Chapter 21 The Girvan district — continued. Llandeilo-Caradoc sedimentary (Girvan) Series

After the completion of the volcanic episode in Lower Silurian time, and the deposition of the Arenig cherts and overlying sediments, these various accumulations were elevated and exposed to denudation. During this process the cover of conformably overlying sediments was removed, and the lavas, agglomerates, and accompanying plutonic rocks were laid bare. The detritus of these older rocks furnished the materials of the conglomerates (Kirkland and Benan) at the base of the Girvan series, the green matrix of the conglomerate being made up of triturated igneous materials, and the pebbles being composed of Arenig chert, black shale, lavas, and tuffs, together with serpentine, gabbro, dolerite, and even the granite of Byne Hill. These conglomerates, which are associated with the Stinchar Limestone, rest unconformably everywhere to the north of the Stinchar valley on all rocks older than themselves, sometimes on the serpentine and gabbro. Further, relics of the Stinchar Limestone group with its characteristic fossils occur as small outliers on the plateau of intrusive igneous rocks at Bougang and near Knockdolian Barns. Indeed, from the manner in which different members of the Benan Conglomerate and Stinchar Limestone group overlap each other, and successively rest on the volcanic plateau, it appears that the latter must have been subjected to unequal movement and subsidence towards the close of Llandeilo time.

In the historical account of Professor Lapworth's researches in the Girvan area (Chapter 2), a description has been given of the order of succession of the fossiliferous rocks of the Girvan series which he established. During the recent examination of that region by the Geological Survey, his views regarding the physical relations of the beds and the sequence of the groups have been confirmed, save in some minor points. To his paper on "The Girvan Succession", of which ample use has been made in the preparation of this description, the reader is referred for more minute details than are here given. In the sequel we adopt the general classification which he propounded.

The Barr Series

The members of this series are typically developed on the north side of the valley of the Stinchar, near the village of Barr, where the characters of the various sub-divisions and their relations to each other are displayed in numerous natural sections. Here they present the following succession in descending order:

The dominant feature of this series is the massive conglomerate of Benan, so remarkable in its lithological characters and so strikingly developed on the elevated ridge forming the Benan, Kirkland, and Auchensoul Hills, north of the Stinchar valley. The peculiar dark-green matrix, derived from the disintegration of the basic igneous rocks, which, when exposed to atmospheric agencies along the banks of the stream, decomposes readily and frequently crumbles away under the touch of the hammer, and further, the varied assemblage of well-rounded boulders from the Arenig volcanic plateau, are features that cannot fail to impress the geologist. From the Benan Hill to the terraced slopes of the Milljoan and Milton Hills this remarkable conglomerate can be traced along the strike for a distance of six miles, while from the village of Barr it stretches northwards to the sources of the Water of Assel and Penwhapple Burn. It reappears in many other remarkable sections, as will be shown in the sequel, but wherever it occurs its peculiar characters are readily recognised. The section chosen by Professor Lapworth to illustrate the sequence of the Barr series — that of the Benan Burn — is by far the best in the whole Girvan area (Figure 111).

Benan Burn. — [NX 23955 92173] This stream, which drains the southern slopes of the Benan Hill and joins the Stinchar River about 2½ miles to the south-west of the village of Barr, displays each subdivision of the series, and furnishes excellent opportunities for studying the zones in detail (Figure 111). Ascending the burn from the alluvial terraces of the Stinchar, the observer, after traversing for a short distance a deposit of boulder clay, reaches the lowest member — the Kirkland Conglomerate. No great thickness of this band is here visible. It is better developed on the adjacent slopes of Craigbickerae Hill [NX 23458 92075], to the south-west. The matrix of this conglomerate is usually of a purple colour, differing in this respect from that of the Benan band, which is green, and its texture is usually also much harder. It is likewise traversed by innumerable veins and strings of calcite. But as regards the constituents of the matrix, and the characters of the included pebbles, the conglomerate of Kirkland resembles that of Benan. It includes representatives of the various types of Arenig lava, together with blocks of gabbro, dolerite, granite, black shale, radiolarian chert, and sandy greywacke, all of which can be identified as having been derived from the Arenig volcanic plateau and its overlying cover of sedimentary material.

The Kirkland conglomerate (2a), which dips north-westwards, is immediately succeeded by purple sandstones and grits, with partings of sandy shales forming a transitional zone between the conglomerate below and the overlying calcareous flagstones (2b). The latter zone (Orthis confinis flags) is one of the important fossiliferous horizons in the Barr series, and forms the lowest sub-division of the Stinchar Limestone group. These impure calcareous flagstones, owing to the irregular distribution of the calcareous matter in them, graduate into calcareous sandstones, and often contain nodules of limestone, which, when dissolved out, give rise to a honey-combed surface. Weathering with a brown or purple colour, they form a striking contrast to the nodular bands of white limestone in the overlying zone. The included fossils are almost wholly brachiopoda, and the most abundant species is Orthis confinis (Salter), with which are associated Orthis alternata (Salter), Strophomena grandis (Salter), &c. Dipping towards the north-west, at an angle of 25°, the Orthis confinis flags are immediately followed by the prominent zones of the Stinchar Limestone (2c). The lower sub-division is composed of nodular limestone, with numerous partings of shale, which, in several sections, are highly fossiliferous, yielding the characteristic form Maclurea Logani, with Tetradium Peachi and other organisms. The upper member consists of flaggy compact limestones, which are sparingly fossiliferous, though here charged with Saccamina and the peculiar Girvanella, the latter being so abundant as to constitute a prominent constituent of the rock. These limestones are admirably seen at the fork of the Benan Burn [NX 23916 92642] and in the branching streams, as well as in the limestone quarry where the streams unite. Though the characters of the bands may here be studied, yet the exposures are not so suitable for obtaining fossils as at Minuntion Quarry [NX 22462 91262], and other places, where the rocks have been extensively worked (Figure 111).

At the head of the Benan Burn, four rivulets unite to form the main stream, which furnish excellent sections of the two highest members of the series. The compact limestones above the fork dip to the north-west at 37° underneath a band of greenish, hard and nodular mudstones (2d). Though finely laminated, they weather in a concretionary form, break under the hammer with an irregular fracture, and likewise effervesce freely with acid. They have yielded a few graptolites and brachiopoda, but only after prolonged search. The best exposure is in the west branch of the Benan Burn [NX 23816 92708], to the west of the limestone quarry [NX 23840 92728], where the following assemblage of fossils was obtained by the Survey:

Glossograptus Hincksi (Hopk.)

Diplograptus rugosus (Emm.)

Diplograptus euglyphus (Lapw.)

Didymograptus sp.

Dicranograptus sp.

Dicellograptus sp.

Climacograptus Schärenbergi (Lapw.)

Climacograptus bicornis (Hall.)

Turrilepas sp.

Lingula ovata (M'Coy.)

Lingula Ramsayi (Salt.)

Siphonotreta macula (M'Coy.)

Siphonotreta scotica (Dav.)

Professor Lapworth records the forms given in the annexed list from the same locality:

Clathrograptus cuneiformis (Lapw.)

Diplograptus rugosus (Emm.)

Didymograptus superstes (Lapw.)

Siphonotreta micula (M'Coy.)

Glossograptus Hincksi (Hopk.)

Acrotreta Nicholsoni (Dav.)

Cryptograptus tricornis (Carr.)

The green mudstones are overlain by the Benan Conglomerate (2e), which forms numerous small escarpments on the slopes of the Benan Hill, and dips north-westwards at an angle of 40°, which decreases to about 15° further up the hill.

Sections along the line of outcrop of the Stinchar Limestone group on the north side of the valley of the Stinchar, afford clear proof of the same succession of strata. By means of natural exposures and quarries, the limestone can be traced more or less continuously for a distance of three miles from Auchensoul Burn to Minuntion. Along this line the strata dip persistently to the north-west, as in Benan Burn.

In the Kirkland Burn [NX 24469 92607], the section across the higher beds is not quite continuous, for it is truncated by small faults, but the Benan Conglomerate, graptolite shales, and limestone occupy their relative positions. From the horizon of the limestone a nearly continuous sequence may be traced downwards to the Kirkland Conglomerate.

In the neighbourhood of Auchensoul [NX 25845 93035], various exposures of the fossiliferous zones are to be found, (1) on the slopes of Kirkdominae Hill [NX 25073 93007], (2) in Kirkdominae Burn [NX 25435 93040]?, (3) in the Auchensoul Burn [NX 25792 93229]. At some of these places the beds are much disturbed and traversed by faults, which affect the continuous sequence, but not the relative position of the strata. The fossiliferous zones of the Stinchar Limestone are visible in several old quarries on Kirkdominae Hill [NX 24795 92974], where Nidulites favus (Salt.), Lingulella Davisi ? (Salt.), Cleidophorus sp. were collected by the Geological Survey. From the thin seams of calcareous shales, interleaved in the limestones at this locality, Professor Lapworth records Leptaena sericea, Orthis caligramma, O. biforata, Turrilepas sp., Cythere aldensis, Asaphus, Encrinurus; while Mrs. Gray has obtained Cythere sp., Dindymene.

In the prolongation of this series from the Benan Burn southwest to Minuntion [NX 22373 91311], similar evidence may be obtained in support of the general sequence of the beds, while, at the same time, certain data present themselves, suggestive of a gradual overlap of the Benan conglomerate on to the flaggy limestones. Indeed, indications of this overlap make their appearance not far to the west of the coomb-shaped hollow at the head of the Benan Burn. For as the upper boundary of the limestone and the lower limit of the Benan Conglomerate are followed along their lines of outcrop to the south-west, they gradually approach each other, though their actual contact is not visible, the intervening zone of graptolitic mudstones gradually disappearing from view. Professor Lapworth has called attention to similar phenomena to the west of the Auchlewan Burn [NX 23237 91985], in an old quarry [NX 22972 91957]?, where the junction between the conglomerate and the limestone is visible, which he thus describes:

"The flaggy and compact limestones dip into the hill at their normal inclination, and are irregularly surmounted by a solid mass of conglomerate dipping in the same direction, but at a slightly inferior angle. The line of contact is most irregular. The limestone has all the appearance of having been greatly eroded previous to the deposition of the conglomerate. The regular beds of limestone strike abruptly at the lower portions of the pudding-stone, and patches of the former are found adhering irregularly to the weathered basal beds of the latter. The conglomerate itself is almost destitute of anything like bedding, but some of the natural planes of deposition are shown by the linear arrangement of bands of small pebbles. In the lowest beds a few fragments of limestone are visible of the general size of marbles. The appearances seen are difficult to explain on the hypothesis of a fault, owing to the presence of patches of limestone in the conglomerate; but if they are actually due to an unconformability, we have to suppose that within the insignificant distance of about 200 yards the entire thickness of the Didymograptus shales must have been eroded previous to the deposition of the basal beds of the Benan. Conglomerate, and in addition some slight thickness of the compact limestone itself".<ref>"The Girvan Succession". Quart. Jour. Geol. Soc., vol. xxxviii., p. 562.</ref> Professor Lapworth, however, is of opinion that this conjunction of the beds may be ascribed partly to the effect of a fault and partly to certain peculiarities of the basal beds of the conglomerate. Similar phenomena are observable in certain exposures in the valley of the Water of Assel, to be referred to presently.

In the burn of Auchlewan, and on the adjacent slope to the east, the flaggy limestones and Orthis confinis flags are visible; the fossiliferous limestones are likewise seen in the deserted quarries north-west of the farmhouse of that name. By means of occasional exposures they can be traced in a south-west direction to the banks of the Stinchar at Minuntion.

Minuntion Quarry. — [NX 22076 91142]? The section in this quarry, where the rock was formerly excavated for local economic purposes, is of special interest, on account of the excellent exposure of the Orthis confinis flags and the overlying limestones of the Maclurea zone. The characteristic brachiopoda of the former sub-division here occur in profusion, though the species are few in number.

The limestones are also highly fossiliferous. The fossils given in the subjoined list were here obtained from the limestone by the Geological Survey:

Serpulites longissimus (Murch.)

Crinoids.

Illaenus ( Bumastus) Bowmani (Salt.)

Phacops (Acaste) Brongniarti (Portl.)

Remopleurides laterispirifer (Portl.)

Remopleurides Colbii (Portl.)

Leptaena sericea (Sow.)

Leptaena (Plectambonites) quinquecostata (M'Coy.)

Leptaena (Plectambonites) transversalis (Wahl.)

Orthis calligramma (Dalm.)

Orthis confinis (Salt.)

Rhynchonella balclethiensis (Dav.)

Siphonotreta scotica (Dav.)

Strophomena expansa (Sow.)

Pleurorhynchus (Conocardium) sp.

Bellerophon sp.

Holopea sp.

Maclurea Logani (Salt.)

Maclurea M'Coyi (Salt.)

Metoptoma (Tryblidium) sp.

Ophileta (Maclurea) macromphala (M'Coy.)

Murchisonia gracilis (Hall.)

Murchisonia bicincta ? (Hall.)

Murchisonia vitellia (Bill.)

Pleurotomaria qualteriata (Schl.)

Rhaphistoma (Pleurotomaria) lenticularis (Sow.)

Trochus sp.

Cyrtoceras inaequiseptum (Portl.)

From the underlying calcareous flags, the Survey has collected Orthis confinis (Salt.), O. flabellalum (Sow.), O. calligramma (Dalm.), Strophomena expansa (Sow.)

In addition to these fossils from the limestone, Mrs. Gray records the following:

Lingula granulata (Phil.)

Lingula Ramsayi (Salt.)

Rhynchonella nasuta (M'Coy.)

Asaphus gigas (De Kay.)

Cheirurus bimucronatus (Murch.)

Maclurea magna (Lesu.)

Cyrtoceras scoticum (Blake.)

