, with re-entering curves as if plastic when rolled over. Often they have concentric bands of varioles. The matter between these ellipsoids, usually a darker green, is highly complex, and full of banded and angular bodies. This is the fluxion-banded spilite-glass that was afterwards brecciated. Jasper occurs in some of the interstices.

The rest of the inlier appears to be chiefly Gwna Green-schist with a northerly pitch. It is well seen below Ty-coch, and at Hendre, where purple phyllite is incorporated.

Rhyd-y-saint — Between the tongues of alluvium at Rhyd-y-saint some crushed calcaileous schists are seen, showing that a thin wedge of Gwna rocks rises between the Ordovician and the Carboniferous.

The Eastern Inlier

except at its south-western end, on which are broad spreads of drift, is extremely rocky, and the natural clusters of trees about the bosses render it one of the most charming tracts in Anglesey.

Dyfryn to Tyfry — As the country rock of the Western Inlier, and of this one where well exposed, are known to be Gwna Green-schist and mélange, the obscure tracts in the south-west and the smooth bluffs that run on from them above the river as far as Pentraeth Mill have been coloured as such; but there is no direct evidence of this, the few knobs that stand out of drift being chiefly spilite-schist and limestone.

The Serpentine (E9852) [SH 515 767], half a mile west-south-west of Tyfry, is compact and purple-green, with veins and aggregates of fibrous tremolite (E11387) [SH 517 767]. It shows no trace of pyroxenes, but the mesh-work after olivine, with some corroded crystals of chromite. The parent rock must have been a pure dunite. Nothing else is exposed at this boss, but a few yards to the south, a high knob of deformed jasper, chloride schist, and phyllite contains a few seams that appear to be schistose and fibrous serpentine, and if so, the peridotite is involved in the mélange. It is probably, therefore, of the same age (see p. 211) as the larger peridotites.

Tyfry Beds — About 200 yards from the serpentine is a steep craglet composed of hard, pale, flinty rocks (E9822) [SH 514 766]–(E9823) [SH 514 766] that recall those of the Cefni Dingle (p. 350), but well bedded and with some slate and fine grit. The flinty beds are clastic, and appear to be quartz-albite dusts, as do certain rocks at Newborough (p. 378); so it is possible that those of the Middle Region may be infolds of the Tyfry Beds. These now form a succession of straight ridges for a mile to the north-east. They are green and purple grits and phyllites (E9840) [SH 516 773], 9842], evenly bedded, the grits being comparatively fine, but full of albite and spilite. Some lateral dips indicate a symmetrical infoid, but there is a steady vertical cleavage striking 20° more easterly than the bedding does. Locally they are brecciated; beds can seldom, indeed, be followed far, and at a point 400 yards west-north-west of Tyfry begin to break up into lenticles along the cleavage (Figure 68) whose mechanics, therefore, link it with. the foliation of the mélange. A thin strip of that, with a more crystalline matrix like the Gwna Green-schist, appears among the grits 200 yards to the east-north-east. The Tyfry type-grit (E9839) [SH 517 767] (Plate 2), Fig. 6, coarser than the rest, with fragments of tourmaline-mica-schist, is in an isolated nip, well seen 120 yards east by north from Tyfry, lying between finer grits and ashy shales with hardly any cleavage, dipping at 18° north-north-west.

Tyfry to the seaward end — All the way from Tyfry to the seaward end of the inlier there is extraordinary variety; a group of rocks with great petrographical diversity having been so cut into slips by shearing stresses that the outcrops are seldom of the same nature for more than a few yards at a time. No more, therefore, than the points of chief interest can be indicated here. The high foliation dip itself is, indeed, the only constant feature, and to this there are but few exceptions.

The broad basic band is chiefly spilite (E6141) [SH 522 774], but a border on its eastern side about 140 yards in width is chiefly (though not wholly) albite-diabase, in addition to which there is a train of small slips of diabase lying en échelon on a meridian drawn through Fferam-gorniog. As both spilite and diabase pass laterally into chlorite-schist they have been but partially delimited. Just north of the 'Ff ' of 'Fferam', the diabase (E9844) [SH 524 778]. is coarse, with large ophitic plates of augite and small green pseudomorphs that may represent olivine. Its felspars are mere shells of albite enclosing secondary products. In the spilitic remainder of the band, pillowy structure survives well in places north by east from Fferam-gorniog between it and the road. The beautiful hmatised variolite (E9843) [SH 520 775] (Plate 5), Fig. 2 is from a boss 108 yards south-west of the pond at Fferam-gorniog; but variolites hardly less perfect are found at Tyfry (E9875) [SH 518 770], also between and in the limestones at the farm 300 yards to the north-east. They are somewhat inconspicuous in the field. Most of the spilites, however, have passed into dull, undulating green schists, full of purple streaks from hmatisation before deformation, this colouration being often the only means of distinguishing spilite- from diabase-schist. Among them, about 133 yards north of Fferam-gorniog, is the keratophyre (E9874) [SH 522 777], a fine dull-green rock with little spots. The smaller basic strips along the Plâs-gwyn and Rhiwlas tract are largely spilites, but east of a line joining those houses a train of albite-diabases comes on. In one of these (E9830) [SH 523 785] (Plate 5) Fig.4 500 yards north-east of Rhiwlas, though nearly all the felspars are thin shells of albite filled with sericite, a few of the original cores remain, and show that the mineral had been zonal, with centres at least as basic as andesine and probably more so.

Jasper is found everywhere, both in the spilites and the rose-limestones. An isolated boss of it measuring no less than 18 by 9 feet rises in the hollow south of the crag where the variolite (E9843) [SH 520 775] occurs, and there is unusually perfect spherulitic structure (E9877) [SH 520 776] (Plate 5), Fig. 6 210 yards west of Fferam-gorniog, close to which also the little idiomorphic quartzes are found. The typical jaspery phyllite (E9879) [SH 517 772], with very minute clastic micas in a siliceous base that seems almost isotropic, is from the end of the lane that comes from Rhos-cefn-hir. The limestones vary greatly, but most of them are of the rose or rose-green types, the latter often ashy. The large limestone of Tyfry shows, round the farm by the aforesaid lane, rude bedding-like structures dipping at low angles to the north-west. Here (E6142) [SH 519 770] it is in part a grey dolomite, but at Tyfry is rose-green with brown dolomitic and white calcitic veins. In and about the woods it contains the beautiful breceias with spilite fragments described on p. 83. Their best exposure is a little boss with two trees on it just outside the western corner of Tyfry wood (E6147) [SH 515 767], but excellent blocks of them abound in the old walls. On the western margin of the spilites, 400 yards west-south-west from Fferam-gorniog, is a rudely-banded, gneissose-looking calcite-limestone; but that at the farm itself is a rose-green dolomite (E9870) [SH 471 725] with many small fragments of spilite-glass; and on a boss 480 yards to the southwest one of the largest masses interdigitates with the Tyfry grits. By the farm about a quarter of a mile from the end of the inlier is a rose-dolomite rich in jasper (E9816) [SH 581 819], (E9878) [SH 538 796] with the sub-spherulitic oval bodies (p. 85). The last boss of all, which boldly overlooks the low country by the bay, is a spilite-schist with much jasper and jaspery phyllite (E9880) [SH 539 797].

Mélanges — A strip of Gwna phyllite-and-grit-mélange occurs within the Fferam-gorniog spilites just north of the position of the variolite (E9843) [SH 520 775]. But there is much more in the north, and to the east of Rhiwlas is good Gwna Green-schist. The state of anamorphism (E9850) [SH 533 792], (E9933) [SH 532 787] is about the same as in the eastern Middle Region.

In many places are mélanges even more extraordinary (good localities for which are between Rhiwlas and the Three Leaps; by the gneissose limestone; and a little east of Fferam-gorniog) in which grit and phyllite, spilite and diabase, limestone and jasper, are all sheared out in thin sigmoidal overlapping lenticles, and so carded together that every rock may contain inclusions of every other rock. Had such a product been subjected to higher pressures and temperatures, so that the whole became recrystallised, it is easy to see that a gneiss would have resulted whose origin would have been well-nigh undiscoverable.

The Aethwy Region

The Gwna Beds of the east

This large region will be considered in the following order:

1. The Menai coast (with the adjacent road and drive sections), from Gallows Point to Llandysilio Church islet
2. The adjoining heights, from Cadnant to Red Hill
3. The interior
4. The eastern margin, from Baron Hill to Careg-onen
5. The Llanddona highlands

Some inliers of the Penmynydd Zone will also be described here.

The Menai Coast

This affords a continuous dip-section (interrupted only by the Arenig Beds of earth Ferry) from Gallows Point to Cadnant creek, after which the rocks are almost as well exposed along the shore and rocky isles for the rest of the way. The high-road and Baron Hill drive, which run just above the shore, will be considered along with it.

The best part is from Gallows Point to Garth Ferry, a cliff 20 to 50 feet in height, richly overhung by woods; but an ebbing tide should be selected, some parts being difficult to study or even traverse at high water. From Glyn-y-Garth to Cadnant a boat is desirable, as the walls of the private grounds are an obstacle.

Gallows Point to Garth Ferry — The first rock seen is a basic schist among Gwna Green-schist. Its western edge is ill-defined, so it may possibly be a spilitic tuff. The Gwna Green-schist is typical, its quartz-augen being venous, but some of them soon show clastic grains, though veiled by vein-quartz, and less conspicuously clastic than on the weathered bosses inland. There is good flattened folding, but the dominant structure is an undulating foliation-dip at moderate angles. The large basic band near the of 'Pen-y-parc' (split by a wedge of pale schist close to the plexus of little dykes) is a good example of its class; heavy, dark-green, platy throughout, and containing little aggregates of granular ternary albite. Siliceous green-schist then rises, in which is inconspicuous clastic matter, and a six-foot quartz mass that seems metasomatic, as well as a thin basic schist that shows faint signs of diabasic texture. After the gap with dykes is a rather platy fine siliceous schist, but gritty matter follows, almost as platy; quickly, however, becoming a pencil-schist' with venous augen. Beyond another dyke (which hades west) is true autoclastic mélange, full of lenticular grits, one of which, a yard in length, can be seen in the act of breaking up sigmoidally into smaller augen (Figure 5), and, a little before a natural arch is reached that leads to the old bathing-house beach, are fine sections in mélange. But the matrix is far better foliated than in the Middle Region. By the arch are some red grits.

