Richey, J.E. and Thomas, H.H. 1930. The geology of Ardnamurchan, north-west Mull and Coll. Memoir for geological sheet 51, part 52 (Scotland). Edinburgh: [HMSO for the Geological Survey] Ardnamurchan Central Complex 1:25,000 geological map. British Geological Survey, 2009.
Chapter 11 Tertiary volcanic vents, Centres 1 and 2, Ardnamurchan
Extensive volcanic vents, mainly filled with agglomerate, are met with in the eastern part of the Ardnamurchan igneous complex. They pierce Tertiary basalt lavas, Mesozoic sediments and Highland schists, but are traversed by cone-sheets belonging to Centres 1 and 2, and are therefore of comparatively early date. Though they are demonstrably later than only an inconsiderable thickness of basalt lavas with which they are actually in contact, there is little room for doubt that they belong to a later period than the widespread Plateau Group of the adjoining parts of Mull and Morven. In the first place, the vent-materials of volcanic origin differ markedly in composition from these basic lavas. The agglomerates are largely composed of fragments of trachyte and more acid rocks, together with blocks of big-felspar basalt, while pitchstone lavas are associated with the vents of the Ben Hiant district. Further, no intercalation of such materials is known from the lava-succession of Morven and Northern Mull. It is true that in the upper part of the Plateau Group in Central Mull intercalated flows of big-felspar basalt have been noted. But the long duration of the pre-cone-sheet vent period in Ardnamurchan, during which no typical plateau basalt is known to have been erupted, seems to indicate its distinctness from the period of the plateau lavas. On Ben Hiant, for example, two successive vents have been mapped in which agglomerates are interleaved with recurring beds of tuff, and also, in the case of the more southerly vent, with a few flows of pitchstone lava. The infilling of these two enormous cavities must therefore have extended over a considerable time.
The vents earlier than cone-sheets may be subdivided into two groups, the Ben Hiant Vents and the Northern Vents (see
The explosive character of acid magma has been well exemplified in the repeated volcanic outbreaks of Central Mull,<ref>E. B. Bailey in Tertiary Mull Memoir, 1924, pp. 202, 203.</ref> and is also manifested in all the Ardnamurchan vents. It may be noted that at a later period certain of the basic ring-dykes of Ardnamurchan were completely or partially brecciated and then granulitized or recemented by granophyre, and that the brecciation is to be attributed to explosive gases derived from the granophyre magma.
Vent-walls are clearly exposed in the case of the Ben Hiant Vents. On the other hand, country rocks bounding the agglomerates of the Northern Vents are often much brecciated, and the exact position of the vent-walls is in such places difficult to determine. Marginal brecciation is most in evidence at interior contacts of the vent-agglomerate with basalt lavas, and it is clear that the shattered basalt lavas are earlier than the agglomerates. It is likely that these interior masses of basalt lava have slipped down into the vents. Brecciated country rocks of all kinds also form a large proportion of the materials of the agglomerates. In the Ben Hiant agglomerates, on the other hand, fragments of country rocks are little in evidence, and basalt lava only has been noted.
It will be seen on
The Ben Hiant Vents lie to the south-east of the outer bounding wall of the Northern Vents, but there is reason for connecting both vent-complexes with the same igneous centre. Both belong to the same period in the igneous cycle, and in both the contained volcanic materials are similar. Further, the two Ben Hiant Vents lie along a north-east line as if referable to Centre 1.
Ben Hiant vents (earlier than cone-sheets)
Ben Hiant, the highest peak in western Ardnamurchan, has long been known to geologists from the writings of Prof. J. W. Judd and Sir Archibald Geikie. Judd's publications date from 1874 to 1890,<ref>J. W. Judd, The Secondary Rocks of Scotland. Second Paper. On the Ancient Volcanoes of the Highlands and the Relations of their Products to the Mesozoic Strata, Quart. Journ. Geol. Soc., vol. xxx., 1874, pp. 261–264; and The Propylites of the Western Isles of Scotland, Quart. Journ. Geol. Soc., vol. xlvi., 1890, pp. 373–380.</ref> Geikie's results are given most fully in his 'Ancient Volcanoes of Great Britain'.<ref>Sir A. Geikie, The Ancient Volcanoes of Great Britain, vol. ii., 1897, pp. 278–280, and pp. 318–322.</ref> Striking features of this hill are its magnificent cliff-sections of agglomerate and a great central mass of dolerite (the Ben Hiant Intrusion). With regard to these two rock-bodies, Judd and Geikie arrived at divergent conclusions. Judd interpreted the central dolerite mass, in part at least, as a pile of lava-flows, and the agglomerates as underlying them and resting transgressively across the denuded edges of the plateau basalts and older stratified rocks. Geikie rightly contended that the dolerite was intrusive, and pointed out that the outcrops of agglomerate were in some cases clearly seen to lie within vent-walls. His conclusions are in the main confirmed by the recent detailed survey. No more than two large vents can, however, be determined, whereas Geikie has indicated in his illustrations a number of small vents. The outcrops of agglomerate which he investigated are separated from one another by the later, intrusive dolerite, and they probably all form part of one large vent, here termed the Southwest Vent. The second vent indicated by the recent mapping lies north-east of, and in contact with, the South-west Vent, and is called the North-east Vent.
