Chapter 19 III. — The Girvan district. Arenig-Llandeilo Volcanic Series of Ballantrae and associated sediments

In approaching the study of the stratigraphical arrangement of the volcanic rocks of the Ballantrae region, and their relation to volcanic zones occupying similar horizons throughout the Southern Uplands, it is obviously of importance to begin with those sections that reveal a sequence from the Arenig lavas and tuffs with the overlying cherts to those blue-black mudstones which yield an undoubted assemblage of Glenkiln (Upper Llandeilo) graptolites. Several sections of this nature occur on the shore between Glen App and the valley of the Stinchar; some at the margin of the main volcanic area north of the Currarie Glen, and one at Portandea, near Glen App, where the lavas and tuffs form a small inlier among younger sediments.

Area between Glen App and Ballantrae

Portandea Section (Sheet 7 of the Survey Map). — [NX 04775 75326] About two miles round the shore to the north of Glen App, the relations of the strata are clearly displayed at Portandea — a small bay, where the sea laves the base of a cliff from 150 to 300 feet in height. On the north side of this bay a sloping beach, from thirty to forty yards broad, merges upwards into a raised beach that separates a ridge of rock on the seaward side from the main line of sea cliff. The hollow occupied by the raised beach runs northwards for a distance of 350 yards (Figure 99).

On the north side of the bay and along the eastern limit of the sandy beach, the volcanic rocks appear for a space which from north to south measures about 100 feet. The lowest visible bed rising from the sandy beach is an amygdaloidal diabase-lava of a green tint with small vesicles, and showing the pillowy structure already so often referred to and so conspicuous on the Downan shore, to be described presently. It encloses occasional blocks or patches of red mudstone and chert, and the pillow-shaped masses of lava are surrounded by calcareous matter. This bed of lava, which is about twenty-one feet thick (1 B), is overlain by a band of fine tuff, followed by alternations of coarse and fine agglomerate or breccia (1 Ts.), which contain numerous small angular fragments of red chert. Higher up the cliff the agglomerate is succeeded by twenty-seven feet of fine-grained slaggy lava resembling the lower bed.

Overlying this volcanic zone, red radiolarian cherts and mudstones (1 and 2 C, (Figure 99)) are visible on the cliff and on a prominent stack to the south-east of the volcanic rocks. The upper bed of lava dies out southwards, and the red cherts and mudstones, which rest directly on the agglomerates, are succeeded by green mudstones; the united thickness of the red and green beds being upwards of forty feet. They dip to the south-east, and are followed in normal order by green mudstones and greywackes, which include a band of dark or blue-black mudstone that breaks with an irregularly conchoidal fracture (2a). This recognisable bed, exposed about fifty yards to the south-east of the volcanic rocks, is charged with graptolites characteristic of the Glenkiln division as well as with small brachiopods; the former, though stunted in development, are nevertheless well preserved and easily determinable. The fossils given in the annexed list were here obtained:

Dicranograptus zic-zac var. minimus (Lapw.)

Caenograptus pertenuis (Lapw.)

Diplograptus euglyphus (Lapw.)

Climacograptus Schärenbergi (Lapw.)

Dicellograptus sextans (Hall.)

Dicellograptus moffatensis (Carr.)

Dicellograptus patulosus (Lapw.)

Dendrograptus sp.

Lingulella lepis (Salt.)

Acrothele granulata (Linnr.)

Obolella sp.

Discina sp.

Sponge-like body.

The occurrence of fragments of Dendrograptus in this assemblage is worthy of note, as this genus rarely appears in the Glenkiln Shales throughout the uplands; still it has been recorded from that horizon in the Snar Water, near Snar Castle (see p. 321), in the Leadhills area.

The strata in the section at Portandea, from the Arenig volcanic rocks, radiolarian cherts, and mudstones, to the blue.-black mudstones with Glenkiln graptolites, form one limb of an arch which is truncated by a fault that runs in a north and south direction along the hollow occupied by the raised beach at the base of the cliff. The rocks that form the seaward ridge west of the raised beach consist of green mudstones and greywackes with a south-east dip.

About 150 yards to the S.S.E. of this section, the red cherts and green mudstones are again brought to the surface by two isoclinal folds. Along the west side of the westmost fold, a north and south fault truncates the green cherts and brings them in contact with the overlying green mudstones and greywackes. From the amount of debris of the dark graptolitie mudstone at the base of the cliff it is clear that this band must occur at several localities on the adjoining slopes.

In a grassy field about 500 yards to the north-east of Portandea [NX 05095 75406], a small quarry hole shows the dark graptolitic mudstone belonging to the same horizon and yielding Dendrograptus, Dicranograptus minimus, Dicellograptus sextans, and Climacograptus coelatus.