Professor Lapworth likewise obtained Orthis biforata, Triplesia Grayi, and Illaenus latus.

In the foregoing paragraphs we have described the outcrop of the Stinchar Limestone group, on the north side of the valley of the Stinchar, from Auchensoul Burn to Minuntion, along the northern limb of an anticlinal fold, where the general dip of the strata is towards the north-west. We will now refer to certain outcrops of the limestone at Dularg, on the south side of the valley opposite Auchensoul, where the calcareous series dips towards the south-east.

Dularg. — [NX 25928 92908] In the bed of the river Stinchar, below Dularg, the purple sandstones and conglomerate (Kirkland), the Orthis confinis flags, and the Maclurea limestone zone are met with in a highly disturbed condition. Though the strata are contorted, their general dip is to the south-east. Again, in the quarries at the farmhouse of Dularg, the limestones appear with a south-east dip, but here they have a purple tint, are much hardened and veined, and have been truncated on the south side by a powerful fault, traceable along the south side of the valley, which throws down a patch of Lower Old Red Conglomerate with its accompanying lavas. Other small exposures of reddened limestone are met with on the south side of this strip of conglomerate.

It is clear that along this part of the valley of the Stinchar the members of the Barr series are arranged generally in the form of an anticline, with minor folds on the south-east limb. The limestone group and associated strata are not traceable for any distance on the south side, owing to the compound anticline being obliquely truncated by the powerful fault just referred to. It is highly probable, however, that the Ballantrae volcanic series lies at no great depth below the present level of the river Stinchar, between Auchensoul and Minuntion, underneath the Kirkland conglomerate. This supposition is strengthened by the fact that as we descend the valley from Minuntion to Kilpatrick and Pinmore, a distance of about a mile, the Arenig lavas and intrusive rocks appear, succeeded by coarse conglomerates, which include the Maclurea Logani zone of the Stinchar Limestone. The latter is met with at the northern margin of the alluvium on the north side of the Stinchar, about 300 yards east of the junction between the Stinchar and Water of Assel.

Stinchar Limestone Group in Assel Valley. — [NX 21929 93658] When the observer crosses the prominent ridge formed by the Benan, Kirkland, and Auchensoul Hills northwards to the Water of Assel, he finds numerous outcrops of the Stinchar Limestone beds, which are there brought to the surface apparently by means of isoclinal folds and faults. They are traceable at intervals from Craigwells to Dupin on the south side of the Water of Assel, and further to the south-west they are met with on Shalloch Hill, north-west of Daldowie. They appear further north between Tramitchell and Brockloch.

At the north-east limit of these exposures, an important section is to be seen in a deserted limestone quarry about 200 yards south of Craigwells Cottage [NX 25689 95318] and about half a mile E.N.E. of the farmhouse of Brockloch [NX 24557 95061], where the strata have a persistent dip to the south-east. The highest bed visible on the right-hand side of the quarry is the Benan Conglomerate, which towards the base contains numerous nodules or pebbles of the Stinchar Limestone with its characteristic fossils. The conglomerate is underlain by eight feet of green shales (graptolite shales), which yielded to Professor Lapworth Lingula, Acrotreta Nicholsoni, Diplograptus, &c., and which rest upon the compact and nodular limestones of the Stinchar group. No beds lower than the limestone are exposed on the right-hand side of the quarry, but on the left side nodular calcareous sandstone appears, charged with Orthis confinis. It contains limestone nodules, with an occasional pebble of altered lava and chert, and rests unconformably on a mass of decomposing gabbro or basic rock (Ballantrae volcanic series).

In the same quarry a short distance to the south, the graptolite shales are interleaved in nodular conglomerate limestone, the sequence in descending order being: (1) Benan Conglomerate at the top, five feet; (2) shales and nodular limestone, seven feet; (3) graptolite shales, ten feet six inches. In this case also the basal beds of the Benan Conglomerate contain nodules or pebbles of the Stinchar Limestone.

Further to the south-west, the limestone quarry about 700 yards east of Brockloch Farmhouse [NX 25361 94945]? shows the Benan Conglomerate resting apparently on an eroded surface of the limestone. Here the highest bed consists of Benan Conglomerate, containing the usual assemblage of pebbles at the base, and numerous nodules or blocks of the Stinchar Limestone, derived to all appearance from the eroded surface of the compact limestone. On the left-hand side of the quarry calcareous flagstones are visible with Orthis confinis.

From this quarry the limestone can be traced at intervals south-westwards to the outcrops visible on several streams near Dupin farmhouse [NX 23974 94119], of which the three most important may be briefly referred to. The eastmost burn [NX 24545 94325], about 550 yards E.N.E. of the farmhouse, furnishes evidence of the appearance of the Stinchar Limestone group along an isoclinal fold, in which the strata dip to the south-east. In the centre of the isocline, the Orthis confinis flags are met with and readily yield the zonal form. They are succeeded on both sides by the white limestone of the Stinchar group. On the north-west limb of the fold the graptolite shales plunge with an inverted dip to the south-east beneath the limestone; on the south-east side of the arch the section ceases with the limestone, no higher beds being visible.

In the Dupin Mid Burn, about 250 yards east of the farmhouse [NX 24391 94274], the beds are disturbed and traversed by small faults, but there is a clear sequence from the Benan Conglomerate through calcareous shales to the compact and nodular limestones beneath. Like the sections already quoted, the basal bed of the Benan Conglomerate contains rounded nodules or pebbles of limestone, suggestive of local erosion, though Professor Lapworth is inclined to regard the relationship as probably due to faulting. The limestones are underlain by barren sandy grits.

About 400 yards west of the farmhouse of Dupin [NX 24169 94054], the most complete section is visible in Dupin Glen, the beds ranging from the Orthis confinis flagstones to the Benan Conglomerate. In the upper part of the stream the Benan Conglomerate is well seen dipping south-east at an angle of 37°, and containing the usual assemblage of Ballantrae igneous rocks, together with pebbles of Stinchar Limestone. Underneath lie compact and nodular limestones, which yield to the Geological Survey Maclurea Logani. These are underlain by nodular calcareous flags, from which Professor Lapworth obtained Orthis confinis, Strophomena expansa, Illaenus, &c. Northwards, green conglomerate appears like that of the Benan Hill, so that it is highly probable that the northern limb of this fold may here be faulted out (Figure 111).

From the evidence supplied by the various exposures along the line of Dupin and Craigwells, it is probable that they do not form one continuous outcrop, but occur on different folds. In one instance (Dupin East Burn) the isoclinal fold is still preserved; in others it is probable that the northern limb is truncated by faults.

Shalloch Hill. — Westwards from the outcrops just described between Craigwells and Dupin, the representatives of the Stinchar Limestone group appear along the northern margin of the inlier of volcanic and plutonic rocks on Shalloch Hill, and are exposed in two quarries at the edge of the cultivated ground. In the western opening [NX 21843 93370] the limestones with Maclurea rest upon calcareous flags, green sandy shales, and grits, from which Orthis confinis was obtained. These underlying strata are seen reposing, to all appearance, unconformably on a mass of gabbro to the south. The view that such is the relation of the rocks here receives support from the fact that, about 200 yards south of this quarry, in the heart of the volcanic rocks, a small outlier of the Orthis confinis flags comes in contact with the lavas and agglomerates. This evidence is suggestive of overlap.

In the eastern quarry [NX 22040 93433], the junction of the limestone group with the igneous rocks to the south is not visible, but it is evident that the limestones are much folded and traversed by faults. North of these quarries, a mass of gabbro is exposed, which belongs to the older Ballantrae volcanic series, and in the lower portion of the Auchenmady Burn the Benan Conglomerate succeeds. On the south-east side of the volcanic "inlier", the Benan Conglomerate is faulted against the igneous rocks, and on Daldowie Hill it appears to rest unconformably on them.

From the evidence now adduced, which is by no means complete, it would seem that the existing relations of the Stinchar Limestone group to the Arenig volcanic rocks of Shalloch Hill may be accounted for by overlap, folding, and faults.

Little Letterpin. — [NX 19933 92695] About half a mile to the south-west of the inlier of volcanic rocks at Shalloch. Hill and Daldowie, the Stinchar Limestone is exposed in an old quarry near the farm at Little Letterpin, on the west side of the Assel Water, associated with the Benan Conglomerate and containing Maclurea Logani and corals. In the core of this fold the underlying platform of the Ballantrae volcanic series appears as already described.

Tramitchell. — [NX 23379 94399] On the north side of the Water of Assel a good development of the Stinchar Limestone has been laid open in the well-known quarries at Tramitchell, where the rock is extensively quarried; indeed, next to Craighead, this place furnishes the chief supply of lime for the district. Here the beds appear along a faulted anticline, the relations of which are thus described by Professor Lapworth.

"At a first glance it would appear that at least 100 feet of compact limestone are developed at this locality, but this apparently abnormal thickness is delusive, and the limestones are actually of no greater vertical extent than elsewhere. A long strike fault bounds them to the north, throwing down against them the highest zones of the overlying Benan Conglomerate. The limestones, which include also the whole of the Maclurea zone, are crumpled up against this fault in a broken anticlinal form, and dip with several small step-slips, visible in the quarry face, steadily to the southwards, as if passing below the Benar Conglomerate of the flats of Assel. The sharp anticline in the neighbourhood of the fault is well seen at the eastern extremity of the exposure. Some of the more impure and nodular beds on the floor of the quarry take on an oolitic structure, a very rare phenomenon among the limestones of the region".<ref>Quart. Jour. Geol. Soc., vol. xxxviii., p. 576.</ref>

The following fossils were here collected from the limestone by the Geological Survey:

Girvanella problematica (Nich. &Eth.)

Saccamina Carteri (Brady.)

Tentaculites ornatus (Sow.)

Atrypa hemispherica (Sow.)

Orthis calligramma (Dalin.)

Maclurea Logani (Salt.)

Pleurotomaria sp.

Cyrtoceras sp.

Mrs. Gray's list contains the following additional forms from this locality: Orthis confinis, Maclurea magna; and Professor Lapworth further collected Ecculiomphalus (Cyrtolites) Bucklandi and Strophomena corrugatella (See (Figure 111)).

The evidence now brought forward regarding the structural relations of the members of the Barr series in the valley of the Stinchar and in the Water of Assel proves (1) that in the valley of the Stinchar, the Stinchar Limestone group is arranged in the form of an anticline, truncated on the south side by a powerful fault; (2) that in the valley of the Assel the limestone group is brought to the surface by various sharp anticlinal folds in the midst of the Benan Conglomerate.

The original relations of the Benan Conglomerate and Stinchar Limestone group to the volcanic and plutonic rocks forming the main volcanic area of Ballantrae have been largely effaced by prominent strike faults. Nevertheless, the marked unconformability that separates the younger fossiliferous strata of the Girvan succession from the older volcanic plateau can be traced (1) round the north-east slope of the Aldons Hill, (2) near Knockbain west of Millenderdale, (3) between Byne Hill and Kennedy's Pass.

On referring to the geological map of the Ballantrae region (Sheet 7), it will be seen that the Benan Conglomerate forms a continuous belt along the northern margin of the volcanic area between Kennedy's Pass [NX 14802 93142] and Byne Hill [NX 18039 94920], reappearing on the southern slope of the latter eminence. Southwards by Millenderdale and Aldons the conglomerate forms a similar continuous belt, lapping round the volcanic plateau. It is clear that these localities exhibit the relics of two great anticlines, the cores of which are composed of the ancient volcanic and plutonic rocks. Again, along the valley of the Stinchar from Daljarrock [NX 19783 87966], by Craigneil Castle [NX 14711 85328], to the mouth of the Water of Tig [NX 11127 83648], a narrow belt of the unconformable fossiliferous series is thrown down by two parallel strike faults into the midst of the volcanic rocks. These include the Stinchar Limestone group, the Benan Conglomerate, and possibly sediments of still younger date (Balclatchie horizon).

We shall begin our description with the sections visible in the quarries of Aldons, and trace the beds round the margin of the volcanic area towards Millenderdale and Knockbain.

Aldons. — [NX 19656 89484] From the elevated ridge of the Kirkland [NX 24833 93597] and Benan Hills [NX 23499 93197] the great conglomerate can be traced continuously westwards across the valley of the Assel Water north of Pinmore House to the slopes of the Aldons Hill, where the Stinchar Limestone group is well exposed. The limestones appear on both sides of a synclinal fold, the southern limb being truncated by a fault, which is prolonged eastwards to the river Stinchar near Pinmore Bridge.

The quarry [NX 19725 89593]which has been opened on the north-west side of the syncline supplies an excellent section of the limestone group. The lowest visible beds, which dip at a high angle to the southeast, away from the volcanic rocks, consist of purple sandy calcareous flags with scattered pebbles, decomposing readily into a sandy and muddy deposit. These are overlain by thin-bedded, flaggy and compact limestones, that yield some of the characteristic fossils of the series. The following forms were here collected by the Geological Survey:

Cythere (Cytheropsis) aldensis (M'Coy.)

Bellerophon sp.

Holopea sp.

Maclurea Logani (Salter.)

Maclurea magna (Leseur.)

Murchisonia sp.

Raphistoma sp.

Turbo sp.

From the same place Mrs. Gray has obtained the accompanying additional fossils:

Cythere aldensis var. major (Jones.)

Cythere grayana (Jones.)

Cythere wrightiana (Jones & Roll.)

Primitia barrandiana (Jones.)

Harpes sp.

Orthis calligramma (Dalm.)

Orthis intercostata (Port.)