Beyond the beach the large limestone is well exposed, mostly massive, but including several feet of purple and green-schists; as well as lumps of jasper. (Figure 165) represents the base. a zood deal simplified. Some purplish banded rocks are hgematised ashy calcareous beds with fragments of albite and keratophyre. The large basic masses that now come down to the coast are of great importance, as they are evidence of the nature of the many basic sehists of the region which have been completely reconstructed. The first is mostly a chloritic schist, but contains large cores in which are sheared ellipsoids with remains of concentric structure, and little spherulites. Mélange rises from beneath it, in which are basic lenticular bands a foot or two thick, some only two inches thick, but with sharp junctions. Beyond these is a flaggy grit with fragments up to a quarter of an inch in diameter of salmon-tinted acid igneous rocks. A fault bounds them, and the next basic schist has obscure ellipsoids only. Pebbly grit again follows, rich in the same fragments. They are (E11196) [SH 585 744] albite-quartz-felsites and albitic hypabyssal rocks like those of the Skerries Grits (p. 60), with keratophyres. The matrix is full of secondary chlorite and white mica. Beyond a spur of grit a great scree from road-widening makes a landmark; and then follows another sheared spilitic lava with lenticular Jaspers and remains of ellipsoids, but with the dark skins between them now become a contorted schist. Yet only a yard away perfect little spherulites have survived in it. few yards further on ellipsoids about a foot in diameter are perfectly clear, with dark skins in which are spherulites (E11194) [SH 585 745]–(E11195) [SH 585 745]. The augites have disappeared but the felspars,are fairly preserved, the rock resembling the typical Newborough spilite (E9895) [SH 396 645]. The felspar spherulites resemble those of (E9956) [SH 398 650], (E10339) [SH 398 650], and the base was probably a glass. Then, on a spur of cliff all these volcanic structures are seen better still; yet at the foot of the spur are sheared into a platy green schist with no trace of them perceptible. Another spilite follows. Thence to Garth Ferry, along a straight stretch of coast where exact positions were difficult to get, is Gwna Green-schist with a little grit, and several basic bands, one or two of which retain traces of ellipsoids. The general structure is lenticular, with undulating dips, and a few folds.

The Road and Drive — Parallel sections at successive levels of about 80 and 180 feet are afforded by the Beaumaris road and the, Baron Hill drive, besides which the wooded face of the plateau and its edge at the 300-foot contour are hardly less craggy, so that the behaviour of the rocks can be studied through a depth of 300 feet. The larger basic bands are cut by several of these parallel sections; and we find, first, that their margins practically coincide with foliation planes, showing the foliation-dip to be locally a true dip; and secondly, that the masses wedge-out as often downwards as they do upwards.

The rocks are very fresh and good in the cuttings on the road, the analysed specimens of Gwna Green-schist and spilite-schist (E9911) [SH 586 745], (E9913) [SH 587 745] having come from 760 and 850 yards north-east of the Ferry Inn respectively. The spilite-schist is platy here and original structures hardly visible. The sections have been altered and some of them a good deal improved since the ground was mapped, by the straightening of the road. This is the case with the fine cutting in the bath-house limestone; here a beautiful rhodochrosite-dolomite with some white mica (E9932) [SH 584 744]. Some of its jasper is tabular. Smaller lenticular limestones are to be seen close by (Figure 166).

Just inside the gates of the drive a large lenticular limestone appears at the top of the cliff among Gwna Green-schist which is dipping at very low angles, and low dips, even of overfolding, are prevalent for some way. About 310 yards from the gate is a fine group of lenticular limestones (Figure 78), (Figure 79), one of which is contorted. Just beyond, the jaspers are sheared in among a micaceous Gwna Green-schist as thin lenticular seams (E9965) contorted with it, forming, indeed, a member of the foliated complex, as described on p. 88. A massive green rock not far from the gates retains broken large augites and albites with traces of ophitic relations, and was evidently an albite-diabase. There are also excellent sections in mélange.

Garth Ferry to the Church Islet — Returning to the coast beyond the Ordovician grits, the first rock is a siliceous and micaceous Gwna Green-schist in which clastic grains are just discernible. But the grade of metamorphism has risen, and original structures can hardly be made out all the rest of the way. Before Glyn-y-garth terrace interrupts the section a strong platy chloritic schist appears, but it seems only sub-basic, is interbanded with micaceous green-schist, and may be a spilitic tuff. Beyond the terrace are many torn strips of similar sub-basic schist.

The coast under the grounds of the next house (Glan-y-Menai on the six-inch map) is of importance because for nearly a quarter of a mile the Gwna Green-schist becomes so micaceous that in a region of the Penmynydd Zone it would not have been separated out. The section is troubled by a quartz-vein, which has been searched for copper, but this is not a boundary, for on its outer side a complete passage can be seen from nemablastic Gwna Green-schist into this Penmynydd type. The position is on the strike of the glaucophane-schist of the wood west of the Ferry Inn (see p. 119) (E9526) [SH 578 740], (E11088) [SH 578 740] which rests upon mica-schist also of Penmynydd type. On the same strike is the drive of the new house immediately above the high road at the Ferry, and at the foot of this drive is Gwna Green-schist rather more micaceous than usual, followed by the same still more micaceous, until at the turn of the drive, just on the strike of the bed below the glaucophane-schist, is good mica-schist but 'with typical Gwna local structures. Mica-schist has lately been exposed also on the roadside by the post office. The tract is undoubtedly a small inlier of the Penmynydd Zone, with the belt of transition exposed both on the shore and in this drive.

A little point, east of Craig-y-don house, shows a good junction, with alternations, between basic and siliceous schist, and then a fine section follows in rock largely composed of lenticular, folded pegmatoid quartz-augen as much as three inches thick, in. which are albite and anorthoclase. Thence to Cadnant Creek is Gwna Green-schist, with occasional basic schists, containing red albite-aggregates like (E10031) [SH 554 718]. The two Craig-y-don islets are of a rugged nemablastic siliceous type, with some similar basic rocks. Ynys Gaint (accessible by a causeway except at high water) affords fine sections of the same rugged siliceous schist, with undulating constricted augen, which on the outer summit are contorted, and the whole rock strongly gnarled ((Plate 8), also (Figure 6)). The type is dominant on the succeeding islets and rocky shores under the village and past the Suspension Bridge as far as the Church Islet, where the change to the Penmynydd Zone comes on (see pp. 124, 366).

But it has already begun, for glaucophane is found in the basic schist on the shore east-south-east of the island church (E10208) [SH 553 716] (p. 120). The polyclinally folded Gwna rocks in the pine wood east of the islet-causeway are extremely siliceous, and the sections are some of the best for a study of the pencilly pitch that results from nemablastism. They also show signs of the approaching change, mica being better developed than in the typical green-schist (E6093) [SH 554 718], (E10033) [SH 554 718]– (E10034) [SH 554 719]. The quartz of these augen may be studied in (E9861) [SH 559 722]–(E9862) [SH 559 722], (E9985), from the Smithy islet and fiom north of the 'y' of 'Llandysilio', at both of which places it contains albite. In the pine wood are two little outliers of chlorite-epidote-schist (E10031) [SH 554 718]–(E10032) [SH 554 718], with the aggregafes of reddened ternary albite and quartz that are generally to be found in these rocks near the Penmynydd Zone in the Aethwy Region.

The Heights from Cadnant to Red Hill

On the high brow that looks down on the woods north-west of Plâs Cadnant are fine examples of the Mélange with lenticular grits, eases being seen of sharp folding of a phacoid on itself (Figure 77), and many (especially the red grits) that show clearly a development of nemablastic structure at the ends, as well. as others where venous is effacing clastic texture. One of them (E10787) [SH 55 71] contains a little microcline, and garnet with polygonal outlines, curiously cased in chlorite and quartz. In the ravine (E10786) [SH 55 73] is a compact rock with generally low refractive index that seems to be one of the quartz-albite dusts. Along the rocky heights to Pen-y-parc lake are many similar sections (with grits that contain much albite (E10788) [SH 572 748]) and others (Figure 167) that show basic schists and limestones lying as lenticles within the mélange. On and south of the open common of Cyttir Llandegfan are perhaps the best localities for a study of the later cross-corrugation (Figure 80) described on p. 200, which trends here about west-north-west. At the, farm 300 yards east of Bryn-meurig is a rose-dolomite (E6091) [SH 583 745] with juper and purple phyllite; and a curious variety of the spilitic suite—composed of a crowd of idiomorphic augites in a faintly schistose matrix of chlorite, epidote, and leucoxene, with chlorite-albite veinlets, which may be an augite-tuff (E9845) [SH 583 745]. About the next farm the lavas have passed into leucoxene-chlorite-schist (E9846), which contains lenticular jaspers, partly bleached.

About Pen-y-parc lake the fine banded siliceous schist [c.f. (E9824) [SH 569 756]] is well developed, and shows the most regular folding seen anywhere among the Aethwy Gwnas, crossed by the later folds. Mélange comes on again at once, with foliated grits (E10792) [SH 59 74], whose foliation is occasionally folded over sharply within the lenticle, about 120 yards south-east of the lake. By the woodside farm north-east of the lake is a grit (E6091) [SH 583 745] containing a granitoid fragment. In the midst of this mélange lies the large quartzite of Pen-y-parc (E6096) [SH 590 750], (E10802) [SH 59 74], a lenticle which can be traced down to the wood, followed by two smaller ones a few yards further east. The old quarry by the farm-house affords a fine section, on whose north-east cliff some isoclinal folding of schist and quartzite was to be seen 20 years ago (Figure 168). At the edge of the wood, 233 yards east of Pen-y-parc, is a chloritic rose-dolomite (E9802) [SH 593 750] with trachytic lapilli and groups of clear secondary albite. The large chlorite-epidote-schist (E9812) [SH 592 755] with which it inosculates, is cut off by the fault that partly, bounds the Baron Hill outlier. In the midst of it, just north of the road, is a singular brick-red rock (E9970) [SH 592 754] with a fine schistose matrix of rf. ind. about 1.536, in which are many good-sized phenocrysts of albite, evidently a sheared albite-trachyte.