With regard to the dolerite, although its scarp-featuring suggests that it is built up of sheets, as both Geikie and Judd concluded, a close examination fails to locate any sheet-junctions where these are to be expected. The featuring is here considered to be due to weathering along various planes of weakness (see pp. 160–1). Judd's misinterpretation of the dolerite as a pile of lavas appears to be mainly due to his having overstressed certain petrographical evidence. He had knowledge of a glassy rock (pitchstone) on Ben Hiant, which he has included with the dolerite on his map.<ref>J.W. Judd op. cit.1890, fig. 1, p. 374.</ref> As we now know, the pitchstone is associated with the agglomerates, not with the dolerite. It occurs in sheets, which are interpreted as lava-flows within the South-west Vent. Judd also considered the pitchstone to be lava, comparing it with the pitchstone lava of the Sgùrr of Eigg,<ref>J.W. Judd op. cit. p.376</ref> but thought that he recognized a significant petrographical resemblance between it and the dolerite, because the latter usually has a sub-vitreous (variolitic) ground-mass. Nevertheless, though the essentially intrusive nature of the Ben Hiant Dolerite must be maintained, there is evidence that the mass may have consolidated under little cover (see p. 168).
As already mentioned, the agglomerates of Ben Hiant are inter-stratified with beds of ash and the pitchstone lavas. In the Southwest Vent they are contained within high vent-walls. Further, fragments of country rock are, with the exception of one mass, only found in the agglomerates in the immediate vicinity of a vent-wall. It is therefore concluded that this vent possessed an open crater, the walls of which cannot have been less than 1100 ft. high, the greatest height above sea-level at which the bedded vent-materials are now on view.
Walls of North-east Vent
The north-eastern agglomerates, which for reasons stated below are concluded to lie within an older vent, have well-exposed contacts with vent-walls of basalt lava. The contacts are seen alongside two streams on the steep eastern face of Ben Hiant, overlooking the Kilchoan-Glenborrodale road, and are inclined downwards towards the interior of the vent at angles of 35 and 50 degrees respectively. In the more southerly stream it is of interest that a composite dyke cutting the lavas is truncated by the vent-margin
To the north, the agglomerates are bounded by basalt lavas that are exposed in a prominent cliff overlooking lower ground floored by the Moine Schists (
Walls of South-west Vent
It will be seen in
East of Maclean's Nose, close to high-water mark, the agglomerates are in contact with reddish felspathic Moine Schists. Land-slipping has occurred in this vicinity, but there is evidence that the rock-mass showing this contact is undisturbed. It is traversed by a north-west dyke and by a north and south line of crush, both of which are in line with a corresponding dyke and line of crush in the adjoining cliffs of agglomerate. It seems unlikely that land-slipping could have taken place without altering the orientation of the dyke and crush-line. The vertical junction of agglomerate with the basalt lavas is seen at the top of grassy scree-covered slopes, in an angle of the cliffs where a projecting buttress of flatly bedded basalt lavas ends against still higher cliffs of agglomerate. The junction is sharp and clean-cut, and the reddish-brown spheroidally-weathering lavas bordering it are easily distinguished from the agglomerate in contact, which is fine-textured and greyish in colour. Farther north-east the edge of the vent is again exposed in two stream-sections. In the sides of a steep gully formed by the first stream, agglomerate and flatly bedded tuff abut against a steep wall of basalt lava. The actual junction, however, is disturbed, as though the vent-materials had slipped down at their contact with the vent-wall. In the second stream, beyond a fault against which the lavas terminate, a small mass of intrusive dolerite of Ben Hiant type is interposed between agglomerate and Moine Schists.