In his description of this section, Professor Lapworth refers to the existence of a small fault visible at the east end of the quarry, whereby the more gently inclined Benan Conglomerate is let down against the underlying limestones. As this fault is traced along the face of the quarry to the west, certain thin-bedded greenish-grey shales appear below it, which he regards as the representative of the Kirkdominae band and the corresponding zone of Craigwells. From this highly fossiliferous zone he obtained casts of fossils of the genera Agnostus, Illaenus, Remopleurides, Lingula, Acrotreta, and Leptaena, together with Cythere and fragments of encrinites. Professor Lapworth likewise inserts a small fault between the limestone series and the Arenig lavas on the north-west side of the quarry, but the actual contact is not visible, and it is not improbable that this junction may be an unconformable one with overlap. For a short distance to the north the conglomerate can be traced lapping round the lavas and dipping eastwards at a high angle, the boundary line forming a recognisable feature.

Along the south side of the syncline at Aldons, the limestone is visible in several quarries, where it is much disturbed by the fault that brings it in contact with the volcanic rocks. It is likewise seen in the railway cutting south of Pinmore Bridge [NX 20163 89714] as a faulted mass between the volcanic rocks and the overlying conglomerate to the north.

Millenderdale (Figure 112). — Northwards from Aldons towards Chapelcroft [NX 19318 91487], along the boundary line between the volcanic rocks and the overlying Benan Conglomerate, a good opportunity is afforded of studying the relation between the two. Though there is here no stream section deep enough to show the conglomerate actually resting on the lavas the two rocks are found close to each other; the conglomerate folding round the dome of volcanic rocks in such a way as to indicate that the junction is an unconformable one. A similar relation obtains along the margin of the volcanic area west of Chapeleroft towards Millenderdale. About half a mile west of the former place a thin band of limestone occurs in the conglomerate near the base of the series [NX 18513 91081]. The limestones, however, do not appear in force till we reach Millenderdale Farmhouse, where they are visible in certain old quarries [NX 16994 90608]. In the one to the east of the farmhouse Maclurea Logani was obtained, and in that on the west side of the farm steading, Girvanella problematica, Tetradium Peachi, and Saccamina Carteri were collected from the nodular flags and compact limestones.

The unconformable relation of the conglomerate series to the Arenig volcanic rocks is laid bare in a stream to the south-west of Millenderdale, and about mid-way between that point and Knockbain. This section has been figured by Professor Lapworth, and the Ethological characters and physical relationships of the beds have been indicated by him. Above the fork E.N.E. of Knockbain [NX 16703 90158], coarse conglomerate is associated with thin bands of shale. These shales, from which Professor Lapworth obtained Diplograptus rugosus, Cryptograptus tricornis, &c., are underlain by conglomerates and sandstones passing downwards to the coarse brecciated conglomerates that rest unconformably on the volcanic rocks. The general dip of the strata is towards the north-west, at an angle of sometimes about 70°. A careful examination of the conglomerate shows its local origin, for the pebbles consist of the surrounding volcanic and plutonic rocks, and the matrix has been derived from the disintegration of basic materials. This section tends to confirm the belief that the boundary line between the volcanic and the conglomeratic series, extending from Aldons to Millenderdale, is in the main an unconformable one.

Westwards, in the Knockbain Burn, south of the Knockbain farm-steading [NX 16247 89989], green mudstones like the graptolitic mudstones appear, and still further west, at the head of a small burn about half a mile south of Barchlewan, a small exposure of shales has yielded Climacograptus Schärenbergi, Diplograptus foliaceas, &c. These shales have an inverted dip to the south-east, and are separated from the Arenig lavas by a thin mass of conglomerate.

Byne Hill and Kennedy's Pass. — [NX 17993 94896], [NX 14818 93129]Along the northern margin of the volcanic area between Byne Hill and Kennedy's Pass, the Benan Conglomerate well displays its normal lithological characters (Figure 112). It is likewise met with on the south-east slope of Byne Hill, resting on a mass of serpentine, where, to the east of Drumfairn Farmhouse [NX 18193 93989], casts of Orthis were obtained from a band of sandstone which is interleaved in the conglomerate, but is of small extent, since it is abruptly truncated on the east side by a fault that throws down an outlier of the Old Red Sandstone. On the north-west flank of the eruptive mass of granite and gabbro on Byne Hill, the Benan Conglomerate dips to the north-west at an angle of 60°, the junction being to all appearance an unconformable one. Here the massive conglomerate is occasionally associated with beds of sandstone and grit. But westwards, together with the overlying Balclatchie Mudstones, it is evidently faulted against the igneous rocks on the west side of the Fell Hill, south of Ardmillan Burn. Thence, along the northern flank of the Pinbain Hill, it dips north-westwards from the volcanic platform as if the junction were an unconformability. But on the shore at Kennedy's Pass [NX 14820 93181], the [fault is well seen that trends W. 5° N. and throws down the conglomerate and overlying Ardmillan Flags against the brecciated lavas. On the beach, below the rock-cutting marked "Kennedy's Pass" on the Ballantrae Road, a thin basalt dyke has been injected along the line of dislocation. At the south end of the Pass, where there is an ancient terrace, the present beach is occupied by brecciated lava, showing pillow-structure, followed on the seaward side by agglomerate containing blocks of various lavas, and some of a more acid type than the normal diabase of this region. The section of the Benan Conglomerate in Kennedy's Pass, which has been made classic by the descriptions of Murchison and the older observers, displays to great advantage, in cliff-section, this remarkable rock with its tumultous assemblage of pebbles of diabase, gabbro, dolerite, granite, radiolarian chert, and black shale — all of local origin (Plate 15), (Plate 16).

On the beach to the south-west of the Pass, a small detached patch of Benan Conglomerate, grits, and green mudstones is thrown down by a fault that runs parallel to the coast-line. The green mudstones are sparingly fossiliferous, but the following form were obtained' after a considerable search: Leptaena tenuicincta, L. quinquecostata, Ophileta compacta, and Bellerophon sp. Craigneil Castle, Colmonell. — Reference may now be made to the narrow band of the unconformable series thrown down by two parallel faults into the midst of the volcanic rocks, along the lower portion of the valley of the Stinchar, from Daljarrock by Craigneil to the mouth of the Water of Tig. The most important sections in this band occur on the south side of the Stinchar. The limestone, together with conglomerates and sandstones, is exposed in two quarries, the larger one at Craigneil Castle, and the smaller north of the road near Craigneil Farmhouse. At the latter opening the nodular limestones are underlain by brown calcareous flags with Orthis confinis, both Zo dipping to the south-east. On the south-east side of the quarry at Craigneil Castle, the sandy calcareous bands with Orthis eonfinis reappear with an inverted south-east dip, as if-resting on the limestone. Here the nodular limestone (Maclurea bed) and the compact limestone are visible. From these data we may infer that the members of the Stinchar Limestone group in the neighbourhood of Craigneil form an isoclinal fold, with the Orthis confinis beds on both sides. The various zones at Craigneil yielded to the Geological Survey Orthis confinis, Maclurea magna, and Saccamina Carteri. In the Cambridge Catalogue the following further forms are recorded from the same locality: Leptaena sp., Maclurea Logani, Heliolites favosus, and Orthisina, scotica.

Westwards from Craigneil [NX 14303 85264], by the side of the road to Salachan, and in the adjoining fields green mudstones and shales appear, which at certain localities are fossiliferous. They are either vertical or highly inclined to the north-west, and their relations to the limestone series are not apparent. A band of mudstone, on the south side of the road, about 500 yards W.S.W of Craigneil Farmhouse [NX 13857 85189], furnished specimens"of Ampyx sp., Diplograptus rugosus, Turrilepas scotica, Pinnoccuris Lapworthi, Acrotreta sp., and Ctenodonta laevis, some of which occur in the Balclatehie Mudstones that overlie the Benan Conglomerate, to be described presently. Green mudstones and conglomerate appear in the Knockdhu Burn; and still further west, near the mouth of the Water of Tig at Heronsford, the Benan Conglomerate is visible.

Bougang. — [NX 11400 85491] The limestone quarry at Bougang, situated about 300 yards south-west of the farmhouse of that name, and about two miles west of the village of Colmonell, is one of the fossiliferous localities of the Stinchar Limestone which has become widely known through the researches of the older observers. Here the beds are quite unaltered and highly fossiliferous, and yet they occur as a small isolated patch in the heart of the southern belt of serpentine. They consist of green sandy shales, which contain occasional pebbles of serpentine, alternate with flaggy limestone, and dip to the south-east, succeeded by the nodular limestones with partings of shale. The relations of these strata to the surrounding mass of serpentine are not visible, but the presence of pebbles of that rock in the shales clearly points to the limestone group being of later date. The following assemblage of organisms was here collected by the Geological Survey:

Tetradium, Peachi (Nich.)

Asaphus (Isotelus) rectifrons (Portl.)

Cheirurus bimucronatus (Murch.)

Illaenus (Bumastus) Bowmani (Salt.)

Phacops (Acaste) Brongniarti (Portl.)

Remopleurides laterispirifer (Portl.)

Leptaena tenuicincta (M'Coy.)

Nucleospira pisum (Sow.)

Orthis balcletchiensis (Dav.)

Orthis calligramma (Daim.)

Orthis confinis (Salt.)

Pleurotomaria bicincta ? (Hall.)

Pleurotomaria expansa (Sow.)

Bellerophon fastigiatus (Lindstr.)

Maclurea Logani (Salt.)

Murchisonia gyrogonia (M'Coy.)

Holopea sp.

In the Cambridge Catalogue published by Salter and Sedgwick the following additional forms from the same place are noted: Illaenus latus, Orthisina scotica, and Murchisonia angulata.

Knockdolian Barns. — [NX 12556 85849] About a mile and a quarter west of the village of Colmonell, near the farmhouse of Knockdolian Barns, on the north side of the road, another small outlying mass of the Stinchar Limestone is likewise unaltered. It consists of nodular limestone with partings of shale yielding Maclurea and Saccamina Carteri, but its relations to the adjacent rocks are not exposed.

Dinvin Hill, Pinmacher, and Glendrissock Hill (two miles south of Girvan). — [NX 20354 93604], [NX 19545 93643], [NX 19818 95011] Along the northern slope of the valley of the Assel, the Benan Conglomerate can be traced from the limestone quarries at Tramitchell to the Dinvin Hill, where it covers a broad area owing to reduplication by folding. The general dip of the conglomerate, where obtainable, is towards the south-east at an average angle of 25°, so that much of the plication must be isoclinal. On the north-west slope of the Dinvin Hill two isolated patches of the Stinchar Limestone occur, one about 300 yards to the north-east of Pinmacher [NX 19753 93913], [NX 20037 94181] the second about half a mile to the north-east of that farmhouse. At the former locality the limestone dips to the south-east underneath the mass of conglomerate on Dinvin Hill, and not far to the north it is underlain by conglomerate. The horizon of this band is definitely fixed by the occurrence of Maclurea Logani and Lyopora favosa. The second locality offers but a meagre representative of the calcareous group. The nodular bands of limestone do not here exceed three feet in thickness, and are visibly overlain and underlain by conglomerates which dip in an easterly direction. In the sequel reference will be made to the fact that eastwards a regular ascending series may be traced through the Benan Conglomerate to the overlying Balclatchie grits and mudstones visible in Laggan Gill [NX 20374 94633]. There is likewise clear evidence to show that the outcrops of the Benan Conglomerate and overlying Balclatchie beds in Laggan Gill have been shifted by a fault trending E.N.E. and W.S.W., with a down-throw to the south-east. Hence, beyond Laggan Gill, the same zones reappear in normal order to the north of the mass of volcanic agglomerate and serpentine in Glendrissock Hill. The unconformable junction between the conglomerate and serpentine is visible in a quarry on the hill slope on the north side of Laggan Gill, while to the north the Benan Conglomerate is well seen in numerous exposures, dipping northwards, away from the outlier of Arenig volcanic and plutonic rocks.

Trowier Hill (Sheet 8). — [NX 20542 96053] The western spur of Trowier Hill, situated between the two branches of the Piedmont Burn, that flows northwards by Donne to join the river Girvan near the harbour, is formed of a mass of Benan Conglomerate, in the midst of which serpentine protrudes with an associated vein of limestone, as on Glendrissock Hill. No outcrop of the Stinchar Limestone has been detected on this hill. The conglomerate seems here to rest unconformably on the Ballantrae plutonic rocks.

Dow Hill (Sheet 7). — [NX 19280 96102] Further down the Piedmont Burn an anticlinal arch of the Benan Conglomerate may be observed on the west side of that stream, on the Dow Hill, a mile and a half south of Girvan Station, where it is followed in normal sequence by highly fossiliferous Balclatchie Mudstones, to be referred to presently. The southern limb of this fold is abruptly truncated by a north-east and south-west fault, the prolongation of which is well seen in the Piedmont Burn.

Craighead (Sheet 14 of Survey Map). — [NS 23428 01362] On the north side of the Girvan valley, and at the eastern margin of the "inlier" of Arenig volcanic rocks, one of the celebrated exposures of the Stinchar Limestone occurs at the Craighead quarries, which have long been the chief source of lime for the district. While it is doubtless true that the typical sections in the Benan and Kirkland Burns, in the valley of the Stinchar, show more clearly the sequence and relations of the sub-divisions of the Barr series, yet the exposure at Craighead surpasses all others in the Girvan area for the prolific character of the fossiliferous zones. For this reason the locality has figured largely in the palaeontological literature of the Girvan district (Figure 109).

Reference has already been made to the fact that the "inlier" of Arenig volcanic rocks on Craighead Hill with the overlying radiolarian cherts and green mudstones is abruptly truncated on the south-east side by the great strike fault that throws down the Carboniferous rocks of the Girvan valley. At the eastern termination of this "inlier" two detached masses of Stinchar Limestone come into immediate contact with the lavas. Of these, the larger and more important exposure occurs to the north of Craighead Farmhouse, and about a mile east of Killochan Station, where two quarries have been excavated along the line of outcrop. The second and smaller mass lies about 100 yards further to the west [NS 23212 01163], separated from the main body of limestone by the Arenig lavas, and truncated on the south by the great boundary fault.