The Interior

The greater part of this (away from the Penmynydd march-land, which is not well exposed) is composed of mélange. Spots of special interest only will be indicated. The grey dolomite of Pedair-groeslon (E10801) [SH 551 740] and analysis] closely resembles the Cemaes limestones in general aspect. It lies in Gwna Green-schist, in which no graphite-schist or quartzite has been found. A train of thin purple phyllites runs from Pedair-groeslon to Llansadwrn, which are not true jaspery phyllites, but (E9953) [SH 555 741] the albitic type mentioned on p. 68. Good exposures of folded mélange full of lenticular grits (E6086) [SH 559 764], (E9811) [SH 559 764], (E9934) [SH 559 764], (E9809) [SH 581 819], (E9825) [SH 559 764], (E9801) [SH 559 764], and analyses], and green-schist (Plate 9), Fig. 2 are to be seen at Llansadwrn. Their quartz includes needles of apatite and zircon, and (E9801) [SH 559 764] which is red, contains granite and micro-pegmatite. (Figure 169), which was drawn in 1896, by the 'S' of 'Smithy', had become overgrown in 1911, but there are quarries by the road-fork between the church and Tai-lawr, and good bosses towards Ucheldref, where four quartzites appear. By the footpath north of the S ' is a deformed spilitic lava with jasper, which retains traces of original structures.

An interesting group appears 266 yards west of the roadside corner of Hafotty Covert. In mélange whose matrix (E9968) [SH 558 770] is rather more crystalline than usual, is a chloritic rose-dolomite (E6097) [SH 558 770] which contains the type-jasper (E11224) [SH 557 770] (Plate 5), Fig. 5, with haematite aggregates in the cores of quartz-grains. This limestone contains lapilli of haamatised spilite; and close to it is a six-inch bed of spilite-tuff, one lapillum of which is porphyritic, with a twinned albite a quarter of an inch long. The little ravine at Cremlyn is cut in siliceous nemablastic schist (E6088) [SH 574 772] with a large rose-dolomite containing jasper (E10004) [SH 571 772] now slightly schistose. A few yards to the east is a purple schist that simulates a jaspery phyllite, but is really a porphyritic spilite, haematised and schistose. Haematisation is clearly older than deformation, and some seams of granular ternary albite are unhnmatised (E10005) [SH 571 772]. There is another rock like it (E6087) [SH 565 773] south of Hafotty Covert.

The Eastern Margin

This is a zone of siliceous Gwna Green-schist, highly nemablastic, though with ashy grits near Nant woods (E10799) [SH 600 774], and remarkable for the train of great sigmoid lenticular basic masses that strike north from Bod-gylched to the sea, and are well exposed about Carwad and Bryn-cogail. They are platy green-schists, which, as the rose-limestones and jaspers accompany them, are doubtless for the most part modified spilitic lavas. At Bod-gylched one of them (E6085) [SH 581 770] seems to be a modified albite-diabase; while another (E10797) [SH 585 771] is a fine epidote-chlorite schist with alkali-felspar and some amphibole; and a thin one at the north end of Cremlyn alluvium is blue, as if with glaucophane. But they need further petrological study.

The jasper (E9847) [SH 589 788] that underlies the limestone between Carwad and Bryn-cogail is pale, micaceous, and a link with the jaspery phyllites, but though highly reconstructed contains rhombs of carbonates. On Llaniestyn common, just west of the church, the micas of the green-schist are unusually developed, with which is doubtless connected the nature of the large basic mass running northward from the church (see p. 119), where the reconstructed lava seen at the church farm- (E9691) [SH 586 797] strikes at the glaucophane-schists that appear on the bosses south-east of Ty-du, whose powder yields glaucophane in moderate development. The pitch undulates remarkably about Llaniestyn, and 300 yards east of the church farm the cross-folding is a distinct over-drive northwards up a southerly pitch.

The schist of the coast at Careg-onen is nemablastic and siliceous (E9790) [SH 581 819], 9810], but its basic bands are of intermediate character and do not seem to represent the spilites. They may be impure tuffs, but need further study. Still more doubtful are the dull-green rocks at Pen-yr-allt, Llanddona (E9797) [SH 575 802] which may have been somewhat silicified near the lead-veins (see Chapter 19).

The Llanddona Highlands

This will be a convenient name for an extremely rugged tract, 400 to 500 feet in height, with a scenery that in its wild western parts adjoining Mynydd Llwydiarth recalls that of the Lewisian Gneiss of Scotland. The Gwna Green-schists, which strike across the heights, are very siliceous, corrugated, and strongly nemablastic (E9848) [SH 574 778]. Untwinned albite is not uncommon.

The most remarkable feature is the extraordinary number of little lenticular strips of basic schist. In a quarter of a square mile near Ty'n-y-mynydd-'east' (to be distinguished from the western farm of the same name) 100 of these have been laid down upon the six-inch maps, and had the .0004 scale been used many more could have been shown, yet the map would still have been but a rude abstract of nature. These basic rocks need further study. So far as is known, they are (E9831) [SH 551 783]–(E9832) [SH 555 782] (Plate 9), Fig. 3 chlorite-epidote-schists with some development of minute actinolite and seams rich in granular ternary albite (see p. 77) often slightly reddened, as in (E10031) [SH 554 718]. As these are characters of spilite-schists, it is to be supposed that most of them are derived from spilites. Yet (unless it be the source of their siliceous seams) no jasper has been observed in them; so albite-diabases may be largely represented.

The powder of the large basic schist above Wern yields abundant glaucophane in good development, and the rock is freely exposed. That a rise in crystalline grade appears in the siliceous Gwna schists has been mentioned on p. 125, and can be seen easily by traverses down the rugged slopes from the south. But the complete passage to the Penmynydd Zone is cut out by a considerable rupture that bounds the glaucophane-schist on the south-east, bringing chlorite epidote-schists against it, and giving rise to a strong steep feature.

The rugged Llanddona highlands afford, on their innumerable bosses, the best possible sections for a study of the structures of the nemablastic schists. The various types of quartz that separated at successive stages of the metamorphism can be seen almost anywhere, especially by the hollow -some 200 yards north of Ty'n-y-mynydd-east. North-west of Rhos the close connexion between nemablastic pencils and the corrugation of locally thickened seams of quartz may be studied. A strong straight ' slack' or hollow evidently along a fault crosses the hills at the parish-boundary, and at this, west-north-west of Ty'n-y-mynydd-east, a-clinal folding is combined with a vertical dip (Figure 54), (Figure 55). On its eastern side, polyclinal folding (Figure 57), (Figure 58), (Figure 59), (Figure 60), is to be seen in many places, though often confused by the local thickening of the quartz-seam. Sometimes there is evidence of torsion, for, 200 yards north-east of that farm (and elsewhere) the nemablastic lines pass obliquely across the crests of anticlines, as if there had been some shifting along the strike at the time of the crumpling. The angle of pitch varies a good deal, as at the southward end of the great slack, where it changes from 5° to 20° in 34 yards. About 170 yards to the north-north-west a complete phacoid of basic schist is cut through as in (Figure 170), where the pitch is at a gentle angle into the crag-face. Near the middle of the slack on its western side the annexed plan (Figure 171) shows a small basic schist crossing the strike. It is inconspicuously folded on the same scheme as the coarse and ruggedly gnarled siliceous schist. The plan and section (Figure 172) show the effects of pitch on similar bands. One of the best sections that exhibit the relations of the masses to folding and foliation is about 150 yards north of the south end of the great slack (Figure 173). Both basic and siliceous schists are folded together, the pitch being away from the eye at a gentle angle. It will be noticed that the true dip is on the whole in a direction opposite to the foliation-dip and to the 'plunge of the axes of the little isoclinal corrugations.

The Penmynydd Zone of the Aethwy Region

By far the best exposures are on Mynydd Llwydiarth, but as there are some structural peculiarities at that hill, the region will be described in the following order. First: the eastern and south-eastern margins from Llandysilio and Castellior to Newborough, then the north-west margin from Newborough to Plâs-gwyn, then the centre from Newborough to Trefor; then Mynydd Llwydiarth. Some inliers from the Zone have already been described on pp. 361, 364–5.

The eastern and south-eastern margins

On the Menai shores by the island church the gradual on-coming of the Penmynydd Zone, though not laid bare in continuous exposure, is quite manifest on the group of rocky knolls and islets. Heralded by the increasing size of the micas in the nemablastic siliceous Gwna rocks in the wood (E10033) [SH 554 718]–(E10034) [SH 554 719], (E6093) [SH 554 718] (see p. 124), the passage may be said to begin on the shore east-southeast of the church, where the basic schist (E10208) [SH 553 716], and analysis],(p. 78) contains a little glaucophane. The line, however, has been drawn, where, north-west of the church, unmistakable holocrystalline mica-schist first appears, which is a compact albite-quartz type where it adjoins the basic schists. At the little headland north of the church islet those are in about the same crystalline condition as (E10208) [SH 553 716], true transition rocks, with ternary albite (E10035) [SH 552 719], (E10037) [SH 552 719], (E10037) [SH 552 719]. Beyond them it has, in common with the adjacent Gwna rocks, a somewhat nemablastic structure. The -shore is a good section under the woods and railway bridge as far as the base of the Carboniferous 'rocks. On the church islet the types are transitional, but all the other islets are crystalline mica-schist (the western ones highly so) (Figure 174), and worthy of note because two 'of them are very near to Carnarvonshire and yet show not a trace of decline in metamorphism.

One more transitional rock should be noted—the narrow basic mass along the road at Pedair-groeslon (see p. 119). North of Yr-allt this resembles the green chlorite-epidote-schists with reddened albite usual in the Gwna tract, but west by south of that farm becomes a good glaucophane-schist. Whether the two types are continuous is not known, but they strike at each other, and the green chlorite-epidote-albite-schist contains (E6092) [SH 552 733] large hypidiomorphic glaucophanes, leaving little doubt of the continuity. The siliceous schist adjacent to the blue glaucophane-schist on its western side has the aspect and the nemablastic pencils of Gwna Green-schist, from which it differs only in a better development of mica, and is a passage,-type to the Penmynydd Zone.