North-east of the last-mentioned stream the vent-margin bends somewhat abruptly, runs uphill to the north-west, and forms a north-eastern boundary to the South-west Vent. Along this margin a vertical junction of fine-grained agglomerate with basalt lava is exposed just south-west of where another stream has cut a channel in the lava-escarpment. It will be seen from
Since the Ben Hiant Intrusion separates the outcrops of vent-agglomerate on the north side of the hill, both from each other, and from the large outcrop to the south-west the walls of which have just been described, it cannot be definitely decided whether all these agglomerates lie within one vent or in several. The disposition of the outcrops would seem to suggest that they all belong to one vent (see Memoir-map).
No actual contact is seen between agglomerate and wall on the north side. In the steep banks of a stream south-east of Beinn na h'Urchrach, agglomerate outcrops a few yards from a bounding wall of Lower Lias limestone, and marmorization of the limestone was noted by Sir A. Geikie.<ref>Sir A. Geikie, The Ancient Volcanoes of Great Britain, vol. ii., 1897. p. 280, footnote.</ref>
Materials infilling the Ben Hiant Vents
Agglomerates
The agglomerates consist of angular fragments varying from a foot or more in length down to small particles, together with bombs of large size. These constituents seldom show any signs of bedding, though their stratified nature is evident since the agglomerates are interleaved with beds of tuff. Angular blocks of trachyte are perhaps most plentiful, but others of rhyolite and dacite are also usually present. All these rock-types may be found at any level in the 1000 ft. of agglomerate exposed in the South-west Vent, though it is often noticeable that at any particular place one type greatly preponderates over the others.
Vesicular structure is especially well developed in the darker, trachytic types. The vesicles do not show any concentric arrangement, and whether the fragments are shattered portions of bombs, or of lava-flows within the crater, is not certain. The vesicles are unfilled by amygdale-minerals, and the fragments would appear to have been broken off from their parent masses soon after solidification and before such minerals of late formation had time to crystallize. In contrast to the relatively small fragments of the more acid rocks, there are large sub-rounded blocks of big-felspar basalt
Fragments of country rocks are absent or at any rate very rare in the Ben Hiant vents, except in the immediate vicinity of a vent-wall. For example, fragments of amygdaloidal basalt lava are abundant in the North-east Vent on its eastern side next to the inclined wall formed of these lavas. Again, at Maclean's Nose, amygdaloidal basalt fragments are restricted to the proximity of the vent-wall, here composed of Moine Schists. Since the basalt lavas occur in place at a height of some hundreds of feet above the latter locality, these fragments must have fallen down thus far at least into the vent. Blocks of schist have nowhere been noted, though angular quartz, which is plentiful in some of the tuff-beds, may have been derived from the comminution of schists by explosion (p. 136).
An exception to the above rule is an elongate mass of brecciated olivine-basalt (S2107)
Bedded tuff
The various larger fragments in the agglomerates are, as a rule, bound together by others of smaller dimensions. Sometimes they are set in a matrix of fine ash or tuff. The deposition of the tuff often continued after the showers of larger fragments had ceased, so forming a distinct bed of tuff. Sometimes the upper surface of such a bed is well defined, with agglomerate abruptly overlying it
Tuff-beds are well seen along the cliffs south-east of Maclean's Nose, where certain of them are indicated on
The tuffs are often excessively fine grained. Microscopic examination shows that finely divided angular quartz is an ingredient (S22293)
Pitchstone lavas
Three separate outcrops of pitchstone occur within the South-west Vent, south-east of the Ben Hiant Intrusion (see
On account of their internal structure the pitchstone sheets have been concluded to be lava-flows. The evidence is best seen in the lowest outcrop east of Stallachan Dubha, which is crossed by a stream-gully affording a practically complete cross-section. The sheet consists structurally of lower, middle, and upper portions, of which the lower and middle portions display perfect columnar jointing. In the lower portion the hexagonal columns are arranged at right angles to the lower surface of the sheet, which rests upon slightly hardened tuff. In the middle portion the columns change abruptly in their direction and are arranged more or less parallel to the base of the sheet. These are of much smaller diameter than the upright columns below. Towards the top of the middle portion, small, oval-shaped amygdales are