The sequence and relations of the beds exposed in the two quarries traversing the more easterly mass have been described by Professor Lapworth, who states that "we have in this locality a section similar in all its details to that typical of the Stinchar calcareous series of the district to the south of the Girvan valley, the natural members of the succession corresponding precisely in both districts in their position in the vertical series and in their petrological and palaeontological characters, and differing merely in local thickness". <ref>Quart. Jour. Geol. Soc., vol. xxxviii., p. 583.</ref> The physical relations of the beds, as shown by the same observer, are clearly displayed in the south quarry of the more easterly mass of limestone. The unconformable junction between the lavas and conglomerates at the base of the calcareous series is visible at the south-west end of the quarry. The beds of basal conglomerate and grit are vertical, and in places slightly inverted along the line of contact with the volcanic rocks; they are overlain by nodular limestone, charged with corals and an occasional Maclurea, and are traceable round the cliff that forms the west and north face of the quarry. Next in order fine compact limestone appears on the floor of the quarry and in the little cliff to the south where it is truncated by ten feet of grey and black graptolite shales, which Professor Lapworth correlates on mineralogical grounds with the Didymograptus shales of the Auchensoul band. From these shales Mrs. Gray has obtained Cryptograptus tricornis and Diplograptus foliaceus. Overlying these bands come greyish-yellow shales, in which Professor Lapworth identified casts bf the genera Encrinurus, Ampyx, Trinucleus, Leptoma, Stirophomena, Orthis, and Cythere. The next strata consist of conglomerate with limestone nodules.

The succession of beds now described is abruptly truncated by a fault which brings in the nodular limestones of the Maclurea zone, exposed in the adjoining quarry to the northeast, and underlain by nodular green and purple sandstones, resembling the Orthis confinis sandstone of the Stinchar valley. The calcareous strata visible in these two quarries excavated in the easterly mass of limestone at Craighead cannot be traced westwards round the inlier of Arenig volcanic rocks; indeed, the feature along the margin of the lavas is suggestive of a fault that may throw down higher members of the unconformable fossiliferous rocks of Girvan. Recently a large quarry has been opened in the westerly detached mass of limestone, about 100 yards to the west of the quarries at Craighead Farm, in which nodular and massive limestones are exposed, containing Maclurea and abundant corals.

The rich variety of organic remains obtained from these exposures naturally aroused the attention of palaeontologists and geologists who were desirous of fixing the horizons of these beds fluided by its palaeontologists, the Geological Survey assigned the Craighead Limestone to the Caradoc formation <ref>Explanation of Sheet 14, one inch, p. 9.</ref> a view which was at first maintained by Mr. Davidsonn <ref>Monograph of the Silurian Brachiopoda (1884.71).</ref>and subsequently abandoned by him after the publication of Professor Lapworth's researches on "The Girvan Succession".<ref>Supplement to the foregoing monograph (1882.84).</ref> Professor Nicholson, from an examination of the corals, inferred that the Craighead Limestone is of Lower Silurian age, corresponding, perhaps to the upper part of the Trenton Limestone or the beds of the Cincinnati and Hudson River formations of North America; while Mr. Etheridge, jr., on the other hand, from a consideration of the evidence supplied by the trilobites, concluded that it might be of Caradoc age.

In order that the reader may form a clear idea of the nature of the palaeontological evidence on which these conclusions were based, two lists of fossils obtained from the Craighead Limestone are annexed, the first by the Geological Survey, and the second, a supplementary list by Mrs. Gray. Owing to her prolonged researches she has been able to record about 100 species of organic remains from that locality.

List of fossils from the Craighead Limestone in the collection of the Geological Survey of Scotland:

HYDROZOA.

Diplograptus rugosus (Emm.)

ACTINOZOA.

Lyopora favosa (M'Coy.)

Prasopora Grayae (Nich. & Eth.)

Stenopora fibrosa (Goldf.)

Streptelasma craigense (M'Coy.)

TRILOBITA.

Calymene Blumenbachi (Brong.)

Encrinurus punctatus (Brinan.)

Encrinurus punctatus var. arenaceus (Salt.)

Illaenus (Bumastus) Bowmani (Salt.)

Lichas hibernicus (Portl.)

BRACRIOPODA.

Acrotreta Nicholsoni (Dav.)

Crania siluriana (Dav.)

Discina perrugata (M'Coy.)

Lingula quadrata (Eichw.)

Lingula Ramsayi (Salt.)

Leptaena Etheridgei (Dav.)

Leptaena sericea (Sow.)

Leptaena sericea var. rhombica (M'Coy.)

Leptaena (Plectambonites) transversalis (Wahl.)

Leptaena youngiana (Dav.)

Orthis balcletchiensis (Dav.)

Orthis (Platystrophia) biforata (Schl.)

Orthis calligramma (Dalm.)

Orthis Dalmanella) elegantula (Dav.)

Orthis (Dinorthis) flabellulum (Sow.)

Orthis girvanensis (Dav.)

Orthis sowerbyana (Dav.)

Orthis (Dalmanella) testudinaria (Dalm.)

Orthis vespertilio (Sow.)

Rhynchonella aemula (Salt.)

Rhynchonella Lapworthi (Dav.)

Rhynchonella nasuta (M'Coy.)

Rhynchonella Peachi (Dav.)

Rhynchonella portlockiana (Dav.)

Rhynchonella Thomsoni (Dav.)

Rhynchonella Weaveri, (Salt.)

Stropleomena corrugatella (Dav.)

Stropleomena (Rafinesquina) deltoidea (Conr.)

Stropleomena expansa (Sow.)

Stropleomena Grayi (Dav.)

Stropleomena imbrex var. semiglobosina (Dav.)

Stropleomena retroflexa (Salt.)

Siphonotreta scotica (Dav.)

Triplesia Grayae (Dav.)

GASTEROPODA.

Bellerophon acutus (Sow.)

Pleurotomaria sp.

CEPHALOPODA.

Orthoceras politum (M'Coy.)

Orthoceras sp.

In addition to the foregoing list of fossils in the collection of the Geological Survey, the following species are recorded in Mrs. Gray's list from Craighead:

FORAMINIFERA.

Saccamina Carteri (Brady.)

HYDROZOA.

Climacograptus bicornis (Hall.)

Climacograptus Schärenbergi(Lapw.)

Cryptograptus tricornis (Carr.)

Diplograptus foliaceus (Murch.)

Diplograptus sp.

Dicranograptus ramosus (Hall.)

ACTINOZOA.

Favosites girvanensis (Nich. & Eth.)

Fistulipora pilulea (Nich & Eth.)

Halysites sp.

Heliolites Grayi (M. Edw.)

Lindströmia subduplicata (M'Coy.)

Monticulipora gimvanensis (Nich.)

Streptelasma aggregatum (Nich. & Eth.)

Tetradium Peachi (Nich. & Eth.)

Propora Edwardsi (Nich. & Eth.)

Thecostigites scoticus (Nich. & Eth.)

CRINOIDEA.

Glyptocrinus ap.

ANNELIDA.

Cornulites sp.

TRILOBITA.

Cheirurus bimucronatus (Murch.)

Trinucleus sp.

Sphaerexochus mirus (Beyr.)

BRACHIOPODA.

Lingula attenuata (Sow.)

Orthis turgida (M'Coy.)

Rhynchonella Salteri (Dav.)

Rhynchonella sub-borealis (Dav.)

Rhynchonella scotica (Dav.)

Strophomena rhomboidalis (Witck.)

Strophomena llandeiloensis (Dav.)

Triplesia Grayae (Dav.)

LAMELLIBRANCHIATA.

Ambonychia radiata, (Emmons.)

Ambonychia amygdalina (Hall.)

Orthonota contracta (Hall.)

Pleurorhynchus dipterus (Salt.)

GASTEROPODA.

Bucania profunda (Hall.)

Helminthochiton sp.

Holopea paludiniformis (Hall.)

Ophileta sp.

Cyrtolites sp.

Platyceras sp.

Metoptoma sp.

Bellerophon punctifrons (Emm.)

Maclurea Logani (Salt.)

Maclurea macromphala (M'Coy.)

PTEROPODA.

Conularia elongata (Portl.)

Conularia Sowerbyi (Defr.)

Ecculiomphalus Bucklandi (Portl.)

Ecculiomphalus sp.

Hyolithes sp.

CEPHALOPODA.

Cyrtoceras scotium (Blake.)

Orthoceras bilineatum (Hall.)

Trocholites planorbiformis (Conr.)

From the fossils given in the foregoing lists, it is apparent that the forms have a Llandeilo–Caradoc facies. The stratigraphical relations, however, clearly prove that the Craighead limestone, as shown by Professor Lapworth, is on the same horizon as the Stinchar limestone group on the south side of the Girvan valley, which is overlain by Didymograptus shales, containing some Glenkiln graptolites. For these reasons, we regard the Craighead limestone as of Upper Llandeilo age.

Along the north side of the Craighead volcanic area, the Stinchar Limestone group is cut out by a fault, but at the southwest termination of the "inlier" round the mansion-house of Trochraigue [NS 21149 00384] a considerable development of conglomerate and Treen shales is to be seen, which on palaeontological grounds probably belong to the horizon of the Benan Conglomerate and the calcareous shales of the Stinchar Limestone. Professor Lapworth has suggested that the shales and mudstones which are visible at the little pond near the old toll-bar at Trochaigue are probably on the horizon of the Trinucleus mudstones of Drummuck.<ref>Quart. Jour. Geol. Soc., vol. xxxviii., p. 620.</ref> But in the course of the recent revision by the Geological Survey, the fossils given in the annexed list were obtained from these shales (Figure 109), in a burn a few yards to the north of the little pond. Green shales are there interstratified with greywacke bands, both dipping towards the N.N.W., but the beds are also traceable up the stream to the wood south of the mansion-house:

Ischadites, showing internal structure.

HYDROZOA.

Dictyonema reteforme (Hall.)

Dicranograptus ramosus (Hall.)

Diplograptus foliaceus (Murch.)

CYSTOIDEA.

Echinosphaerites granulatus? (M'Coy.)

CRINOIDEA.

Crinoid stems.

ACTINOZOA.

Petraia bina (Lonsd.)

Ptilodictya dichotoma (Portl.)

TRILOBITA.

Asaphas sp.

Encrinurus punctatus (Brun.)

Illaenus (Bumastus) Bowmani (Salt.)

Phacops (Acaste) Brongniarti (Portl.)

Trinucleus sp.

BRACHIOPODA.

Acrotreta Nicholsoni (Dav.)

Atrypa scotica ? (M'Coy.)

Discina perrugata (M'Coy.)

Leptaena sericea (Sow.)

Orthis calligramma (Dalm.)

Orthis crispa (M'Coy.)

Orthis Bouchardi var. Balcletchiensis (Dav.)

Orthis flabellulum (Sow.)

Orthis girvanensis (Dav.)

Orthis intercostata (Portl.)

Orthis vespertilio (Sow.)

Rhynchonella Salteri (Dav.)

Rhynchonella sp.

Siphonotreta micula (M'Coy.)

Strophomena (Rafinesquina) expansa (Sow.)

Strophomena (Leptaena) rhomboidalis (Wilck.)

Strophomena sp.

LAMELLIBRANCHIATA.

Ambonychia sp.

Pterinea sp.

Pteurorynchus dipterus (Salt.)

GASTEROPODA.

Pleurotomaria (Raphistoma) lenticularis (Sow.)

Pleurotomaria sp.

CEPHALOPODA.

Orthoceras pendens (Blake.)

With one or two exceptions all the forms in this list have been recorded from the Stinchar Limestone group. It is highly probable, therefore, that these shales are stratigraphically not far above the horizon of the limestone.

Green shales, having a north-west dip, are likewise met with' in knolls in the policies between the mansion-house and gardens, which are overlain by conglomerate, visible in the woods to the north of the policies at Trochraigue, and also in the fields between the woodlands and the farmhouse of Ladywell. Previous investigators have regarded this conglomerate as the westward prolongation of the band at the base of the Llandovery formation; but in an old quarry in a wood due north of Trochraigue Mansion-house, the following graptolites were obtained in grey shales interleaved in this conglomerate: Dicranograptus ramosus, Diplograptus foliaceus, and Cryptograptus tricornis. It may probably, therefore, represent the Benan Conglomerate.

Thunderton and Glenlochrie, near Kilgrammie (Sheet 14). — [NS 24927 02186] About a mile to the north-east of the Craighead quarries, additional exposures of the limestone group are met with in a shattered condition close to the great fault between the Silurian and Carboniferous rocks. The strata are visible in several quarries between Thunderton Hall and Glenlochrie Farmhouse, where they rest unconformably on a small patch of Arenig lava. The basal conglomerate is succeeded by sandy calcareous bands yielding Orthis confinis, which are in turn overlaid by nodular limestones of the Maclurea zone.

Geological horizon of the Barr Series

The distribution and physical relations of the Benan Conglomerate and underlying Stinchar Limestone group in the Girvan region, having now been described, several points remain relating to the geological horizon of the beds to which special attention may here be directed.