The Great Glaueophane-schist, the largest known in the British Isles, extending over more than two square miles, is well exposed from the Column to beyond the Cromlech, at Dyfnia, Felin-engan, and Castellior. With the exceptions presently to be noted, it maintains all the characters, described on pp. 115–18 with great uniformity (E1644) [SH 525 716]–(E1645) [SH 525 716], (E9789) [SH 536 715], (E9829) [SH 536 715], (E9866) [SH 544 741], (E9867) [SH 544 741], (E9868) [SH 536 717], (E9924) [SH 547 744], (E10767) [SH 534 716]–(E10768) [SH 536 715], (E10770) [SH 543 741]–(E10771) [SH 543 741]. It is powerfully folded, with a pitch (occasionally high) to the north-east, and the foliation-dips and overfolds indicate thrust from the north-west except at Dyfnia. Little cross-wrenches occur at intervals, and at Castellior near the dyke are themselves overfolds from the south-west. There are also larger cross-folds overthrust from south-west, themselves crossed by little ones at a lower angle (Figure 175). A core with hypo-plutonic texture appears-near the north end of the Dyfnia bosses by the river; and another at the first 'w',of 'pwllgwyngyll', but this (E9853) [SH 532 720] contains no true glaucophane, and is a foliated zoisite-albite-amphibolite with broad plates of faintly-bluish hornblende. Small porphyroblasts of glaucophane may be found 225 yards west of the Cromlech. The large knots of glaucophane-epidosite (E11141) [SH 539 736] which may represent the ellipsoids of a spilite (pp. 117, 120) are well seen at Felin-engan; and on the Column crags, especially alongside the footpath going north-westward through the little wood where the crag looks down upon it. The great glaucophane-schist is nearly split by a long curving wedge of mica-schist, well seen west of Pedair-groeslon, so crystalline as to suggest that part of the zone of passage from the Gwna rocks may be cut out by a slide. Its felspar seems all albite, it is rich in epidote, and is one of the few Penmynydd rocks that contains a little tourmaline (E9827) [SH 545 742]. By the roadside east of the bridge at Felin-engan some hard flaggy siliceous bands occur in the glaucophane-schist which resemble the banded marginal rocks well seen elsewhere, and the loose blocks not far off show that the type must be well developed below the drifts.<ref>Note just before going to press — Can it be that they are of the same nature as the hard siliceous bands in the Engan spilite (see p. 352), but in a much more advanced stage of dynamic metamorphism? It has already been suggested that the glaucophane-schist may represent that spilite. The specimens are not at the present moment accessible, and the point will need further investigation.</ref> Finally; it may be convenient to add that fresh specimens of the glaucophane-schist can be obtained from the roadside quarry 250 yards east-south-east of the Column, about five, eighths of a mile from Llanfair Station.

Trefor and Pen-hesgyn to Newborough — From Wern to Trefor the eastern margin is obscured by drift, but the Trefor inlier is fairly exposed and contains thin glaucophane-schists. Returning south-westward, the mica-schists are well seen at several places between Pen-hesgyn and Bryn-gof, and the line is one where some overfolding from the east has taken place, though the usual westward foliation-dips are not entirely excluded by this. But this folding calls for further study. The analysed specimen of mica-schist (E9912) [SH 530 736], (p. 112, cf. also (E11372) [SH 562 731]–(E11373) [SH 562 731]), was from this tract, at the bosses on the north side of the road near Braint farm; it contains twinned albite and much epidote. The analysed hornblende-schist (E9914) [SH 520 734] from the large knob 400 yards west of Sarn-fraint bridge, is rough rather than platy. Beds, a yard or so thick, of the usual hard mica-schist occur in it. From the same basic band, and not far off, must have come the interesting variety collected by Blake (E10766) [SH 520 735], containing a blue-green amphibole with low extinction angles allied to but not glaucophane; while, further south (E10769) [SH 515 726] the band contains a little pale glaucophane.

The two hornblende-schists that run towards Bryn-celli-ddu are but moderately exposed. A gently curving anticlinal axis of pitch runs nearly west-south-west past the Carnedd, beyond which the pitch is southerly as far as Pont Dic, where the normal northerly direction is resumed. There are good exposures along the river between Crug and Dic bridges, the schists striking sharply at the Carboniferous boundary. A good deal of flaggy rock occurs here as well as the normal type, and hard fine seams adjoin the thin hornblende-schists, a good junction (Figure 176) being visible on the easterly one opposite Gwydryn. By the footpath north of Pont Dic is a pink albite-mica-schist rich in garnet (E9918) [SH 487 684]. The great basic masses that begin at Bodlew are but scantily exposed for some three miles: indeed the tract on the south side of the river from Trefwri to Tre-anna has been coloured on hardly any better evidence than that of the Dwyran strike and the contents of the drifts. A chain of scattered bosses ranges from Llysllew to Pont-mynach, along which the hornblende-schist has a tinge of blue and has yielded glaucophane at Bodrida by the roadside, though opposite Bryn-gwyn (E10764) it is normal. The adjacent mica-schist, mostly of the usual types, contains, a little south of Bodlew (E10747) [SH 481 680], a large broken compound felspar that is chiefly microcline. The Dwyran ground is rocky, the schist fine and blue enough to look like a true glaucophane-schist, but four powders examined yielded no good glaucophane, the amphibole being partly green, partly indefinite with low bi-refringence. These rocks call for much more examination. On the western side of the river several thin bands of compact mica-schist lie within them, and (90 yards north-north-east of a small house) there is a singular breccia that seems ancient. From Llangeinwen Church to the dunes they are obscured by drift.

Along the Braint both foliation-dip and fold-axes have been very high, but at Newborough they are first overdriven from the southeast and then roll over on an anticline. The mica-schist (E10742) [SH 40 64] is normal. The large basic mass of Newborough is glaucophanic in its north-eastern parts, but between the village and the southern dunes (E9971) [SH 416 651], (E10763) [SH 425 652], where there are good sections, is a typical hornblende-epidote-schist rich in granular albite, and with singular spherical groups of epidote that may possibly represent spherulites. Perhaps a spilite, and a diabase have been sheared together.

The north-western margin

Newborough to Llangaffo — Along the rocky hills that look down upon the Malldraeth Marsh the Penmynydd rocks are well exposed. By a little farm half a mile west of Tyddyn-pwrpas, Newborough, the mica-schist has broken down into a singular schistose breccia, probably a product of the last movements of the Complex. The marginal mica-schist varies a good deal, but contains plenty of fresh albite (E10743) [SH 416 649] even when fine. At Bryniau some varieties (E9917) [SH 427 672] are dark with oxidised biotite. The long, broken train of basic rocks is largely green hornblende-schist, but glaucophane begins to appear, and thence to the railway the broader bands all contain glaucophane. From Tyddyn-fawd as far as the Glan-morfa bosses there is most intimate inter-felting of basic and siliceous rocks, the seams being only half an inch or less in thickness, the acid ones occasionally (E9915) [SH 431 678] not very compact, but containing rounded small porphyroblasts of albite. On the bosses north-east of Tyddyn-fawd these banded complices are powerfully folded, the glaucophane-schist being traversed by a second foliation (a very rare phenomenon in the Penmynydd Zone) which, however, almost dies out in the acid seams (Figure 90). Connected with this may be the fact that a quartz-vein filling an old fault in the glaucophane-schist is itself crossed by the foliation (Figure 177).

Llangaffo Railway Cutting is of exceptional importance because, between Plâs Llangaffo bridge and the marsh, it lays bare 466 yards of continuous section. on whose northern cliff the relations of the several basic and acid rocks (E6110) [SH 451 694], (E6130) [SH 451 694], (E8529) [SH 444 683], (E8530) [SH 444 683], (E8531) [SH 444 683], (E8532) [SH 444 683], (E8533) [SH 444 683], (E8534) [SH 444 683], (E8543) [SH 444 683], (E8544) [SH 444 683], (E8545) [SH 451 694], (E8546) [SH 451 694], (E9190) [SH 451 694], (E9557) [SH 450 694], (E10765) [SH 450 694] are seen quite clearly. The .0003 plan ((Figure 178), reduced from a .0004 plan) will be a guide on the ground. The mica schists are on the whole rather flaggy, but highly crystalline. The western basic bands are hornblende-schist, but the eastern one is a tolerably coarse foliated diorite with deformed bodies that were evidently phenocrysts of felspar. On its margins (as well as along some internal planes of shearing) it passes into hornblende-schist, thin bands of which alternate rapidly with mica-schist. Rapid alternation occurs also near the other basic masses, and there is a thin acid band in the midst of the diorite. About 12 yards west of the diorite some dark knots appear within the mica-schist, which are locally thickened bands that alternate marginally with it, and are intensely contorted (in their thinner parts) on horizontal axes. Their nature is uncertain, as they are now (E9190) [SH 451 694] crushed quartz-epidote, and chlorite-epidote aggregates, with distorted flakes of white mica, and some carbonates. Near the west end of the larger hornblende-schist, their foliation sweeps across that of siliceous mica-schist without any disruption plane, as if they had appeared at a late stage of the Complex. With this exception, the relations of all the rocks are those of common and conformable foliation. Wedges of siliceous mica-schist plunge down. from above into one of the western hornblende-schists, but they are sharp synclinal nips in a compound anticline upon which the hornblende-schist is rising. Where the mica-schist adjoins any of the basic rocks, it assumes the compact marginal modification so often alluded to (p. 121). Some of such, high up 'on the cliff close to the diorite, appear from below to be tongues of acid matter intrusive in the basic. But on close inspection they are found to be true bands, conforming to the foliation of the basic schist, rapidly folded along with it, and with a faint internal parallel banding that is folded also in the same way. No junctions in the cutting Can be regarded as original, but the close association of the hard bands with margins of basic rock suggest an early thermal induration that conferred powers of resistance to deformation.

Llangaffo to Berw — At Hendre-gadog is a schist with 'spangle' micas oblique to the foliation; and about Treferwydd are coarse, gneissose-looking mica-schists with dark bands that contain a biotite with variable axial angle. The pitch here is locally high, and even vertical with sharp folding. There are cases where the overfolding of soft beds in a nip (Figure 179) is from a direction opposite to that of the main fold.