found which are lined with bluish chalcedony and centred with drusy quartz. In one place there are giant amygdales a foot in length, in which calcite is developed within the drusy quartz and chalcedony. At a higher level still, the pitchstone is unjointed and amygdales occur in increasing numbers till finally, towards the top of the sheet, they become very small and irregular in form, so that the rock may be termed a pitchstone-slag. The actual top is unfortunately not seen, for a gap of a foot or so separates exposures of the pitchstone-slag, above the left bank of the stream, from overlying agglomerate and spheroidally-weathering tuff. The three-fold structural sub-division of the sheet, however, seems sufficient evidence that we are dealing with a lava-flow and not an intrusion. The asymmetry negatives an intrusive origin, while the three-tier structural arrangement recalls that of the columnar basalt lava of Staffa.<ref>G. A. Burnett in The Geology of Staffa, Iona, and Western Mull, Mem. Geol. Surv., 1925, p. 65.</ref>This structural arrangement cannot be in the nature of an accident, for it is seen to be repeated in the upper of the two pitchstone sheets of the uppermost outcrop. In this case, lower and middle tiers of columnar pitchstone, exactly similar to those just described, are capped by amygdaloidal unjointed pitchstone, as is well seen at one point on the south-west side of the outcrop just east of a stream
The columnar portions of the pitchstone sheets are often intruded by thin sills of stony pitchstone (S21459)
Unlike the intrusive sheets of sheath-and-core pitchstone in Mull,<ref>E. M. Anderson and E. G. Radley, The Pitchstones of Mull and their Genesis, Quart. Journ. Geol. Soc., vol. lxxi., 1915, pp. 205–217.</ref> stoniness has not been developed along the joint-planes of the pitchstone lavas of Ben Hiant. This seems all the more remarkable, since the two rocks are practically identical in chemical composition (see p. 84). The reason for this difference we do not know. Stoniness, however, is developed as a secondary character in the Ben Hiant pitchstones where they come into contact with intrusive masses. For example the uppermost pitchstone becomes stony next to the great Ben Hiant Intrusion, a quarter of a mile south of the summit (S21466)
Northern vents (earlier than cone-sheets)
An irregular outcrop of vent-agglomerate and tuff occupies about half the countryside between Ben Hiant and the north coast at Faskadale.<ref>The Faskadale exposures and their abundant content of acid fragments have been described: Sir A. Geikie, Ancient Volcanoes of Great Britain, vol. ii., 1897, p. 278.</ref> As already mentioned, it has a roughly arcuate form, as if grouped about Centre 1 of
The agglomerate of this northern series of vents, besides containing abundant debris of basalt lavas, is rich in fragments of Mesozoic sediments and of gneiss or schist. It also includes numerous small blocks of acid rocks that are often porphyritic. These acid fragments cannot be attributed to the breaking up of any exposed country rock, and their presence suggests that the vent-explosions were connected with acid magma. E.B.B., J.E.R.
The following localities may be given for masses of Mesozoic sediments of unusual dimensions:
- At a point 400 yds. north-east of the mouth of the Achateny Water there is a block of false-bedded calcareous sandstone with obscure fossils, which measures 100 ft. in both directions. The bedding is nearly vertical. Similar sandstone is not known in the part of the Mesozoic sequence that remains intact east of the vent, but it is fairly abundantly represented by smaller blocks (up to 15 by 20 ft.) in the neighbouring agglomerate. The sandstone may be of Inferior Oolite age.
- A strip of brecciated black shale occupies the foreshore 500 yds. south-east of Ardtoe Island, north of Kilmory. Again, rock of this type is unknown to the east outside the vent. (There is some uncertainty as to whether the fossils listed on pp. 40–41 came from this mass or from another farther east).
- There is a good exposure of sandstone in a stream close to Kilmory school, and several others of sandstone and black shale within 500 yds. of this locality, especially towards the north-west and south-east. Individually these exposures give an impression of Mesozoic outcrops in situ, but thorough examination of the evidence leaves no doubt that the sediments occur as masses involved in vent-agglomerate.
- The Loch Mudle Fault, 1000 yds. east-north-east of Braehouse, runs along a little stream, and is drawn on the map as separating basalt lava on the east from vent-agglomerate on the west. An exposure shows 30 ft. of Lower Lias limestone dipping steeply away from a substantial outcrop of gneiss and towards the lavas. Both the limestone and the gneiss are probably enveloped in agglomerate, which, close at hand, contains small blocks of like materials.