1. It has been shown that at several localities throughout the main volcanic area of Ballantrae the basal conglomerates of the Barr series rest unconformably on the volcanic and plutonic rocks of Arenig age. Similar unconformable junctions occur where the volcanic rocks form "inliers" among younger Silurian strata, as at Craigwells, Glendrissock Hill, Trowier, Craighead, and other places.
2. The nature of the matrix of the conglomerates and the Ethological characters of the pebbles conclusively prove that the materials have been locally derived from the denudation of the underlying contemporaneous and intrusive igneous rocks, together with the Middle Arenig black shales and overlying radiolarian cherts, green mudstones, and greywackes.
3. The Stinchar Limestone group forms an important phase in this conglomeratic series, the highest zone of which (p. 485) consists of graptolite mudstones yielding a graptolite fauna, from which most of the characteristic zonal forms of the Glenkiln Shales are absent. Some of the graptolites which do occur are elsewhere confined to the Glenkiln division, while others are common to the Glenkiln and Hartfell groups.
4. In the Girvan area lying to the north of the Stinchar valley, where the fossiliferous strata of Girvan are separated by a marked unconformability from the older volcanic rocks, no band has been found to contain the characteristic assemblage of zonal Glenkiln graptolites. Such a band, however, occurs, with many of the zonal Glenkiln forms, at the southern margin of the Ballantrae volcanic area (Portandea, Currarie), where there is a perfect passage from the Arenig lavas and overlying radiolarian cherts to the blue-black mudstones with Glenkiln graptolites.

From these data we infer that the local elevation of the area now lying to the north of the Stinchar valley took place in Llandeilo time, and was continued during a considerable interval which is elsewhere represented by the deposition of the greater portion of the Glenkiln Shales (Upper Llandeilo). Towards the close of the Llandeilo period submergence again ensued, and as the volcanic ridges slowly sank the coarse conglomerates accumulated round the old shore line. It is highly probable, therefore, that the conglomeratic series (Kirkland and Benan) and the associated limestone group belong to the close of the Llandeilo period — a classification which may partly account for the Caradoc fades of some of the organisms found at Craighead and other localities.

During the lapse of time represented by the unconformability at the base of the Barr series, continuous sedimentation was in progress in the region lying to the south of what is now the valley of the Stinchar. The blue-black mudstones (Portandea, Currarie), charged with many of the zonal Glenkiln graptolites, the green mudstones, shales, greywackes, and pebbly grits of the"Tappins group" were then deposited on the sea-floor.

Hence to the south-east of the village of Barr, a gradual intercalation of fine conglomerates sets in, composed of materials similar to those in the Benan Conglomerate and in the mudstones and shales of the "Tappins group". Though a fault does extend for a considerable distance along the south side of the valley of the Stinchar, on the southern margin of the volcanic rocks and the Benan Conglomerate, yet this dislocation can hardly be of any great amount, for the strata to the south-east of the fault are known to belong to the conformable Llandeilo series ("Tappins group"). It is interesting, too, to observe that at various localities among the conglomeratic sediments south of the Stinchar valley (Corsewall Point, Glen App, &c.) coarse boulder conglomerates appear, which contain an assemblage of pebbles similar to those in the Benan Conglomerate, including the different types of Arenig lava, the basic and plutonic rocks, radiolarian cherts, black shales, and greywackes. It is highly probable that these ancient gravels may lie on the horizon of part of the coarse boulder conglomerate of Benan. If this correlation be correct, then the strata intervening between the Glen App conglomerate and the radiolarian cherts at Portandea, north of Glen App, may approximately represent the interval marked by the unconformability north of the Stinchar valley.

The straight feature which stretches along Glen App northeastwards by the Water of Muck, the Howe of Laggan, to the valley of the Stinchar at Aldinna marks a line of fault which was originally detected by Professor James Geikie in the course of the geological survey of that region. It is the south-westward prolongation of the great structural fault which bounds the Silurian Tableland on the north and throws down the younger Paleozoic formations against it. The continuation of this fault on the west side of Loch Ryan, where it crosses the promontory to Dally Bay, has already been referred to (p. 411).

Caradoc Formation

The strata belonging to the Caradoc formation that follow in normal order the great conglomerate of Benan have a wide distribution throughout the Girvan region. They occupy that portion of the elevated plateau on the south side of the Girvan valley between the Water of Assel and the belt of Llandovery strata, extending from Saugh Hill, near Girvan, by Camregan to the slopes of Hadyard Hill. They are likewise traceable along the shore from Kennedy's Pass northwards to Shalloch Mill, sweeping upwards to the Ardmillan Braes and Dow Hill. On the north side of the Girvan valley they form a large part of the "inlier" of Craighead and Mulloch Hill.

At the base of the Caradoc strata, and forming, as Professor Lapworth has suggested, a transition zone between the underlying Benan Conglomerate and the overlying graptolite flagstones of Ardwell and Penwhapple Glen, lies an interesting group, the Balclatchie beds, which, on account of their highly fossiliferous character, have figured prominently in the literature of the Silurian rocks of Girvan. Since the upper limit of the Benan Conglomerate is a line easily traceable in the field, and marking the close of the great conglomerate deposits, we have grouped the Balclatchie beds as the basal group of the Caradoc division. With this slight modification, we have followed the various sub-divisions of the Ardmillan series established by Professor Lapworth, as given in the annexed table in descending order:

Generalised Section of the Caradoc Strata (Ardmillan Series and Balclatchie Group).

v. Drummuck Group

3. The sandstones and starfish beds of Quarrel Hill.

2. The trilobite mudstones of Lady Burn and Drummuck.

1. The sandstones and grits of Auldthorns.

iv. Barren Flagstone Group

Flagstones, shales, and mudstones.

 iii. Whitehouse Group

2. Green and purple mudstones, with calcareous seams.

1. Shales with bands of calcareous grit.

ii. Ardwell Group

3. Green grits and mudstones (cascade beds).

2. Thin-bedded flags and rusty shales.

1. Iron-stained shales and mudstones (Knockgerran).

i. Balclatchie Group

2. Fossiliferous grits and conglomerates.

1. Nodular mudstones, highly fossiliferous.

We shall describe first of all the distribution of the Caradoc strata on the south side of the Girvan valley, and thereafter, their development in the Craighead "inlier".

i. Balclatchie Group

Balclatchie Bridge, Penwhapple Burn. — About two miles to the north-west of Barr, on the high-road between that village and Girvan, the passage is well seen from the Benan Conglomerate to the overlying mudstones and grits of the Balclatchie group. Here the highest beds of the conglomerate are traversed by the Penwhapple Burn, where it has carved a small gorge [NX 25588 96903] between the Green Hill to the north and the Doon Hill near Balclatchie. On the cliff by the roadside eastwards from the bridge the successive beds of conglomerate appear charged with pebbles of the usual volcanic and plutonic rocks, lying in a matrix derived from the disintegration of basic materials. They dip to the E.N.E. at an angle of about 60°, and are succeeded by a zone of dark green nodular calcareous mudstones, with certain marked lithological characters. At the first glance they recall in their appearance and mode of weathering the graptolitic mudstones that overlie the Stinchar Limestones in Benan Burn. Where they are fresh, they break under the hammer into angular fragments with a conchoidal fracture; where weathered, their joints are coated with yellow iron-oxide. They are exposed by the roadside, in the bed of the Penwhapple Burn (which joins the Girvan. Water near Killochan), and also in a small cliff on the south bank of the stream. The rich variety and remarkable preservation of the organic remains obtained from these mudstones are, however, their striking features. Upwards of 100 species have been recorded from the exposures of these nodular mudstones at this locality. In order to furnish the reader with a complete enumeration of the fossils, two lists are annexed, the first embracing the forms in the collection of the Geological Survey, the second comprising additional species obtained by Mrs. Gray. The first list is as follows:

Nidulites favus (Salt.)

HYDROZOA.

Climacograptus sp.

ACTINOZOA.

Ptilodictya dichotoma (Portl.)

CIRRIPEDIA.

Turrilepas scotica (Nich. & Eth.)

PHYLLOCARIDA.

Peltocaris sp.

Pinnocaris Lapworthi (Eth. jun.)

TRILOBITA.

Acidaspis Grayae (Eth. jun.)

Acidaspis hystrix (Wyv. Thom.)

Agnostus agnostiformis (M'Coy.)

Ampyx Hornei (Eth. & Nich.)

Ampyx Maccallumi (Salt.)

Asaphus (Isotelus) gigas (Dekay.)

Barrendia sp.

Bronteopsis scotica (Salt.)

Bronteus Andersoni (Eth. & Nich.)

Cheirurus gelasinosus (Portl.)

Cheirurus sp.

Cybele verrucosa (Dalm.)

Illaenus (Bumastus) Bowmani (Salt.)

Illaenus (Dysplanus) Thomsoni (Salt.)

Illaenus sp.

Lichas hibernicus (Portl.)

Phacops (Acaste) Brongniarti (Portl.)

Proetus girvanensis (Eth. & Nich.)

Remopleurides Barrandei (Eth. & Nich.)

Remopleurides laterispirifer (Portl.)

Salteria primaeva (Wyv. Thom.)

Staurocephalus globiceps (Portl.)

Staurocephalus unicus (Wyv. Thom.)

Trinucueus Macconochiei (Eth. & Nich.)

BRACHIOPODA.

Acrotreta Nicholsoni (Dav.)

Discina oblongata (Portl.)

Discina perrugata (M'Coy.)

Leptaena (Plectambonites) quinquecostata (M'Coy.)

Leptaena (scissa)=llandeiloensis (Dav.)

Leptaena sericea (Sow.)

Leptaena sericea var. rhombica (M'Coy.)

Leptaena (Christiania) tenuicincta (M'Coy.)

Lingula attenuata (Sow.)

Lingula ovata (M'Coy.)

Lingula Ramsayi (Salt.)

Orthis balcletchiensis (Dav.)

Orthis (Dalmanella) elegantula (Dalm.)

Orthis sowerbyana (Dav.)

Paterula (Discina) balcletchiensis (Dav.)

Siphonotreta micula (M'Coy.)

Strophomena corrugatella (Dav.)

Strophomena (Rafinesquina) deltoidea (Conr.)

Strophomena imbrex (Panel.)

Strophomena (Leptaena) rhomboidalis (Wick.)

LAMELLIBRANCHIATA.

Ctenodonta anglica (D'Orb.)

Ctenodonta sp.

GASTEROPODA.

Bellerophon acutus (Sow.)

Bellerophon bilobatus (Sow.)

Bellerophon dilatatus (Sow.)

Ecculiamphalus Bucklandi (Portl.)

Murchisonia gyrogonia (M'Coy.)

Raphistoma aequalis (Salt.)

PTEROPODA.

Hyolithes sp.

CEPHALOPODA.

Lituites ap.

Orthoceras politum (M'Coy.)

Orthoceras subundulatum (Portl.)

The following supplementary list shows the additional species obtained by Mrs. Gray.

Ischadites Koenigi (Nich. & Eth.)

HYDROZOA.

Dicranograptus tardiusculus (Lapw.)

Diplograptus rugosus (Emm.)

CRINOIDEA.

Glyptocrinus sp.

Myelodactylus sp.

ACTINOZOA.

Retepora sp.

TRILOBITA.

Acidaspis Lalage (Wyv. Thom.)

Cheirurus Sedgwicki (M'Coy.)

Deiphon Forbesi (Barr.)

Harpes sp.

Lichas Grayi (Flet.)

Sphaerexochus mirus (Beyr.)

BRACHIOPODA.

Lingula canadensis (Bill.)

Lingula granulata (Phil.)

Stricklandinia balcletchiensis (Dav.)

Rhynchonella balcletchiensis (Dav.)

Rhynchonella cuneatella (Dav.)

Rhynchonella girvanensis (Dav.)

Rhynchonella nasuta (M'Coy.)

Rhynchonella portlockiana (Dav.)

Rhynchonella nucula (Sow.)

Orthis calligramma var. virgata (Salt.)

Orthis calligramma var. plicata (Sow.)

Strophomena semiglobosina (Dav.)

LAMELLIBRANCHIATA.

Ctenodonta levata (Hall.)

Nucula subacuta (M'Coy.)

GASTEROPODA.

Bellerophon expansus (Sow.)

Bellerophon bilobatus (Sow.)

Bucania sp.

Cyrtolites sp.

Ecculiomphalus laevis (Sow.)

Murchisonia simplex (M'Coy.)

Raphistoma sp.

PTEROPODA.

Conularia elongata (Portl.)

Hyolithes acutus (Eichw.)

Hyolithes operculatus (Salt.)

Hyolithes reversus (Portl.)

CEPHALOPODA.

Endoceras strangulatum (Hall.)

These nodular mudstones are succeeded by green sandstones and conglomerates, which likewise form an important horizon owing to the fossiliferous character of the conglomerates. Like the Benan Conglomerate, the matrix of the beds above the Balclatchie mudstones is largely composed of the detritus of the Arenig igneous rocks, and the scattered pebbles are formed of similar materials. The fossiliferous conglomerate is overlain by green sandstones which pass upwards into iron-stained shales with graptolites that form the lowest sub-division of the Ardwell group. The fossils obtained from the Balclatchie Conglomerate by the Geological Survey, are given in the following list:

Illaenus (Bumastus) Bowmani (Salt.)

Acrotreta Nicholsoni (Dav.)

Leptana (Christiania) tenuicincta (M'Coy.)

Orthis calligramma (Dalm.)

Orthis balcletchiensis (Dav.)

Orthis sowerbyana (Dav.)

Rhynchonella nasuta (M'Coy.)

Rhynchonella nucula (Sow.)

Strophomena corrugatella (Dav.)

Strophomena (Rafinesquina) deltoidea (Cour.)

Strophomena semiglobosina (Dav.)

The following are the additional species obtained by Mrs. Gray.

Fistulipora favosa (Nich. & Eth.)

Ampyx sp.

Cheirurus bimucronatus (March.)

Sphaerexochus mirus (Beyr.)

Lingula canadensis (Bill.)

Leptaena sericea var. rhombica (M'Coy.)

Leptaena (Plectambonites) transversalis (Dalm.)

Leptaena youngiana (Dav.)

Orthis Bouchardi var. balcletchiensis (Dav.)

Orthis Lapworthi (Dav.)

Orthis turgida (M'Coy.)

Rhynchonella balcietchiensis (Dav.)

Rhynchonella cuneatella (Dav.)

Rhynchonella girvanensis (Dav.)