Some of the best sections of the zone are between Berw and Bwlch-gwyn. On the rocky moor about Bryn-tirion are long straight trains of the venous quartz-phacoids, overlapping en échelon (Figure 181), themselves often split by bays of mica-schist, but not sensibly affected by the folding (which is at intervals, in short hitches, as in (Figure 180)), and on a boss 100 yards south-east of the 178-foot level one of them (Figure 182) truncates granoblastic lenticles. At the northern cottage in the moor is flaggy moine-like rock (E9916) [SH 471 718] with small idiomorphic garnets; and 90 yards south of the 178-foot level are rounded porphyroblasts (E9919) [SH 470 716] chiefly of albite but some of micro-perthite. The hornblende-schists near Berw are of the platy type with occasional feathers' needles, many small lenticular albite-pegmatites, and at a railway cutting 120 yards south of Plâs-berw plates of a white mica with a wide axial angle, probably paragonite. In and alongside of these hornblende-schists, south and east of Plâs-berw, is perhaps the most minute inter-felting of basic and acid schists to be seen in Anglesey, the alternating bands ranging from an inch or half an inch in thickness down to mere films, and remarkably even and parallel. The white seams are granoblastic albite and quartz with a few garnets, the dark seams chiefly straight prisms of hornblende with many of blue-polarising zoisite, some of which have cores of epidote (E9885) [SH 460 704].

Holland Arms — North of the main road the mica-schist is as highly crystalline as anywhere in the Penmynydd Zone, so that the survival close by of original structures is remarkable. It is rapidly folded, sometimes a-clinally, on axes pitching north-east (most of the foliation-dips being high), with corrugated quartz-phacoids (Plate 11), all well seen west of the little wood north of the church. The lenticular orthoclase-pegmatites are better developed here than anywhere, especially in the little wood itself. The important knob discovered by Dr. Callaway (p. 122) whence came the specimen (E8485) [SH 474 724] with felsitic texture, and the analysed specimen (E8486) [SH 474 724], as well as (E8487) [SH 474 724], (E8488) [SH 474 724], (E8489) [SH 474 724], (E8490) [SH 474 724], (E8491) [SH 474 724], (E8492) [SH 474 724], (E8493) [SH 474 724], (E8494) [SH 474 724], (E9170) [SH 479 726], (E9171) [SH 479 726], (E9172) [SH 479 726], (E9173) [SH 479 726], will be easily found by the reduction from the .0004 map (Figure 10). The slides (E8496) [SH 471 724], (E8497) [SH 471 724], (E8498) [SH 471 724], (E8526) [SH 471 724], (E8527) [SH 471 724], (E8528) [SH 471 724], (E1548) [SH 48 71], (E1549) [SH 48 71] are from the immediate neighbourhood; as well as those of the hornblende-schist (E8536) [SH 474 724], (E8537) [SH 474 724], (E8538) [SH 474 724], (E8539) [SH 474 724], (E8540) [SH 474 724], (E10776) [SH 478 728], (E11380) [SH 478 726]–(E11381) [SH 478 726], of which (E8537) [SH 474 724]–(E8538) [SH 474 724] contain beautiful albite-quartz seams.

The hornblende-schist with its hard adinolic inclusions (E9884) [SH 477 725], (E9925) [SH 480 728] can be obtained fresh (occasionally with 'feathery' porphyroblasts) from the roadside quarry close to Graig-fawr. But it is far better studied on the great natural bosses at and south-west of Bwlch-gwyn. Fine and well-foliated for the Most part, its contrast with the coarse gneisses across the fault is pronounced, yet it contains cores of dioritic matter a yard wide, never, however, so coarse as the gneisges. The lenticular albite-pegmatites here attain a foot in length, and pass into seams which transgress the foliation. Some of them are, as has been remarked on pp. 115, 199, slightly foliated, indicating a pause in the anamorphic process. The hard fine inclusions that may be zoisitic adinoles (p. 121) are greatly developed close to Bwlch-gwyn, where they are sometimes flaggy with fissile partings, containing also pegmatitic eyes, and a network of thin veins that do not pass outside them. Many are large enough to be shown on the .0004 maps., There is also a larger acid mass, which is nearer in character to the ordinary mica-schist.

The Centre

The tract from Newborough to the main line of railway, though deeply drift-laden, seems to be chiefly normal mica-schist. There is a curious variety (E10745) [SH 470 680] that is albitic with vermiculite aggregates. The compact felsitic rock (E9480) [SH 475 704] that resembles (E8485) [SH 474 724] but contains quartz-albite pseudomorphs of porphyritic orthoclase (p. 123) was obtained from a quarry in a field 500 yards west of Myfyrian (Myfyriant-isaf of the six-inch map) near Gaerwen Junction. The adjacent mica-schist, though rather fine, is well foliated and even folded, with quartz-augen two feet long. At and south-east of Bridin the quartz-augen (E9863) [SH 513 724], (E9864) [SH 508 730], (E9865) [SH 508 729] may conveniently be studied. On the boss overlooking the south side of Bridin farm-yards the three types of quartz (p. 111) are well seen; and some very thin trains of mica-flakes are sharply doubled on themselves inside one of the fine granoblastic bands (Figure 183). On Penmynydd are fine exposures of rather massive and granular types. The Zone was named from this place because no other member of the Mona Complex is present there. On the boss by the pitch-arrow to the west of Elusendai alluvium a small vertical hornblende-schist is folded a-clinally (Figure 56). At Bryn-eryr a hornblende-schist is finely folded (Figure 50) isoclinally.

Between Pen-y-garnedd Inn and the marginal dioritic Gneisses the mica-schist is generally normal, except (as usual) close to the basic schists; but the basic schists themselves are interesting because, though their amphibole is chiefly green, they contain glaucophane in places. The Pen-y-garnedd band is a green hornblende-epidote-schist (E9828) [SH 540 770] with albite and a pale mica a quarter of a mile north of the Inn, and so minutely corrugated as to show this in thin section; but just south-west of the Inn it is faintly blue, contains glaucophane, and shows large folds. The Careg-landeg band, 475 yards south-west of the house, is green, but contains glaucophane, some needles of which also occur in the inclusions of mica-schist (Figure 184) which it contains at the same place. Blake's slide (E10784) [SH 527 766], an albite-hornblende-epidote schist with delessite, and containing another amphibole allied to glaucophane, is evidently from the same band. Just outside the corner of the woods east by south from the 's' of 'Leaps' is a dark schist with a good black streak, which may be graphitic, but the dark mineral has not been investigated. It is the only such rock known in the Aethwy Region, and is unlikely to be the Gwna graphite-schist, as none of the associates of that rock have been found there. A venous origin is quite possible.

Mynydd Llwydiarth

Mynydd Llwydiarth (Folding-Plate 8), which in spite of its moderate height of 520 feet is extremely rugged, far more so than many greater mountains', is the best place in the Island for a study of the Penmynydd metamorphism. It rises rather abruptly at a strong craggy feature, developed along a cross-fault, the largest of a group of six that shift the main junction of the Mona Complex with the Ordovician rocks. The mountain is divided by a dislocation into two sharply contrasted parts. All its upper portions have the prevalent north-easterly strike, and are composed of acid and basic rocks of the prevalent types; but about the tarn, between them and the Gwna rocks, is wedged a long triangular tract (which may be called 'The Tarn Wedge') of compact acid schists and glaucophane-schists that strike directly at the rocks on either side (Figure 185).

The Summits — The mica-schist of alb summits (E6083) [SH 546 792]–(E6084) [SH 546 793], (E9849) [SH 546 793] is all coarsely crystalline and rich in well-developed white mica, the seams of which, usually thin, may be an inch or two in thickness. The dominant schist is of the lenticular type, but there is, especially about the eastern summit, a good deal of the more evenly-banded kind. Both contain abundant venous quartz, involved in the folding; that of the lenticular schist being in stout augen, often rod-like, that of the other schist being in tolerably regular thin seams. This venous quartz continually tends to become granoblastic at its edges, which is the beginning of incorporation. The mica-schists are nemablastic in thin seams only, they are for the most part granoblastic, the grains being less deeply sutured than usual, and are very rich in albite in good sized oval crystals often showing cleavage and but rarely twinned (E9849) [SH 546 793], with much minute epidote and a little garnet.

Twenty small hornblende-schists pass across the summit, east of which are two larger ones, the greater of the two ranging along the whole length of the mountain and giving rise to a dark line of rugged escarpmental crag. They are normal green-hornblende-epidote schists with granular albite (E11376) [SH 550 794]–(E11377) [SH 545 789], one of them (E10785) containing large clear porphyroblastic albites in which are needles of secondary actinolite and veinlets of epidote. At the margins of the large ones are zones of interfelting, in which the mica-schist assumes the compact adinolic modification, the finest section of which in Anglesey is below the little cottage on the crags about half a mile north-east of Ty'n-y-mynydd-'west' (to be distinguished from the eastern farm of the same name—see p. 364) (p. 121) called Gigfran on the six-inch maps (Figure 185) where the felting resembles a strong banded gneiss. Some of the junctions are sharp, but some of the adinolic bands are shredded through and through with hornblende-schist.

The great mica-schist of the summits is overfolded from northwest, pitching to north or north-east at varying angles, often as high as 40° to 60°, so that the outcrops of bands are very curious, but the folding lacks a smooth decisive sweep. In the hornblende-schist, when visible, the folding is more decisive, and very sharp at the dip-section in the gap north-west of the cottage on the crag (Gigfran) (Figure 51). The pitch varies, and about 100 yards north of Gigfran is vertical, so that the movement was locally horizontal (as is often the case in the Lewisian Gneiss of Scotland) and extremely violent (Figure 61).

The Tarn Wedge (Figure 185) — Along the foot of the great hornblende-schist escarpment runs a narrow hollow of green sward, beyond which we find a banded series dipping to the north-east and striking abruptly at the escarpment. At the junction with the Gwna Green-schist there is an equally abrupt change of strike; so it is evident that the triangular tract about the lake must be wedged in between a diverging pair of ruptures.