There are only three exposed contacts of the northern vent-agglomerate with its outer wall. Where the edge of the vent reaches the coast, half a mile north-east of Kilmory, agglomerate is in contact with Lower Lias limestone (Broadford Beds) and the limestone is brecciated. A quarter of a mile inland a little roadside quarry shows agglomerate in contact with Lower Lias shales (Pabba Beds). The agglomerate overlies the shales, transgresses their bedding at a low angle, and is largely composed of their debris. Seven hundred yards farther south-east, Lower Lias limestone is seen with a vertical junction against agglomerate, and the agglomerate contains fragments of the limestone. The absence of extensive brecciation of the outer wall is very marked in all three localities. E.B.B.
As evidence of recurrent explosions within the complicated Northern Vent or Vent-complex, one may cite certain tuff-filled fissures traversing tuff and agglomerate. A good example occurs in the left bank of the Allt na Mi-Chomhdhail, 550 yds. upstream from the main road, and 1300 yds. due north of Camphouse. Another tuff vein, traversing basalt lava, is exposed in the floor of a roadside quarry 960 yds. south of Braehouse, and can be traced for 25 yds.
The vent-agglomerates, though later than the basalt lavas, seem to be earlier than all the intrusions with which they are visibly associated. They are cut by early major intrusions, cone-sheets, ring-dykes, and dykes. Exposures west of Faskadale Bay afford admirable illustrations of this relationship (see
On the other hand, the agglomerates are later than some quartz-dolerite and leidleite intrusions (see p. 137). For example, they contain blocks of quartz-dolerite with acid veinings on the west shore of Faskadale Bay. Such inclusions afford an interesting demonstration of the occurrence of intrusions earlier than any seen in situ at the present level of denudation.
Glas Eilean Vent (later than Outer Cone-Sheets of Centre 2)
A narrow strip of volcanic breccias has been mapped for two thirds of a mile north-eastwards from the south-east extremity of Kilchoan Bay. The breccias are best exposed on the promontory of Glas Eilean, which is separated from the mainland as an island at high tide, and along the shore to the north-east
A post-vent fault forms the north-west boundary of the vent on Glas Eilean, and is presumably responsible for the down-thrown lavas on this side. The fault-plane between vent-agglomerate and brecciated basalt lava belonging to the vent-wall is here vertical. The lavas are seen to dip steeply away from the fault, as indicated on
In contrast to the well-defined junctions between country rocks and vent-materials where in faulted contact, no exact line of division exists elsewhere. It is a matter of opinion where to draw a line between brecciated country rocks belonging to the vent-wall and similar materials that form the greater part of the contents of the vent. There can, however, be no doubt that the brecciated country rocks mapped as lying within the vent have suffered vertical displacements from their original positions, for the vent-materials are heterogeneous, and fragments of different rocks are found side by side. For example, on Glas Eilean they consist partly of Jurassic limestone and sandstone not seen in situ in the vicinity, partly of the basalt lavas and schists that form the vent-walls. In addition, fragments of cone-sheets are abundant, and others of porphyritic basalt and of an acid compact rock are occasionally to be found. An interesting feature on Glas Eilean is the occurrence of large brecciated masses of Jurassic limestone traversed by cone-sheets. So little relative movement of the brecciated fragments composing each mass has taken place that the original relationship of the plexus of cone-sheets cutting the limestone can be easily made out. Coherent masses would seem to have been separated from the vent-wall in the first instance, and to have been brecciated by subsequent explosions while lying within the vent itself.
The explosions responsible for the brecciation are to be ascribed to an acid magma, of which the included acid fragments are a sign. Also, at one point, on the east side of Glas Eilean, a light yellow compact material, found under the microscope to be an acid tuff (p. 140), infills all the cracks and crevices of the breccias. To the unaided eye the veinings appear exactly similar to the net-veining characteristic of the margins of many ring-dykes in Ardnamurchan (see
From what has been said above, coupled with the fact that not a single cone-sheet cuts the breccias, it is certain that the vent is later than the Outer Cone-sheets of Centre 2, or, at any rate, that portion of the belt of cone-sheets where the vent is situated. The parallelism between vent and adjacent cone-sheets seems to mark the vent as connected with Centre 2. It needs no great assumption to conclude that such a linear vent may be the superficial expression of a linear intrusion, below ground, guided by a ring-fissure, as already remarked in an introductory chapter (p. 77). J.E.R.