Rhynchonella sp.

Strophomena retroflexa (Salt.)

Pleurorhynchus dipterus (Salt.)

Bellerophon sp.

Cyrtolites orbicularis (Lindst.)

Raphistoma aequalis (Salt.)

Maclurea matutina (Hall.)

Cyrtoceras sp.

Phragmoceras sp.

Barbae, Daldowie, and Millenderdale. (Sheets 7 and 8). — From Balclatchie Bridge south-westwards to Tramitchell, considerable difficulty has been found in tracing the outcrop of the Balclatchie group, for in that district the line marking the upper limit of the Benan Conglomerate is very irregular, owing to the reduplication of the strata by folding. Hence long tongues of conglomerate and grit penetrate in anticlinal form the graptolitic shales and mudstones, while the latter lie in synclines of the underlying beds. Professor Lapworth has obtained fossils in grits at various localities along this line of outcrop — as, for instance, near the limestone quarry at Tramitchell [NX 23169 94435], near the farmhouse of Barbae [NX 22486 94525], and in quarries west of the Dalfask Burn. Regarding the outcrop by the roadside near Barbae Farmhouse he states that the grit "there abounds in fragments of quartz, and, except for its decidedly gritty matrix, has few points of resemblance to the typical Balclatchie grit. It contains a few fossils in some shaly seams at its summit: Leptaena, Bellerophon, and Ctenodonta were here collected by myself<ref>Quart. Jour, Geol. Soc., vol. xxxviii., p. 589,</ref>. Another exposure of a similar fossiliferous grit occurs in the Water of Assel, about 700 yards N.N.E. of Pinmerry [NX 24446 95468] , where numerous casts of fossils occur, including corals, brachiopods, and encrinites.

On the western slope of Daldowie Hill [NX 21561 92846], A rapid reduplication of the Benan Conglomerate and overlying mudstones, similar to that near Balclatchie, is observable. So marked is the folding that the observer cannot fail to realise how largely the great development of the conglomerate between the Stinchar and the Assel Water may be due to the plication of the beds. From the shales on Daldowie Hill Professor Lapworth obtained Strophomena corrugatella, Leptaena sericea, Ctenodonta, and a form of Ampyx. Immediately to the east of these highly folded strata (Balclatchie Shales and Benan Conglomerate) occurs a small inlier of volcanic agglomerate bounded on the east side by an outcrop of Stinchar Limestone.

Beyond the valley of the Assel in the direction of Millenderdale, fossiliferous grit, evidently on the horizon of the Balclatchie Grit, is found in quarries on the north side of the road near Meikle Letterpin [NX 20158 91707]. Again, in the south branch of Millederdale Burn, north-west of Knockbain Farmhouse [NX 16084 90280]?, greenish mudstones dip to the north-west at high angles and immediately overlie the Benan Conglomerate, from which the following fossils have been obtained, indicating the horizon of the Balclatchie Mudstones:

Turritepas scotica (Nich. & Eth.)

Illaenus sp.

Remopleurides sp.

Leptaena sericea (Sow.)

Lingula attenuata (Sow.)

Ctenodonta laevis (M'Coy.)

Bellerophon fastigiatus (Lindst.)

Cyclonema sp.

Laggan Gill (Sheet 7). — [NX 20374 94633]On the south-west side of the gorge of Laggan Gill, and about 400 yards west of the farmhouse of Laggan, the Balclatchie shales and grits, as shown by Professor Lapworth, plunge with an inverted dip underneath the Benan Conglomerate. Here the transition zone at the summit of the group is crowded with characteristic graptolites and brachiopoda in a state of fine preservation, the commonest forms recorded by Professor Lapworth being: Cryptograptus tricornis, Lasiograptus Harknessi, Climacograptus bicornis, C. Schärenbergi, Diplograptus rugosus, D. foliaceus, together with species of Trachyderma, Cythere, &c. During the recent revision by the Geological Survey the following graptolites were obtained from limestone nodules in shales, passing with an inverted south-west dip beneath the Balclatchie Grit of Laggan Gill, viz.: Diplograptus foliaceus, Dicranograptus ramosus, Climacograptus Schärenbergi, and Orthoceras.

Ardmillan Braes (two miles south of Girvan). — [NX 16697 94282] Next to the exposures at Balclatchie Bridge the outcrop of the Balclatchie group at Ardmillan Braes is deserving of notice on account of the rich variety of its organic remains. As described on a previous page, the successive bands of Benan Conglomerate are traceable along the hill slopes from Kennedy's Pass by Shiel Hill to the declivity south of Ardwell and Ardmillan. Throughout part of this distance, as in the streams S.S.W. of Ardwell, the conglomerate dips to the north-west at an angle of about 50° underneath the graptolite flagstones, but towards the slope south of Ardmillan House the beds are vertical and in some cases inverted. Here the highly fossiliferous mudstones are exposed in old quarries about half a mile E.S.E. of Ardwell Farmhouse [NX 16819 93968], and a continuous section can be followed northwards in several small streamlets from the underlying Benan Conglomerate to the mudstones. As Professor Lapworth has shown, the conglomerate in the stream sections is succeeded northwards by greenish grey grits, that weather with a purple tint and have a highly inverted dip to the S.S.E.; these are followed by calcareous sandstones, which are very fossiliferous on certain horizons, having yielded to him Remopleurides dorsospirifer, Asaphus gigas, Staurocephalus globiceps, Phacops Brongniarti, Orthis calligramma, O. actoniae, Leptaena tenuicincta, Murchisonia, &c. After a short gap in the section, the green nodular mudstones reappear in the quarries and in the streamlets, with their characteristic lithological and palaeontological features.

Regarding the sequence of the strata visible at Ardmillan Braes, Professor Lapworth makes the following statement:; "The relation of the fossiliferous Balclatchie Sandstones to the Benan Conglomerate at this locality is presumptive of a dislocation between the two, and if the natural sequence contains the same members as at Balclatchie, the beds here in contact with the conglomerate must be the highest beds, the fossiliferous mudstones being the lowest strata of the transitional groups here exposed and owing their great thickness to their being arranged in anticlinal form. The hiatus in the succession would naturally be filled with graptolitiferous mudstones, over which follow the shell-bearing gritstones in their proper sequence. If this be the true interpretation of the visible phenomena, it follows that the shell-bearing gritstones should be repeated between these quarries and the graptolitic flagstones visible in the roadway, or must be again cut out by a fault. That the latter supposition is in all probability the correct one is evident from the fact that where the sequence is unbroken, a quarter of a mile to the westward, it is identical with that at Balclatchie, as is also the case in Ardmillan Burn, about half a mile further to the east"<ref>Quart. Jour. Geol. Soc., vol. xxxviii., p. 591.</ref>.

The following list gives the fossils in the collection of the Geological Survey from the Balclatchie beds, Ardmillan Braes:

Stenopora (Favosites) fibrosa (Goldf.)

Petraia elongata (Phil.)

Asaphus (Isotelus) gigas (Dekay.)

Asaphus (Isotelus) rectifrons (Portl.)

Bronteus hibernicus (Portl.)

Harpes sp.

Illaenus Bowmani (Salt.)

Phacops (Acaste) Brongniarti (Portl.)

Phacops Dalmani (Portl.)

Staurocephalus globiceps (Portl.)

Stygina latifrons (Portl.)

Trinucleus sp.

Acrotreta Nicholsoni (Dav.)

Discina oblongata (Portl.)

Discina oblongata var.

Leptaena (scissa)=llandeiloensis (Dav.)

Leptaena sericea (Sow.)

Leptaena (Orthis) Rankini (Dav.)

Leptaena (Christiania) tenuicincta (M'Coy.)

Orthis (Dalmanella) testudinaria (Palm.)

Orthis calligramma (Dalm.)

Orthis calligramma var. virgata (Sow.)

Orthis (Dalmanella) elegantula (Palm.)

Rhynchonella cuneatella (May.)

Strophomena arenacea (Salt.)

Strophomena (Rafinesquina) expansa (Sow.)

Strophomena grandis (Sow.)

Ambonychia sp.

Arca edmondimformis (M'Coy.)

Ctenodonta sp. (4)

Cucullella sp.

Modiolopsis modiolaris (Cony.)

Orthonota sp. (2)

Bellerophon acutus (Sow.)

Bellerophon carinatus (Sow.)

Bellerophon trilobatus (Sow.)

Cyclonema crebristrium (M'Coy.)

Ecculiomphalus minor (Portl.)

Ecculiomphalus scoticus (M'Coy.)

Holopea sp.

Holopella sp.

Maclurea sp.

Murchisonia sp.

Ophileta compacta (Salt.)

Raphistoma lenticularis (Sow.)

Conularia elongata (Portl.)

Hyolithes (Theca) reversus (Salt.)

Hyolithes triangularis (Salt.)

Hyolithes vaginulus (Salt.)

Orthoceras ibex (Sow.)

Orthoceras imbricatum (Wahl.)

The following supplementary list of fossils shews the forms obtained by Mrs Gray in addition to those given in the foregoing list:

Dicellograptus moffatensis (Carr.)

Dicranograptus tardiusculus (Lapw.)

Diplograptus sp.

Glyptocrinus sp.

Ptilodictya dichotoma (Portl.)

Turrilepas scotica (Eth. & Nich.)

Pinnocaris Lapworthi (Eth. jun.)

Agnostus agnostiformis(M'Coy.)

Ampyx Hornei (Eth. & Nich.)

Bronteopsis scotica (Salt.)

Cheirurus sp.

Cybele verrucosa (Dalm.)

Crania sp.

Discina perrugata (M'Coy.)

Leptaena llandeiloensis (Dav.)

Leptaena (Plectambonites) quingue-costata (M'Coy.)

Lingula attenuata (Sow.)

Lingula brevis (Portl.)

Lingula granulata (Phill.)

Orthis calligramma (Dalm.)

Orthis calligramma  var. plicata (Sow.)

Orthis Carausii (Salt.)

Orthis crispa (M'Coy.)

Orthis (Dinorthis) flabellulum (Sow.)

Orthis kilbuchoensis (Dav.)

Orthis sp.

Rhynchonella ardmillanensis (Dav.)

Rhynchonella sp.

Siphonotreta micula  (M'Coy.)

Strophomena compressa (Sow.)

Strophomena corrugatella (Dav.)

Strophomena (Rafinesquina) deltoidea (Conr.)

Strophomena insculpta (Hall.)

Strophomena llandeiloensis (Dav.)

Strophomena retroflexa (Salt.)

Strophomena sp.

Triplesia spinferoicles (M'Coy.)

Ambonychia radiata (Emm.)

Ambonychia sp.

Asicula sp.

Ctenodonta astartaeformis (Salt.)

Ctenodonta varicosa (Salt.)

Goniophora sp.

Bellerophon bilobatus (Sow.)

Bellerophon expansus (Sow.)

Bellerophon sp.

Bucania sp.

Ecculiomphalus Bucklandi (Portl.)

Pleurotomaria trochiformis (Portl.)

Raphistorna sp.

Pterotheca transverse (Salt.)

Orthoceras Avelini (Salt.)

Dow Hill, (Sheet 7). — [NX 19158 96087] In a quarry on the northern slope of the arch of Benan Conglomerate on Dow Hill, a mile and a half south of Girvan Station, the Balclatchie Mudstones occupy their normal position and yield their characteristic fossils. The fossils given in the subjoined list have been collected there by the Geological Survey.

Agnostus agnostiformis (M'Coy.)

Ampyx Maccallumi (Salt.)

Asaphus (Isotelus) gigas (Dekay.)

Cybele sp.

Phacops (Acaste) Brongniarti (Portl.)

Remopleurides Barrandei (Eth. & Nich.)

Remopleurides laterispirifer (Portl.)

Staurocephalus unicus (Wyv. Thom.)

Trinucleus ornatus (Stemb.)

The following additional forms have been obtained by Mrs. Gray from the same locality:

Diplograptus sp.

Glyptocrinas sp.

Ptilodictya dichotoma (Port].)

Ptilodictya sp.

Tentaculites sp.

Turrilepas scotica (Eth. & Nich.)

Ampyx Hornei (Eth. & Nich.)

Cybele verrucosa (Dalm.)

Lichas sp.

Staurocephalus globiceps (Wyv. Thom.)

Trinucleus concentricus (Eaton.)

Discina oblongata (Portl.)

Lingula attenuata (Sow.)

Orthis (Dalmanella) testudinaria (Dalm.)

Strophomena corrugatella (Dav.)

Strophomena retroflexa (Salt.)

Strophomena (Rafinesquina) deltoidea (Conr.)

Rhynchonella sp.

Ambonychia sp.

Ctenodonta astartaeformis (Salt.)

Bellerophon bilobatus (Sow.)

Bucania sp.

Raphistoma sp.

Helminthochiton sp.

Ecculiomphalus Bucklandi (Portl.)

Conularia elongata (Portl.)

Hyolithes reversus (Portl.)

Endoceras strangulatum (Hall.)

Orthoceras subundulatum (M'Coy.)

Ardmillan Shore. — Having indicated the chief localities where the highly fossiliferous Balclatchie beds are visible, we may now describe two prominent sections on the south side of the Girvan valley, which display the three succeeding subdivisions of the Caradoc strata, viz.: the Ardwell group, the Whitehouse group, and the Barren Flagstone group. The first of these, which extends along the shore from Kennedy's Pass to Shalloch Mill — a distance of about three miles — has been chosen by Professor Lapworth as a typical section of the graptolitic flagstone series (Figure 113)). The second occurs in Penwhapple Glen, to the north-west of Balclatchie Bridge.

ii. Ardwell Group

The Ardmillan shore section (Figure 113), [NX 15118 93527], [NX 16906 95300] furnishes at low tide a tolerably continuous sequence of these groups. At the northern limit of Kennedy's Pass the Benan Conglomerate is followed immediately by the basal zones of the Ardwell group. Though every bed at the line of contact is visible, there is here no representative of the intervening Balclatchie group, which is met with, as already indicated, about a mile and a half to the east on Ardmillan Braes.