Some of the smaller basic bands of this tract are green, but the great ones are beautiful glaucophane-schist, the bluest rock known in Anglesey being found above the west end of the lake. Most of it is finely foliated, but on the strong escarpment overlooking the green hollow that comes down from Ty'n-y-mynydd-west, it contains knots of green epidosite like those at the Column, and the great phacoids of imperfectly foliated material several yards long, as well as lesser ones with dioritic texture. But the glaucophane-diorites (E11140) [SH 54 78] described on p. 117 are best seen about 90 yards north-east of the lake, on the south side of the dyke (at the '300' on the six-inch map, and marked 'Gl-Di ' on (Figure 185)), some 30 yards from the alluvial tongue. None have been found quite free from foliation. The relations of the larger cores of diorite have not been seen, but smaller ones a few inches thick are completely exposed, floating in ordinary glaucophane-schist.

The banded acid rocks between these glaucophane-schists are nearly all hard compact adinoles (E9826) [SH 546 790] rich in albite and full of minute granules of epidote. There is, however, more white mica than is usual in this type, and it is visibly a modification of the ordinary mica-schist, strips of which remain and pass into it. It forms here a beautiful synthetic gneiss with the glaucophane-schist itself, whose margin is full of its hard sharp-edged bands up to six inches or more in thickness, both rocks folding together.

The discordance of strike, though very pronounced in the bands considered as wholes, is not absolute, when the minuter structures are considered. For the rocks of the wedge, though dipping to the north-east, are, like those outside it, affected by an overfolding from north-west (Figure 186) which pitches to the South-east of Gigfran. north-east, though the folds are usually on a small scale. South of the lake, moreover, the north-west strike begins to die away, but it is probable that the wedge is here cut off by the cross-fault that bounds the south-west end of the mountain. Northwards, however, the relations of the narrowing wedge are not easy to interpret. For, as has been shown on p. 125, there is at Hafod Leucu (Figure 11), the cottage north-east of the 'h' of 'Llwydiarth', a complete passage from Gwna Green-schist into the Penmynydd Zone. The eastern rupture seems, from the features, to leave the Penmynydd boundary, passing into the Gwna rocks along their strike; and it would also appear that the western one must leave the base of the great hornblende-schist to coalesce with it. The truth is that, north of the large glaucophane-schists, the north-westerly strike dies away and all the rocks begin to strike north together, with conformity of foliation, the strong feature at the base of the great hornblende-schist dying down. It is likely that both ruptures belong to the period of the metamorphism, and are themselves planes of the foliation (not brecciating faults or mylonising, thrusts) formed at a time when the rocks were comparatively pliant, so that rupture could pass into folding,molar into molecular movement, easily and rapidly. Dynamical relations of that kind may often be seen on the small scale (Figure 44), (Figure 46), in crystalline schists. Such planes would not be perceptible as ruptures in the midst of conformable foliation.

Gneiss of Holland Arms and Gaerwen

All but a single narrow strip is basic. The best locality for study is the lofty rugged platform to the north-east of Graig-fawr. The rock varies rapidly in character, some being an almost massive, some a foliated diorite, but most of it a good hornblende-gneiss like that of the Middle Region, comprising a lighter and a darker element, combined in rude lenticular lunips a few yards long, the aspect of the whole recalling the old basic parts of the Lewisian of Scotland. Banding is rare, but well developed on the southern crags. Lenticular albite-pegmatites are frequent, bordered by thin selvages of darker gneiss than usual, and there are thin pegmatite veins, as well as one or two knots of a granite with albite-oligoclase in the middle of the platform. Epidote is common, and it is in the gneisses of this district (E8414) [SH 486 720], (E8473) [SH 480 719], (E8474) [SH 480 719], (E8475) [SH 480 719], (E8476) [SH 480 719], (E8477) [SH 480 719], (E8478) [SH 480 719] (E11379) [SH 478 726] that the remarkable saussuritisation described on p. 130 is so highly developed. The dips tend to be high, and the strike changes continually; besides which there are innumerable mylonising crushes, seldom permitting a band to range for more than a yard unshifted. Doubtless these are the late movements of the Mona Complex, but they are developed on a much greater scale in this gneiss than in the adjacent rocks of the Penmynydd Zone. The silicification described in Chapter 19 is towards the eastern end, west of the Bwlch-gwyn track, and may possibly be Pre-Cambrian. Although the adjacent hornblende-schists contain dioritic knots and lenticular pegmatites, these gneisses are far more deep-seated and variable, and the contrast is so sharp that the boundary must be a fault of considerable magnitude, probably in part at any rate of Post-Silurian age, as a Paleozoic dyke is shifted at it. On the wooded continuation of this platform, which is such a charming feature seen from Holland Arms, the hornblende-gneiss is still more basic, but is of course less well exposed. A strong feature bounds these crags towards the south-east, and near the little farm south-west of Graig-fawr, a lower tract of saussuritised and sheared hornblende-gneiss (E9886) appears between them and the Penmynydd schists.

The acid gneiss mentioned is to be seen just at the edge of the high wood, south of the 're' of 'Pentre', where a wall comes up from the little farm. It is coarse and flaky, of the 'C' type, with lenticular pegmatites, and quite granitoid in parts, an albite-biotite-muscovite-gneiss with leucoxene, and a little garnet and sillimanite (E9887) [SH 474 725], (E11382) [SH 475 724], and analysis]. Its junction with the hornblende-gneiss of the wood, though interfered with by a local. crush, conforms to the foliation-planes. Some 25 feet of it are visible, dipping vertically, at the section. Doubtless it is but a strip, cut away from extensive masses like those of the Middle Region, but in the Aethwy Region it is unique.

The tract which runs from Gaerwen towards Penmynydd is of the same general character as that of Holland Arms, and is well exposed in the places indicated by the drift-lines. Perhaps it is on the whole rather less gneissose. At Gaerwen Windmill (E8414) [SH 486 720] the saussuritisation is also strong. Some of the best sections are among the dykes east of Bwlch-gwyn, where there is good gneiss, with seams of pegmatite that show folding when no other planes do. Beyond this it is evidently brought against the mica-schists by a Post-Ordovician fault, north of which foliated diorite forms a considerable proportion of the mass. No junctions of these gneisses with the Penmynydd mica-schists are exposed.

Basic Gneiss of Rhos-cefn-hir

The long band that runs past Rhos-cefn-hir, Pentraeth, is fairly exposed from that hamlet north-eastward and in Plâs-gwyn woods. Its relations are not seen. The rock resembles those of Holland Arms, being partly a foliated diorite, partly gneissose, but is much crushed and injured. It has (E6109) [SH 523 768], (E10782) [SH 56 73], (E10783) [SH 522 782] the same types of hornblende and epidote, and the saussuritised albites.

The Llanddwyn Wedge

This, though narrow, is of the first importance, Llanddwyn Island and the dunes of Newborough being the only places where the spilitic lavas are to be found free from deformation.

Llangaffo to Newborough

The first tract of Gwna rocks, west of Llangaffo Church, exposed only in the lane-floor, is delimited chiefly on the evidence of strike and features. The rocky tract from near Tyddyn-fawd to Rhedyn-goch is typical Gwna Green-schist (E9964) [SH 418 663], and its basic rocks, now platy chlorite-schist (E9898) [SH 424 671] with purplish seams (doubtless reconstructed spilites), are rich in epidote and granular ternary albite.

The Dunes of Newborough

At the Newborough–Bodorgan road these rocks become less crystalline, but can be traced at intervals among the dunes to Llanddwyn Bay. On a little boss 280 yards north-west of Tir-forgan they are cut at right angles by some four- or five-inch (Figure 187) (E9967) [SH 411 653] pegmatoid pink veins, which are traversed by a marked foliation parallel to that of the schist. The vein-felspar is orthoclase with a little albite. It is surprising to find such veins in a rock so slightly reconstructed; but thin pegmatites are known in thrust masses of Torridon Sandstone that are even less altered. ['Geology North-West Highlands of Scotland' (Mem. Geol. Surv.), p. 576.] The western flank of the Gwna Green-schist is replaced at the Bodorgan road by a schistose mélange in which -almost every member of the group is represented, and this continues as far as the old Woollen Factory. But on the knobs by the streamlet beyond that the spilites begin to escape from the shearing, and to come on in force. This, the easterly of the two great bands of spilite, is well exposed among the dunes at the 'L' of 'Llwyd', where ellipsoids are developed, though not on the great scale of the western band. There are sills of albite-diabase (E11248) [SH 400 645] among the lavas.

Along their west side run the rocks regarded as Tyfry Beds; fine green grits with purple and green phyllites. At the 'a' of 'Mawr' is a beautiful section in folded and evenly-banded beds that appear to have been dusts with seams of broken abites (E9962) [SH 397 643]. One of them is hard, and cherty in the matrix (E10308) [SH 396 642]. They recall the compact rocks of the Dingle at Llangefni, and of the Tyfry tract (pp. 350, 355). The limestone band that follows is a perfect type of the rose-dolomites, brecciated, with fragments of deeply haematised spilite. In it, on a boss 320 yards south-west of the cross-fault and dyke, is the 20-foot jasper (E10307) [SH 394 638], (E10340) [SH 40 64], (E10359), and analysis], part of which is purplish and banded, while another band weathers porously and is crowded with rhombs of a carbonate that are zoned with dusty haematite. Between the dykes at the cross-fault the limestone, there full of purple spilite fragments, shows most remarkable spheroidal structures (Plate 6), which can be seen to be determined by thin shells of spilite. These thicken north-westward, and the margin of the great spilite just beyond has its inter-ellipsoidal spaces filled with ashy limestone. The spheroidal limestone would seem to be a pseudomorph after a spilite-agglomerate. This is by far the finest example of the structure in the Island.