Petrology
Agglomerates, Ben Hiant Vents
One of the most striking features of the agglomerates that occur around Ben Hiant is the generally acid nature of the component fragments. Although a variety of tock-types enter into the composition of the agglomerates, it is patent that trachytes, rhyolites, and dacites constitute the bulk of the material, and point to the former existence of acid extrusive and intrusive rocks of which practically all other records are wanting.
The most prevalent rock-type met with is a bostonitic trachyte (S22822)
The felspar phenocrysts in the microporphyritic types reach a quarter of an inch in length (S22822)
The augite is subordinate to felspar both as regards the number and size of the phenocrysts. It builds small idiomorphic or rounded crystals of a greenish colour, and although prone to serpentinous decomposition is apparently an aluminous variety with a moderately large axial angle. We must not, however, dismiss the possibility of the former presence, in some instances, of a uniaxial augite of enstatite composition (S22291)
The ground-mass consists commonly of a plexus of narrow short microliths of alkali-felspar, most frequently arranged in a fluxional manner, abundant grains of finely divided magnetite, and small limonitic and chloritic patches that appear to represent original biotite. In some instances seemingly original biotite in a fresh condition can be detected in the ground-mass (S22822)
That some of these trachytic rocks were in part glassy and possibly intrusive types, is suggested in some examples (S22825)
A porphyritic structure in the trachytes is not by any means universal, and several instances can be cited of rocks of trachytic aspect which are devoid of porphyritic constituents. In such cases the rock consists of a felted or fluxionally arranged mass of alkali-felspar prisms, with uniformly distributed minute crystals of magnetite, and small ragged flakes of biotite (S22292)
Certain non-porphyritic microlithic rocks of a somewhat more basic composition are also represented (S22445)
From the agglomerate of Maclean's Nose comes a most beautiful fluxion rhyolite (S21446)
In addition, a rock of dacitic composition (S22823)
It contains small isolated and grouped phenocrysts of andesine, numerous small decomposed crystals of augite, and grains of magnetite, in a quartzo-felspathic matrix of variable texture.
The above account is descriptive of those rocks which occur in the agglomerates of Ben Hiant, and which cannot be matched in solid form in the accessible portions of the vent-walls. In petrological types they are most closely paralleled by the trachytic and rhyolitic rocks associated with agglomerates, and described by Dr. Harker, from Fionn Choire in Skye.<ref>A. Harker, The Tertiary Igneous Rocks of Skye, Mem. Geol. Surv., 1904, pp. 58, 59.</ref> Plateau basalts, some with porphyritic olivine (S21447)
Although all the basalt masses show equally the effects of shattering, some evince more intense thermal alteration than others. In the case of certain xenolithic masses in the Ben Hiant Intrusion (S22452)
It is probable that the shattering was accomplished in the vent soon after the rock had consolidated, and that the metamorphism of the blocks was produced by the inherent heat of the crater, prior to their incorporation in the agglomerates. It is impossible to attribute the metamorphism of these masses to the thermal influence of any known intrusions, and further, they show a higher grade of metamorphism than that of the tuffs and other rocks with which they are now associated.
Bedded Tuff, Ben Hiant Vents
The tuffs (S22293)
Agglomerates, Northern Vents
The great mass of agglomerates which stretches southwards from the coast near Achateny towards Camphouse has been made the subject of close study. It is more than usually rich in fragments of the various country rocks that bound the vents, but a representative collection of other types shows a general assemblage similar to that met with in the Ben Hiant Vents to the south. As at Ben Hiant, porphyritic bostonitic trachytes are extremely abundant (S23598)
Spherulitic quartz-rhyolites are also represented amongst the more acid fragments (S23599)
Amongst other fragments collected on the shore near Achateny farm, it is interesting to note definitely intrusive types such as quartz-dolerites, leidleites, and pitchstones. The quartz-dolerites are represented by rocks of moderately coarse type that resemble either the coarser varieties of basic cone-sheets (Talaidh type (S23609)
Another type (S23607)
The origin of these intrusive types is obscure, but although of earlier date, they are certainly related petrographically to the Ben Hiant intrusions.
Pitchstones of Ben Hiant
(Anal. I and II,
The microliths of the ground-mass, scattered indiscriminately throughout the brown glass, are oligoclase and a greenish, presumably aluminous, augite. There is generally no arrangement of the crystallites indicative of flow, but perlitic cracks are a noticeable feature of the glassy base (S22294)
Glas Eilean Vent
In the vent-breccia of Glas Eilean the fragments collected are chiefly of plateau basalts (S24018)