The lowest zones consist of thin dark grey slightly carbonaceous shales with hard grey ribs, from which' Professor Lapworth has collected the following graptolites:; Diplograptus foliaceus, D. pristis, D. rugosus, and Climacograptus bicornis. These strata are followed by flaggy shales passing upwards at the farm of Ardwell into flagstones with partings of dark grey shale. From these beds the same observer obtained in addition to the fossils of the lower zones: Dicranograptus ramosus, Corynoides calycularis, Ecculiomphalus Bucklandi, and an occasional brachiopod. The highest members visible on the beach at Ardwell consist of banded shales with alternating dark and grey seams charged with Climacograptus caudatus and Diplograptus rugosus (3b⁶, (Figure 113)).

Though the section from Kennedy's Pass to Ardwell Bay does not show the Cascade beds, which Professor Lapworth regards as the highest sub-zone of the Ardwell group, it affords excellent opportunities for studying the peculiar types of the graptolitic flagstones and the rapid folding of the strata (Plate 17). This striped flagstone series includes the representatives of only a portion of theo Lower Hartfell black shale of the Southern Uplands, and by means of the zonal form Climacograptus caudatus it is possible to fix the stratigraphical horizon of the beds in Ardwell Bay relatively to the Moffat sequence. Instead of a limited thickness of black shales, we have here a succession of striped flagstones and shales, which is probably not far short of 1000 feet in thickness.

iii. Whitehouse Group

From Ardwell Bay [NX 15991 94219] north-eastwards along the shore to Shalloch Mill [NX 17815 95698], the members of this group are well exposed. They have been arranged by Professor Lapworth in two sub-groups. The lower consists of (1) grey and green carbonaceous shales, yielding Dicellograptus Forchhammeri, followed by (2) grey flagstones with ribs of impure fossiliferous limestone, enclosing specimens of Leptaena transversalis, L. sericea, Orthis calligramma, Strophomena sp.; (3) soft green mudstones with hard siliceous ribs. These sub-zones are displayed on the beach northwards from Ardwell Bay, the beds being vertical or highly inclined, and in some cases inverted. At the third milestone from Girvan, beside the old ruin of Whitehouse, the variegated mudstones, shales, flags, and calcareous bands are well seen that form the upper sub-group and contain certain highly fossiliferous seams. The lowest beds of the upper sub-group are readily recognised from the presence of a zone of bright purple mudstones, conspicuous on the beach at low tide, which are associated with alternating green and purple mudstones and shales. These strata are destitute of fossils, but they pass upwards into calcareous grits, flagstones, and shales, with black shale seams some o the bands being highly fossiliferous.

The fossils given in the following list have been obtained by Mrs Grey from this group in Whitehouse Bay [NX 16412 94655]?.

Dicellograptus Morrisi (Hopk.)

Diplograptus quadrimucronatus (Hall.)

Diplograptus socialis (Lapw.)

Diplograptus truncatus (Lapw.)

Serpulites longissimus (March.)

Turrilepas Peachi (Eth. jr.)

Aparchites leperditioides (Jones.)

Aparchites subovatus (Jones.)

Beyrichia Kloedeni var. infecta (Jones.)

Beyrichia impar (Jones.)

Cypidina Grayae (Jones.)

Primitia girvanensis (Jones.)

Primitia Grayae (Jones.)

Primitia Krausei (Jones.)

Primitia mundula var. fimbriatus (Jones.)

Primitia nuda (Jones.)

Primitia ulrichiana.

Sulcuma prceurrens (Jones.)

Ulrichia girvanensis (Jones.)

Agnostus perrugatus (Barr.)

Ampyx sp.

Bohemillasp.

Cyclopyye armata (Barr.)

Cyclopyye rediviva (Barr.)

Cybele sp.

Dindymene Cordai (Eth. & Nich.)

Dionide Lapworthi (Eth. & Nich.)

Trinucleus sp.

Athyris sp.

Lingula attenuata (Sow.)

Lingula brevis (Portl.)

Orthis sp.

Euomphalus sp.

Hyolithes (Theca) triangularis (Portl.)

Hyolithes (Theca) reversus (Salt.)

Orthoceras bullatum (Sow.)

To the north of the old ruin of Whitehouse, the Upper Whitehouse beds are followed seawards by the Barren Flagstones of the succeeding group, but their strike being nearly parallel to the shore, they can be traced at intervals north-eastwards along the margin of the beach to Port Cardloch [NX 16723 94958], thence across Woodland Point [NX 16980 95288] to Woodland Bay, and onwards to Myoch Bay [NX 17779 95718], opposite Shalloch Mill and south of Craigskelly where a fine development of the upper sub-division has been minutely described by Professor Lapworth (Quart. Jour. Geol. Soc., vol. XXXVIII., p. 599). The relations of the strata are here somewhat complicated owing to inversion and folding. The overlying Barren Fagstones dip south-eastwards underneath the Upper Whitehouse beds, showing how misleading is evidence based merely on local relative superposition. In the centre of Myoch Bay, between Shalloch Mill and the mouth of Byne Hill Burn, the beds are highly contorted, but towards the south-west the sub-zones of purple and variegated mudstones are recognisable, and are succeeded northwards by the richly fossiliferous bands that form the highest sub-division of the Upper Whitehouse beds. The latter, as developed on the beach opposite Shalloch Mill, are thus described by Professor Lapworth.

" The final or fossiliferous sub-group is greatly contorted and broken, but when carefully mapped in detail, the following succession is easily made out:

"(1) Dicellograptus-complanatus zone. Black shales, highly carbonaceous, with a few seams of grey mudstone and calcareous grit, crowded with graptolites (5 feet). Diplograptus socialis (Lapw.) is the commonest form, and occurs in hosts. Less frequent are the forms in the following list:

Climacograptus tubuliferus (Lap.)

Dicellograptus complanatus (Lapw.)

Dicellograptus Morrisi (Hoek.)

Theca triangularis (Portl.)

Lingula sp."

"(2) Dictyonema-zone. Hard Baggy shales (9 feet) with ribs of grey calcareous rocks. In certain zones these beds contain an abundance of Dictyonema, together with numerous fragments of Phyllopoda, Lingulidae, and Diplograptidae.

"The characteristic forms are Dictyonema, Ganocladium, Lingula.

"(3) Dionide-beds. Finally we have a thickness of about six feet of sandstones, containing such a large proportion of calcareous matter that in places they rather deserve the title of impure nodular limestones. They afford a large and varied association of fossils, some of which are beautifully preserved".

The fossils given in the subjoined list have been collected by Mrs. Gray from these fossiliferous bands on the beach opposite Shalloch Mill:

Climacograptus tubuliferus (Lapw.)

Diplograptus sp.

Halysites catenulatus (Linn.)

Halysites sp.

Glyptocrinus sp.

Nematolites Grayi (Lapw.)

Tentaculites anglicus (Salt.)

Fenestella sp.

Calymene Blumenbachi (Brong.)

Calymene sp.

Cybele sp.

Illaenus sp.

Staurocephalus globiceps (Portl.)

Trinucleus sp.

Discina perrugata (M'Coy.)

Leptaena segmentum, (Aug.)

Leptaena (Plectambonites) quinque-costata (M'Coy.)

Leptaena sericea (Sow.)

Leptaena (Christiania) tenuicincta (M'Coy.)

Leptaena (Plectambonites) transversalis (Wahl.)

Lingula attenuata (Sow.)

Lingula brevis (Portl.)

Lingula quadrata (Eichw.)

Orthis (Bilobites) biloba (Linn.)

Orthis calligramma (Dalm.)

Orthis (Dalmanella) elegantula (Dalm.)

Orthis Lapworthi (Dav.)

Orthis sowerbyana (Dav.)

Orthis (Dalmanella) testudinaria (Dalm.)

Orthis vespertilio (Sow.)

Pentamerus shallockiensis (Dav.)

Porambonites intercedens (Pand.)

Crania (Pholidops) implicata (Sow.)

Rhynchonella shallockiensis (Dav.)

Skenidium shallockiensis (Dav.)

Strophomena corrugatella (Dav.)

Strophomena rhomboidalis (Wilck.)

Strophomena shallockiensis (Dav.)

Strophomena simulans (M'Coy.)

Ambonychia radiata (Emm.)

Mytilus gradatus (Salt.)

Bellerophon argo (Bill.)

Bellerophon carinatus (Sow.)

Bellerophon sp.

Cyclonema carinatum (Sow.)

Ecculiomphalus Bucklandi (Portl.)

Ecculiomphalus laevis (Sow.)

Euomphalus funatus (Sow.)

Metoptoma sp.

Murchisonia (Pleurotomaria) bicincta (Hall.)

Ophileta compacta (Salt.)

Ophileta Nerine (Bill.)

Ortostoma discors (Sow.)

Pleurotomaria sp.

Orthoceras arcuoliratum (Hall.)

Orthoceras bilineatum (Hall.)

Orthoceras gracile (Portl.)

Orthoceras ludense (Sow.)

Orthoceras vagans (Salt.)

Near the mouth of the Byne Hill Burn, close to Shalloch Mill, from an outcrop of soft blue flaggy mudstones with black carbonaceous seams, Professor Lapworth obtained an important suite of graptolites characteristic of the Pleurograptus linearis zone, which are given in the following list:

Leptograptus flaccidus (Hall.)

Pleurograptus linearis (Carr.)

Dicellograptus Morrisi (Hopk.)

Climacograptus tubuliferus (Lapw.)

Diplograptus quadrimucronatus (Hall.)

Diplograptus foliaceus (His.)

Regarding these beds the same observer states that they "are of the same general character as those of the majority of the Whitehouse group, to which they unquestionably belong; but the absence of definite ribs of flagstone from among the soft blue shaly mudstones, of which these Byne Hill beds are made up, will not allow us to parallel the little group satisfactorily with any of the zones already described".

iv. Barren Flagstone Group

The next group of strata above the Whitehouse beds, consists of alternations of flagstones and shales with greywacke bands, and is singularly destitute of organic remains. It is traceable along the shore in a nearly vertical position from Whitehouse [NX 16387 94668] to near Shalloch Mill [NX 16387 94668], being seen to advantage in Woodland Bay, where the thick flaggy greywackes of the middle zones are displayed. The shales at the base have yielded Nematolites Grayi — a form which, according to Professor Lapworth, "preeminently distinguishes this group throughout the Girvan region". These are the highest visible beds of the Caradoc formation on the Ardmillan shore, for the promontory at Woodland and the rocky islets at Craigskelly, north of Shalloch Mill, are formed of Llandovery strata, which are here thrown down by a prominent strike fault running parallel to the shore.

Penwhapple Glen. — [NX 23060 98248] The rocky gorge formed by the Penwhapple Burn affords a continuous section of the Ardwell, Whitehouse, and Barren Flagstone groups, which cover a considerable part of the plateau south of the Girvan valley between Millenderdale and the south slope of Hadyard Hill (Figure 114). The richly fossiliferous mudstones and grits at Balclatchie are immediately overlain by carbonaceous shales, charged with Climacograptus Schärenbergi, Lasiograptus Harknessi, and Cryptograptus tricornis, forming the basal zone of the Ardwell beds. Next in order come thin iron-stained shales and flaggy shales (Knockgerran), which, though in places much contorted, dip generally to the north-west, and have yielded to Professor Lapworth: Diplograptus foliaceus, D. rugosus, Climacograptus Schärenbergi, C. caelatus var. antiquus, Corynoides calycularis, and Dicellograptus moffatensis (?). These lower zones contain some calcareous bands and nodules, which have furnished to Mrs. Gray, Mr. M'Kie of Knockgerran, and to Professor Lapworth the following forms: Orthoceras angulatum, O. calamiteum, O. politum, and Hyalostelia girvanense.

The shaly sub-division is followed by more flaggy beds with occasional bands of grit, and these are in turn succeeded by iron-stained shales, yielding Dicellograptus Morrisi, Diplograptus rugosus, and Climacograptus bicornis. Further down the stream, massive green pebbly grits which form two prominent waterfalls, are associated with dark shales and mudstones, including calcareous bands and nodules. These beds are inverted, having a persistent dip to the south-east, as if plunging underneath the graptolitic flagstones to the south. On account of the physical features to which they give rise they have been termed tie Cascade-beds by Professor Lapworth, who has obtained from them the following graptolites, which place the horizon beyond doubt: Dicellograptus Forchhammeri, Lasiograptus margaritatus, Diplograptus pristis, Diplograptus foliaceus, Climacograptus caudatus, and Dicranograptus ramosus (3b⁴, (Figure 114)).

The members of the Whitehouse group extend down the stream from the lower waterfall for a distance of about half a mile, with a more or less persistent dip to the south-east, which is due to inversion of the beds. It is clear, however, that the strata are repeated by innumerable folds, and that the apparently considerable thickness is entirely misleading. The beds of the lower sub-division consist of dark grey shales with black seams and occasional courses of greywacke and cement-stone. From the shales Professor Lapworth obtained the following graptolites, characteristic of the Pleurograptus linearis zone of the Hartfell Shales, which enabled him to correlate the beds with the soft mudstones at the mouth of the Byne Hill Burn on the Ardmillan shore (3c¹):

Pleurograptus linearis (Carr.)

Leptograptus flaccidus (Hall.)

Climacograptus tubuliferus (Lapw.)

Corynoides calycularis (Nich.)

Diplograptus quadrimucronatus (Hall.)

Diplograptus foliaceus (Murch.)

Diplograptus truncatus (Lapw.)