The mass that follows, 200 to 300 yards in width, is the most perfect ellipsoidal spilite in Anglesey. All the characters described on pp. 71–4, save the variolitic texture and the glass, can be studied in perfection on the great bosses of Bryn-llwyd and Cerig Mawr, for, swept by the 'sand-blast of this remarkable desert-like tract (see Chapter 32) they are not only bare of all vegetation but even of a weathered crust, and are as fresh at the surface as within. The best view of the larger ellipsoids is 260 yards north of the 'M' of 'Mawr' (Plate 3) (E9895) [SH 396 645], (Plate 5), Fig. 1, and analysis, but a section just west of the 'M' is almost as good. The smaller ellipsoids are perhaps best seen just south of the M', on the face of a cliff; and the summit above affords unusual advantages for a study of the inter-ellipsoidal relations of the jasper (Plate 4) (E9957) [SH 398 649]; (E10306) [SH 395 640]. Between the dykes by the cross-fault is good hwmatisation in an unusually porphyritic variety (E11252) [SH 396 643] with stout phenocrysts of albite, some of which have turbid cores. Near the 'B' of 'Bryn-llwyd' some dark skins (E10305) [SH 398 646] now much epidotised, with quartz-spherulites, are well developed. Outside the great spilite, bedded tuffs may be seen about a quarter of a mile north of the 'G' of 'Gwddw' (E9966) [SH 392 637], and still better a quarter of a mile north-north-east of that section (E9961) [SH 403 651]. About 150 yards north-east of the Llanddwyn Causeway a rather pale jasper (E9177) [SH 392 636] contains objects that may possibly be the remains of radiolaria. Before considering the structures, a spilite may be noted that abuts upon the Berw fault and forms the dark high knobs of Cerig-duon, for though considerably deformed, it contains better felspar spherulites (E9956) [SH 398 650], (E10339) [SH 398 650] than any other. It is very fine in texture, probably in part vitreous, and has been a good deal haematised (E9896) [SH 398 649].

The ellipsoids of the great spilite of Cerig Mawr and Bryn-llwyd lie with their greatest axes along the strike while their shortest axes are at right angles to that and horizontal, Were the flattening of such resistant bodies due to compression after consolidation, that would undoubtedly manifest itself internally as textural deformation, from which they are completely free. It must therefore be original, acquired under gravity when the stream had ceased rolling and begun to settle. And as the two longer axes would in such case be horizontal, we thus obtain light upon the true dip, which must be vertical. As limestone appears on both sides, the spilite is doubtless in the core of a symmetrical fold, and the vertical dip, together with schistosity which develops on the flanks, indicate that this fold is ruptured. Whether the spilite be on an anticline or in a syncline depends upon whether the succession be or be not inverted in this wedge. But, in either case, the upper side of the lava-flow may be looked for along the flanks, as those face towards the Tyfry Beds.

The deformation on the eastern side is trifling, but on the western side is a zone of complicated shearing in which almost every member of the group is involved, brecciated, ripped into thin strips, dragged past and driven into one another, and rendered schistose (E9897) [SH 398 649]. The Gwna alternating beds appear, torn into lenticular contorted mélange and their matrix converted into green-schist (E9959) [SH 397 648], (E9960) [SH 397 648], in which condition they are thrust as a long wedge into the midst of all the other rocks, whose less resistant members have broken down into a dull purplish and green schist that forms a sort of second matrix to all, even to the Tyfry Beds. As for the phyllite-and-grit mélange, no finer sections are known, even on the Bodorgan Headlands, than some sand-swept knobs to the south-west of the dark high lava-crags of Cerig-duon, where it has begun to corrugate. On the flank of the high dark crag it is seen in contact with the spilites, which are schistose at the junction. Every rock appears as definite fragments in the breccias except the Gwna Green-schist, itself therefore evidently a product of the process. (Figure 188), reproduced from the .0004 maps, gives a little idea of a part of the zone as it is exposed upon a long low boss between Cerig-duon and Bryn-llwyd.

Llanddwyn Island

This romantic isle is the type locality for the volcanic series of the Gwna Beds, and an ideal spot for the study of that varied and interesting group. The geological variety and complexity of the island are indeed remarkable, as may, be seen from (Folding-Plate 15)<ref>The Llanddwyn place-names now used will be found on (Folding-Plate 15).</ref>, which is a six-inch reduction from the .0004 maps; and this geology is laid bare on some three miles of coast and on the rocky knolls, which rise at the Lighthouse crag to a height of about 60 feet. At most states of the tide it is easy of access along the sandy isthmus, and even at high-water along the causeway, save at spring-tide flood with a heavy westerly gale.

The Halen Spilite and the Tuffs — The great Bryn-llwyd spilite passes on to the north-eastern shoulder of Llanddwyn, where its original characters are as beautifully preserved as among the dunes of Newborough, the augite, as there, surviving in some of the cores of the ellipsoids. A few porphyritic felspars are present (E10085) [SH 391 630]–(E10086) [SH 391 630]. The ellipsoids are of the smaller kind, often globular. Their concentric banding and dark skins with spherulites (E10087) [SH 390 630], broken up sometimes into fluxion-breccia, can be studied anywhere on the fine sections about Porth-yr-halen. There also the inter-ellipsoidal jaspers (E10093) [SH 391 630], (E10094) [SH 391 630], (E10095) [SH 391 630], (E10097), often spherulitic, may be seen to great advantage. In the north-east part of the cove is the jasper with large radial spherulites that simulate organisms (pp. 85, 151), and on the top of the boss above is a mass 24 by 4 feet. In the same parts of the cove, limestone fills the spaces, and here may be found the cases where it encloses a core of jasper. On the north shore some 10 yards east of the path-end, is the six-inch vein of axinite (E11389) [SH 391 633] (p. 74).<ref>On p. 75 it is stated (see also Sum. Progr. Geol. Surv. for 1914, Mem. Geol. Surv. 1915, p. 53) that axinite had not been recorded in Wales until its discovery on Llanddwyn. Since that was written it has been recorded by Mr. H. C. Sargent in the Penmaenmawr intrusion (Geol. Mag. 1916, p. 5). Unfortunately that issue of the Geol. Mag. did not reach me until after Chapter 4 of this book had been passed for Press, so the reference could not be inserted there, and has therefore been placed in this chapter.</ref> It is particularly requested that the radial spherulites, the jasper-cores in limestone, and the axinite vein, be not hammered, or if so, with great care, as they would be easily destroyed. A few zones of brecciation traverse even the Halen spilite, and when followed southwards to the Beacon Tower, it becomes schistose, as it does all along its western margin.

From the Breakwater to Ffynnon-y-sais are spilite-tuffs (E10091) [SH 390 634], (E10092) [SH 389 632], (E10358) [SH 388 630], the best that are known in Anglesey. Though coarse and agglomeratic, with ellipsoid 'bombs', they are clearly bedded, with some thin purple phyllite and siliceous seams, as well as one or two flows of spilite only a foot or two in thickness. They should be studied at the cliff's foot near the north end, at low water. They are folded, with a north-north-east pitch, and in them, 20 yards west of the Breakwater, is the ashy rose-green limestone (p. 84) with blocks of spilite. It is bedded with them, and lies between them and what seems to be a six-inch lava. On the map, it is possible that some true spilite has been included with these tuffs, for along the junction the lava breaks down into a schistose breccia that simulates them.

The Tyfry Beds — Brought against the agglomerates west of B. M. 11.2 by a vertical mylonising slide are the Tyfry Beds, which are (E10116) [SH 389 633], (E10117) [SH 388 632], (E10118) [SH 388 632], (E10119) [SH 388 632] hard, banded, gritty slates, with beds of massive grit up to three feet in thickness. The grits are full of broken alhites (that in some cases have clear margins) and fragments of spilites and keratophyres, with some of the characteristic tourmaline-mica-schist. On the bedding-planes, about 200 yards from the Breakwater, are the bodies (p. 151) regarded as annelid castings. The bedding, which is better preserved here than anywhere else, is vertical, but the cleavage crosses it at a narrow angle, and seems to be identical with the foliation of the Gwna Beds. The Tyfry Beds appear also at Porth-y-clochydd, where, on the little point between the two coves, they are sheared in with limestone, and themselves pass into a schistose mélange indistinguishable from that of the Gwna Beds. In the east cove is jaspery phyllite interbedded with fine purple albite-grit (E10113) [SH 388 628]–(E10114) [SH 388 628], and similar purple phyllite (E10115) [SH 387 626] appears in the midst of Tyfry Beds at the Pilots' Houses.

The Western Cliffs and the Abbey — The western coast is chiefly spilitic lava, but being on the strike of the great zone of deformation already seen in the dunes of Newborough, none of it has escaped, and large parts are now a rough chloritic-schist (E10088) [SH 385 625], (E10089) [SH 385 625], (E10090) [SH 387 628], the origin of which, however, is never doubtful, as it contains everywhere the remains of ellipsoids and small strips of jasper. In it also, on Trwyn Ffynnon-y-sail, Porth-yr-ogof, and the Lighthouse Crag, are strips of Gwna mélange full of lenticular grits, whose matrix (E10120) [SH 386 629] is a true Gwna Green-schist, with secondary sericite. The long band coloured as such that runs past the little ruined Abbey from creek to creek is largely conjectural, being exposed only just below the Cross. Much of the wedge in Porth-y-clochydd is also concealed, these two being really the only obscure parts of the isle. At the neck of the Porth-yr-ogof headland thin purple phyllites appear in the spilite-schist, and increase rapidly, so that the .0004 map is but a rude abstract. On the headland are several bands of hard flinty red beds, and similar bed's, with pale green ones, folded and pitching to the north-east, are even better developed on the two islets west of the Abbey ruin. They simulate bedded jaspers, but (E10107) [SH 386 629], (E10108) [SH 386 629], (E10109) [SH 386 629], (E10110) [SH 386 629], (E10111) [SH 386 629], (E10112) [SH 386 628] are far too gritty for that, and must be really purple gritty beds like the finer ones of Porth-y-Clochydd, baked by the two sills of albite-epidiorite (E10106) [SH 386 630], which adjoin them on the headland. The sill on the point is very massive. Transgressive junctions have survived, at which the colour of the purple sediments has been discharged by reduction of ferric to ferrous oxides, but adinoles have not been found.

The Limestones — For a study of the peculiar Gwna limestones there is no place comparable with Llanddwyn, 34 masses being known, and the sea-washed sections at the southern end are exceptionally clear. One or two (E10102) [SH 385 625] are pale grey calcite rocks, but the majority are (E10099), (E10100) [SH 387 624], (E10101) [SH 387 624], (E10103) [SH 386 623], (E10101) [SH 387 624], (E10103) [SH 386 623] rose-coloured rhodochrosite-dolomites, the one that was analysed (E10100) [SH 387 624] from the west end of the Pilot's Cove breakwater being unusually beautiful. Nearly all these rose-dolomites are rich in jasper (E10096) [SH 386 624]. One of them at Porth-y-clochydd, otherwise massive, shows a few feet of bedding, with bands of jasper and volcanic matter. Many of them contain^— fragments of haematised spilite, some to such an extent as to be spilitic tuffs with dolomitic cement rather than ashy limestones. The phenomenon is exceptionally well seen at the Beacon Tower, where the spilitic fragments contain steam-cavities and sub-radiate groups of thin lath-felspars (E10103) [SH 386 623]–(E10104) [SH 386 623] (Plate 9), Fig. 1. A sheared ashy limestone of this kind near (E10100) [SH 387 624] contains blocks of rose-dolomite like (E10100) [SH 387 624] itself. Allied to these rocks, but not calcareous, is a one-inch bed (E10105) [SH 385 623] seen on the reefs at the west end of Porth Twr-bach, of purple rolled- variolite-tuff, important from containing a few fragments of albite-granites like those of the boulders of The Skerries.

Such are the leading features of the remarkable geology of Llanddwyn.

Carnarvon

On the western side of the river's mouth at Carnarvon, just beyond the Toll Bridge, some greenish-grey schists with little corrugations and nemablastic quartz (E9685), (E9686), (E9687) appear along the foreshore. They have been powerfully sheared, and have developed much minute sericitic mica and iron-ores. They also show signs of thermal action, some of them being full of small spots. Apart from this, they recall the aspect of the Gwna Beds when in a low state of anamorphism, but are all fine rocks, no grits having been seen. Their relations to the adjacent shales of the zone of Didymograptus extensus are not visible, but those beds are totally unaffected by the powerful shearing in which these are involved, and contain large clastic micas. It is also worthy of note that some grey phyllites which resemble these are (see p. 250) not uncommon as pebbles in the Cambrian conglomerates of Llanberis. It is therefore probable that the phyllites of Carnarvon belong to the Mona Complex.

The order of the minor sub-divisions of the Bedded Succession

In the footnote to page 169, the reader was referred to this chapter for detailed evidence in support of the relative positions assigned to the minor sub-divisions of the Complex in the Table on page 164. A brief summary will be given here, with references to the pages on which the local detail will be found. The probable order of the sub-divisions of the Gwna Group (which are not stratigraphically arranged in the Table) will then be discussed, and finally, the horizons of development of the Penmynydd metamorphism.

Sub-divisions of the South Stack Series

It has been shown on pp. 156, 257–8 that, at the South Stack Moor, the Holyhead Quartzite is immediately underlain by the Stack Moor beds, and that these are underlain in their turn by the Llwyn beds. The same order is found (pp. 260–1) in the tract west of the North Stack fault at Rhoscolyn. On the Rhos-colyn anticline (p. 262) it is the Llwyn division that adjoins the New Harbour Beds. Between the two faults near Holyhead (pp. 264–7) the Llwyn beds follow the New Harbour Group, and are succeeded by. the Stack Moor beds, upon which rests the Quartzite of Holyhead Mountain. In the Breakwater tract (pp. 270–1) there is a perfectly gradual passage between the Quartzite and the Stack Moor beds, and though the passage between these and the Llwyn beds is cut out by thrusting, the latter appear in the same order as elsewhere. In the Northern Region, the Holyhead Quartzite is not seen, but as the Coeden beds adjoin the Amlwch Beds, and are flaggy throughout, they are correlated with the Llwyn division only, and the Stack Moor division is regarded as cut out by the Carmel Head thrust-plane.

Sub-divisions of the New Harbour Group

At the Rhoscolyn Lifeboat Station and islets (p. 261), on the Rhoscolyn anticline (p. 262), and throughout the Holyhead country between the faults (pp. 264–7) it is always the Celyn division which, with its little basic tuff, adjoins the Llwyn division of the South Stack Series. The same is the case (though the junction seems to be ruptured) at the Breakwater (pp. 268–70), where, also, the Soldier's Point beds appear on the side away from the South Stack Series. At the Tre-Arddur gap the Soldier's Point beds adjoin and graduate into the Celyn beds on the side that is remote from the South Stack Series (p. 263). In the Western Region it is the Soldier's Point member of the group that, escaping from the Bodfardden thrust-plane, adjoins and graduates into the Church Bay Tuffs at the coast (pp. 157, 278–9, 286), at Brwynog and at Llanddeusant (pp. 281, 286). The same is the case (p. 287) at the Garn Inlier. At the Corwas Inlier (p. 294), though the Church Bay Tuffs appear to be cut out, it is the Soldier's Point beds that come against the thrust, and adjoin the (locally tuff-like) Gwna Beds. Passing to the Northern Region, it is (pp. 297, 303) the Bodelwyn division of the Amlwch Beds that adjoins and graduates into the Coeden representative of the South Stack Series; but (at Llanfechell, Llanrhwydrys, and Bull Bay) (pp. 300, 315–17) it is invariably the Lynas division that adjoins and graduates into the Skerries Grits. The Soldier's Point or Lynas division is also, in both regions, the one which contains the spilitic lavas, bedded jaspers, and jaspery phyllites, for the hmmatitic schists in the Celyn beds are mere films. The lavas appear to be on an horizon or horizons not far above the Skerries Group.

Sub-divisions of the Skerries Group

It is always the Skerries Grits that are found adjacent to the Amlwch Beds (pp. 158–9, 300, 315–17), and in Bull Bay (pp. 215, 315) they are seen to lie between the latter and the Church Bay Tuffs, graduating by alternations into both of them. The Church Bay Tuffs everywhere adjoin the Gwna Beds. The Tyfry Beds, adjoining the Gwna Beds, may be supposed to represent only the Tuffs; but it must not be forgotten that, on the Middle Mouse, a grit somewhat of Tyfry type graduates into the Lynas beds, which is the position of the not-far-distant Skerries Grits.

There is therefore conclusive evidence for placing the sub-divisions of the South Stack Series, the New Harbour Group, and the Skerries Group, in the order in which they are set in the Table on page 164.

Sub-divisions of the Gwna Group

Lithologically well-defined though these be, the determination of their true order is a perplexing problem, owing to the excessive disruption of the group, which has produced certain discrepancies between the apparent sequences in the different regions, sometimes even within one and the same region. Little is to be gained, therefore, by setting out these local variations, but the reader Can roughly verify them for himself from the one-inch map. There is little doubt that they are mainly due to the more resistant members of the group, particularly the quartzite, having been repeatedly driven through (as well as over) the other ones. Nevertheless, by combining the shattered sequences found in the Western Region, the Garn Inlier, the Fydlyn Inlier, the Northern Region, the western Middle Region (i.e. in the Penmynydd Zone), Mynydd Bodaf on (much less shattered, but including little more than the quartzite), the eastern Middle Region, the Pentraeth Inliers, eastern Aethwy, and Llanddwyn; probability emerges for the following suggestion:

It will also be seen that, as the beds in the upper parts of these columns are much thinner than those in the lower parts, the Llanddwyn spilite and limestone must be comparatively near to the base of the Church Bay Tuffs and Tyfry Beds.

Horizons of development of the Penmynydd metamorphism

Reasons have been given on pp. 122–8, 155, and 161 for believing the holocrystalline schists of the Penmynydd Zone to represent in the main the felsitic Fydlyn Group, with large portions also of the Gwna Group. Examination of the details given in the present chapter, however, will show that the metamorphism begins to develop on five or six different stratigraphic horizons.

These are very distinct in the Middle Region. Where the main boundary runs out on the Aberffraw coast (pp. 124, 340) the marginal Penmynydd type is a highly micaceous flaser-schist, but on the main line of railway (p. 343) it is a flaggy type, which cannot be on the same horizon. The rapidity of the passage on the coast indicates that (see p. 127) the crystallisation is developing just at the Fydlyn-Gwna junction. On the railway it is probably somewhat within the Gwna Beds. At six or seven of the inliers (p. 345) it undoubtedly develops within a few yards of the quartzite-limestone-graphitic-schist group, and is evidently keeping rather steadily to that horizon over a considerable area. Yet at Porth China, Trecastell, Bodwrog, and other places (pp. 341–4), the same stratigraphical horizon is found in the heart of the crystalline tract. On the other side of the zone, the Penmynydd metamorphism touches (p. 342), but hardly seems to invade, hornfels that is considered to represent some of the Church Bay Tuffs. At several places, indeed (pp. 127, 342), it has invaded the margin of the Coedana granite, and must therefore have eaten its way through the hornfels aureole. But at those places the granite may be in contact, not with the Skerries but with the Gwna Group.

At the western Pentraeth Inlier, the Peninynydd type seems to be well below the Llanddwyn spilites, and as the Engan spilites do not appear to be reached, the horizon of development must be above them.

In the Aethwy Region, the stratigraphic horizon of development<ref>It will be remembered (see p. 227) that the tectonic horizon is supposed to be somewhat higher than in the Middle Region.</ref>, though still variable, seems to be rather more constant, never passing much above or below that of the glaucophane-schists, which, there is some reason to think (see p. 120), represent the Engan spilites. Where the main boundary emerges from the Menai Strait at Llandysilio, the Penmynydd metamorphism sets in some way above the great glaucophane-schist, but soon descends to it, crosses it obliquely, and then passes below it for a mile and a half. In the Tarn wedge on Mynydd Llwydiarth ((Figure 185) and pp. 375–6), it is again a little above the great glaucophane-schists. At Hafod-leucu, however ((Figure 11), p. 125), bending to the west of the apex of the wedge, the margin of the Penmynydd Zone must again descend into rocks beneath all the glaucophane-schists. From the rapidity of the passage (see p. 127) it is believed to coincide with the Fydlyn-Gwna junction. At the Penmynydd inliers of Trefor, Wern, Llaniestyn, and Garth Ferry, the change is developing just at the top of the glaucophane-schists.

Thus the Penmynydd Zone, though composed for the most part of the felsitic Fydlyn Beds, is often permitted to rise into the Gwna Beds, and in one place even as far as the Church Bay Tuffs. Its effects upon the Gneisses are, unfortunately, inaccessible to us. It will be noticed that it rises to higher stratigraphic horizons in the Middle Region than in Aethwy. This is due to tectonic horizon, but has been facilitated by the great attenuation of the Gwna Beds in their western facies.

We have called it a 'Zone' of metamorphism; but perhaps a better metaphor would have been that of a 'Cloud', that being a condition of things that may spread here to higher, there to lower, strata of an atmosphere.