The members of the upper sub-division consist of dark blue-green mudstones, followed by a few feet of variegated mudstones with calcareous bands, the latter yielding some of the fossils which are found on shore in the beds of this horizon. Further down the stream, a succession of Barren flagstones, shales, and mudstones (3d), continues for a distance of three-quarters of a mile. The strata are remarkably destitute of fossils, but near the base of the series certain green mudstones, south of the foot of Laigh Assel Burn, yield specimens of Nematolites Grayi. Owing to inversion, the beds have the same persistent dip to the south-east, as if they passed underneath the representatives of the Whitehouse group, but there can be little doubt that this part of the Penwhapple section displays the equivalents of the Barren Flagstone group of Woodland Bay on the Ardmillan shore. At a point about a quarter of a mile north of Penwhapple Bridge, the Barren Flagstones are truncated by a reversed fault which brings them against graptolite shales of Llandovery age ((Figure 114), 4b¹).

Additional exposures south of the Girvan Valley. — in addition to the two main sections of the Ardwell, Whitehouse, and Barren Flagstone groups, now described, several sections may here be noticed where one or more of these subdivisions is well displayed on the plateau south of the Girvan valley. For our present purpose it will be sufficient to state generally the distribution of some of these sub-groups, referring at the same time to the fossiliferous localities.

In the railway cuttings between Girvan and the valley of the Stinchar to the north of Pinmore [NX 20355 92223], and in various natural exposures near Meikle Letterpin [NX 19861 91512], the flagstones and shales of the Ardwell group charged with their characteristic graptolites are well developed. Again, along the northern margin of the Caradoc area, on the slopes of, the Fauldribban Hill [NX 20616 96799], in the Piedmont Glen, on Trowier Hill [NX 20045 95902]?, and in the various streams near Tralodden [NX 22699 96272], representatives of the graptolitic flagstones appear. At most of these localities, in the dark seams associated with the flags, several of the graptolites found in the Ardwell group are obtainable. In like manner the beds of this group can be traced north-eastwards from Knockgerran [NX 24867 97493] to Dalamford [NX 29144 99339], where they form a narrow strip, bounded on both sides by Lower Old Red Sandstone strata. At Dalamford the black shales, which are there associated with calcareous grits and sandstones, have yielded to Professor Lapworth Climacograptus caudatus, Dicellograptus Forchhammeri, Diplograptus foliaceus, D. rugosus, Lingula, &c., probably belonging to the horizon of the cascade-beds of the Ardwell group.

Caradoc Strata of the Craighead Inlier. — On the north side of the Girvan valley Caradoc strata not only form a large part of the "inlier" of silurian rocks, but possess there a special interest and importance from the presence of richly fossiliferous mulstones with trilobites and the starfish band at the top of the series (p. 508). These form the important zones of the highest group (Drummuck) in the Caradoc series of Girvan. That the representatives of this sub-division have not been detected anywhere to the south of the Girvan valley, probably points to the conclusion that the original development of the Drummuck group was comparatively limited and did not extend southwards over the region that now forms the plateau between the Girvan and the Stinchar.

In the Geological Map (Sheet 14, one-inch), and in the Explanation accompanying that sheet, it was shown by Sir A. Geikie that the Caradoc strata of the Craighead inlier are arranged in the form of an anticline, which is truncated on the south side by the great strike-fault that bounds the Carboniferous strata of the Girvan valley. The trilobite mudstones of Drum-muck and Lady Burn were regarded as the highest beds of Caradoc age in that limited area, being followed by the Mulloch Hill conglomerate at the base of the Llandovery strata.

The subsequent researches of Professor Lapworth revealed that the important Starfish horizon intervenes between the trilobite mudstones of Lady Burn and the Mulloch Hill conglomerate, and they further showed that the core of the arch of Caradoc strata north-east of Craighead is formed of representatives of the Barren Flagstone group. These younger Caradoc beds were believed by Professor Lapworth to be everywhere faulted against the older series at Craighead and Killochan, comprising the Arenig volcanic rocks, radiolarian cherts and mudstones, and the Craighead Limestone group.

The evidence regarding the distribution of the strata between Craighead and Quarrel Hill is to a great extent concealed by the covering of superficial deposits. Indeed, it is confined mainly to some exposures of rock near Blair and to two streams that traverse the southern limb of the anticline between the farmhouse of Farden and Quarrel Hill. The more easterly of these two burns — the one west of Quarrel Hill — gives an excellent transverse section of the representatives of this group. Near the head of the stream the strata, consisting of flagstones and shales and yielding Nematolites Grayi, have an easterly dip on the crest of the arch; further down, towards the little wood above Auldthorns, they are inclined to the south-east at an angle of 70°, and are composed of thin-bedded hard grey micaceous grits with bands of shale. Southwards, beyond the little wood, pale flagstones and mudstones are visible, traversed by small faults that disturb the natural sequence. Near a bend in the burn, north of the farmhouse of Auldthorns, the trilobite mudstones of the overlying Drummock group are exposed, and close to the Carboniferous boundary fault the Mulloch Hill Conglomerate crops out at the base of the Llandovery series. Similar evidence is obtained in the burn near the farmhouse of Farden, where the Barren Flagstones appear on the southern limb of the anticline. Westwards at Blair, however, the northern limb of the fold is reached for in an old quarry near that locality the flagstones, grits, and shales of this group are exposed with a northerly dip.

v. Drummuck Group

The members of this important group can be traced more or less continuously as a narrow band round the outer margin of the arch of Caradoc rocks, from Drummuck on the northern limb of the fold eastwards along the course of Lady Burn, southwards round the dopes of Quarrel Hill, and westwards towards Auldthorns. The lowest beds consist of pebbly grits, sandstones, olive mudstones, and shales. They are exposed on a crag on the crestof Quarrel Hill, where they dip to the east at an angle of 30°. In the burn section to the west, as already indicated, these basal grits are underlain by the Barren Flagstones, while in the streamlets that drain the eastern slope of Quarrel Hill they are seen to be succeeded by the trilobite mudstones. The most important feature of the group, however, is the succession of blue and grey mudstones, which have become widely known owing to the abundance and perfect preservation of their trilobite fauna. Weathering with a yellow or rusty colour, and possessing a marked concretionary or nodular structure, they break under the hammer into angular fragments. They are exposed at intervals along the course of the Lady Burn for about a mile east of Drummuck Farmhouse, where their general inclination is to the north and north-west at angles varying from 20° to 30°. One of the highly fossiliferous localities occurs on the banks of the stream opposite the farmhouse of South Threave, where Mrs. Gray collected the zonal graptolite Dicellograptus anceps, in association with the trilobites. The subjoined list of fossils gives the forms in the collection of the Geological Survey.

Diplograptus truncatus (Lapw.)

Serpulites longissimus (Murch.)

Ampyx rostratus (Sara.)

Calymene Blumenbachi (Brong.)

Calymene tuberculosa (Salt.)

Cheirurus clavifrons (Dalm.)

Cheirurus bimucronatus (Murch.)

Cybele rugosa (Portl.)

Dindymene Cordai (Eth. & Nich.)

Dionide Lapworthi (Eth. & Nich.)

IIlaenus (Bumastus) Bowmani (Salt.)

IIlaenus nexilis (Salt.)

Lichas Geikiei (Eth. & Nich.)

Phacops (Acaste) Brongniarti (Portl.)

Proetus girvanensis (Eth. & Nich.)

Proetus latifrons (M'Coy.)

Proetus procerus (Eth. & Nich.)

Staurocephalus globiceps (Portl.)

Trinucleus Bucklandi (Barr.)

Trinucleus seticornis (His.)

Trinucleus sp.

Discina oblongata (Portl.)

Leptaena sericea (Sow.)

(Plectambonites) transversalis (Wahl.)

Lingula attenuata (Sow.)

Lingula ovata (M'Coy.)

Lingula sp.

Merista cymbula (Dav.)

Orthis (Dalmanella) elegantula (Dalm.)

Orthis (Dinorthis) flabellulum (Sow.)

Orthis rustica var. rigida, (Dav.)

Spirifera crispa (His.)

Ctenodonta sp.

Orthonota sp.

Bellerophon acutus (Sow.)

Bellerophon expansus (Sow.)

Bellerophon bilobatus (Sow.)

Bellerophon carinatus (Sow.)

Bellerophon dilatatus (Sow.)

Bellerophon perturbatus (Sow.)

Bellerophon trilobatus (Sow.)

Holopella obsoleta (Sow.)

Loxonema sinuosum (Sow.)

Loxonema sp.

Maclurea Logani (Salt.) (young of.)

Murchisonia pulchra (M'Coy.)

Murchisonia sp.

Pleurotomaria aequilatera (Wahl.)

Pleurotomaria alata (Wahl.)

Pleurotomaria (Raphistoma) lenticularis (Sow.)

Pleurotomaria turrita (Portl.)

Pleurotomaria sp.

Tryblidium (Metoptoma) sp.

Comularia elongata (Portl.)

Hyolithes (Theca) corrugata (Salt.)

Hyolithes (Theca) triangularis (Portl.)

Hyolithes (Theca) vaginulus (Salt.)

Hyolithes sp.

Orthoceras argus (Barr.)

Orthoceras pendens (Blake.)

Orthoceras politum (M'Coy.)

Orthoceras sp.

In addition to the species collected by the Survey, the following species have been obtained by Mrs. Gray.

Dicellograptus anceps (Nich.)

Diplograptus sp.

Beyrichia comma (Jones.)

Beyrichia Kloedeni (M'Coy.)

Solenocaris solenoides (Young.)

Turrilepas sp.

Dionide sp.

Lichas laxatus (M'Coy.)

Phillipsia parabola (Barr.)

Crania sp.

Orthis mullockiensis (Dav.)

Paterula balcletchiensis (Dav.)

Strophomena grandis (Sow.)

Strophomena rhomboidalis (Wilck.)

Acroculia (Platyceras) haliotis (Sow.)

Bellerophon argo (Bill.)

Bellerophon elongatus (Portl.) = bilobatus (Sow.)

Bellerophon subdecussatus (M'Coy.)

Bellerophon Urei (Flem.)

Chelodes (Helminthochiton) Grayi (Woodw.)

Euomphalus sp.

Holopea lymnaeoides (Forbes.)

Loxonema elegans (M' Coy.)

Murchisonia obscura (Portl.)

Ophileta compacta (Salt.)

Platyschisma helicites (Sow.)

Raphistoma planistria var. parva (Hall.)

Conularia bilineata (Lindst.)

Hyolithes (Theca) acutus (Eiehw.)

Hyolithes (Theca) elegans (Barr.)

Hyolithes reversus (Portl.)

Pterotheca sp.

Endoceras proteiforme var. strangulatum (Hall.)

Orthoceras angulatum (Wahl.)

Orthoceras ludense (Sow.)

Orthoceras vagans (Salt.)

The highly fossiliferous trilobite mudstones of Drummuck are succeeded near the head of Lady Burn by the Starfish beds, the position of which has been defined by Professor Lapworth and Mrs. Gray. The following description of the Starfish strata and of their outcrop in the field is riven by Professor Lapworth. "Near the head of the Lady Burn, we find the purple Mulloch Hill Conglomerate crossing the little stream valley almost at right angles to the course of the burn. Over this intractable rock the waters of the stream leap in a small waterfall, at the base of which the highest known strata of the Trinucleus or Drummuck beds are seen, dipping steadily and conformably underneath the conglomerate at an angle of about 40°. They are soft, blue mudstones, homogeneous, thick-bedded, and more or less concretionary in structure, breaking up under the hammer into irregular and crumbling fragments. Exteriorly they are stained with rusty oxide of iron; interiorly they are pierced by frequent inosculating worm burrows, stained of a dingy red. Fossils are very rare; only an occasional brachiopod is discernible.

"At their base, however, they contain a fossiliferous band, the abundant organic remains of which fully compensate for the barren nature of the beds above. Fragments of this fossiliferous band are exposed in an old quarry opened for procuring materials for the neighbouring stone wall, in which an occasional slab from the fossil seam may even yet be detected. The bed itself is a hard, greenish-grey sandstone, a few inches in thickness, and highly calcareous. It is almost made up of fossil remains, many being in an excellent state of preservation"<ref>Quart. Jour. Geol. Soc., vol. xxxviii., p. 619.</ref>.

The fossils given in the annexed list have been obtained by Mrs. Gray from this locality:

Diplograptus sp.

Halysites catenularia (Linn.)

Protocystites sp.

Protocystites sp.

Glyptocrinus sp.

Tetraster Wyville-Thomsoni (Nich. & Eth.)

Tetraster sp.

Solenocaris solenoides (Young.)

Solenocaris sp.

Ampyx rostratus (Sars.)

Asaphus marginalia (Portl.)

Calymene Blumenbachi (Brong.)

Phillipsia parabola (Barr.)

Staurocephalus globiceps (Wyy.-Thom.)

Trinucleus Bucklandi (Barr.)

Crania sp.

Leptaena sericea (Sow.)

Leptaena transversalis (Dalm.)

Merista cymbula (Dav.)

Orthis Actoniae (Sow.)

Orthis crispa (M'Coy.)

Orthis (Dinorthis) flabellulum (Sow.)

Orthis mullockiensis (Dav.)

Orthis Rankini (Dav.)

Orthis sowerbyana (Dav.)

Orthis sp.

Paterula batcletchiensis (Dav.)

Rhynchonella sp.

Skenidium Grayi (Dav.)

Strophomena (Leptaena) rhomboidalis (Wilck.)

Triplesia spiriferoides (M'Coy.)

Ctenodonta Eastnori (Murch.)

Orthonota prora (Salt.)

Pterinea Sowerbyi (M'Coy.)

Bellerophon bilobatus (Sow.)

Conularia bilineata (Lindst.)

Cyrtoceras victor (Barr.)

Orthoceras imbricatum (Wahl.)