At a point about 300 yards to the S.S.W. of Portandea, another outcrop of the green cherts and mudstones with radiolaria may be seen near the foot of the March Burn [NX 04593 75026]. Higher up the section, near the foot of some cascades, the dark graptolitic mudstone appears. The grey mudstones, shales, and greywackes, which follow the graptolitic mudstones in normal order, have certain peculiar lithological characters, which are recognisable along the south-eastern margin of the Stinchar volcanic area. Indeed, they have been traced from the shore at Portandea north-east to the Tappins Hill, in the valley of the Stinchar, about six miles E.N.E. of Barr. The greywackes have a regular system of jointing, weather with rounded edges, and vary from a few inches to a foot or more in thickness. Though these beds have been carefully searched at various localities they have failed to yield any trace of organic remains. For the sake of convenience, they will be referred to as the "Tappins group" in the description of the rocks on the south side of the Stinchar valley.

On the shore about a quarter of a mile north of Portandea, the red mudstones are arranged at the foot of the cliff, in a series of four isoclinal folds within a distance of fifty yards. One of the folds extends for a distance of 300 yards along the cliff [NX 04797 75516], and passes underneath the sea to the north of Portandea. The cores of the arches are formed of red cherts with radiolaria and mudstones; these are succeeded by green mudstones containing nodules of green and grey chert, sandy calcareous nodules from the size of marbles to blocks a foot and upwards in diameter, and lenticular masses of grit. The nodules of green and grey chert are crowded with radiolaria. These are followed by green mudstones, dark shales, and greywackes, the general dip of the strata being eastwards. Some of the dark shale bands occupy the position of the graptolite shales in the Portandea section just described.

Northwards, on the shore near Turf Hill [NX 04844 75654], and to the west of Glendrishaig House [NX 05034 76211], the red mudstones and cherts are visible on another isocline, where, on the north side of the outcrop, they come in contact with green sandy greywackes, shales, and mudstones which dip to the south-east at 30°. The shore northwards from Glendrishaig to Brakness Hole [NX 05084 76823] presents a constant repetition of green mudstones and greywackes, dipping towards the south-east at angles from 40° to 50°. The strata are cleaved, the planes of cleavage being at a slightly higher angle than those of the bedding.

Glen Currarie, Shallochbraik Burn, and Brakness Hole. — [NX 06133 78161], [NX 06008 77927], [NX 05112 76853] Another inlier of Arenig volcanic rocks and radiolarian cherts is traceable for about a mile and a half from Glen Currarie across Shallochbraik Burn to Brakness Hole, on the shore north of Glendrishaig. Beginning with the section in Glen Currarie at the northern limit of this inlier [NX 06133 78161], we find at the mouth of the glen, green mudstones, flags, and sandy greywackes ("Tappins group",) rolling about at gentle angles and rising northwards to 50° and 60°, the beds being well displayed at the side of a road leading to Currarie Farmhouse. Near the mouth of the Shallochbraik Burn, which joins the main stream about 300 yards up the glen, the red mudstones and radiolarian cherts appear on the south bank of the Currarie Burn. For nearly 400 yards along the glen, above this junction, the stream forms the boundary between the red cherts with radiolaria on the south-east bank and the green mudstones and greywackes on the north-west side of the stream, the graptolitic mudstones not being visible at this locality. Where the red cherts cross the glen, 400 yards above the junction of the streams, they are well displayed, the radiolaria occurring in abundance. About fifty yards above this point the red jaspideous cherts rest directly on dark fine-grained lava, followed by the peculiar green "diabase-porphyrite" which is so prominently developed in the volcanic zone of this inlier. It here forms the floor and walls of a rocky gorge, and is succeeded by a bed of volcanic agglomerate visible on the north bank of the stream. This agglomerate is followed by a mass of "diabase-porphyrite" lava which occupies the section for a distance of 100 yards.

In the south-east limb of the fold the red mudstones and cherts, owing to the reduplication, occupy the stream for 250 yards. In the midst of these cherts, the diabase-porphyrite "appears on a small fold, close to the spot where a grey felsite dyke traverses the section in a W.N.W. direction. The eastmost exposure of the cherts in Glen. Currarie lies about 1150 yards up from the coast line. These rocks are then succeeded by green, mudstones, flags, and greywackes.

The Shallochbraik Burn [NX 06008 77927] likewise furnishes a cross-section of the same isoclinal arch of Arenig volcanic rocks and cherts. On the north limb of the fold, red cherts and mudstones appear at the foot of this burn, and stretch along the channel for a distance of 100 yards. They are much folded where pierced by a dyke of hornblende-felspar rock. Their junction with the Arenig lava is visible at the point where the burn issues from a rocky gorge, the lava being a fine example of the typical "diabase-porphyrite" (porfido verde antique), with large green porphyritic felspars in a green fine-grained matrix. For a distance of from 300 to 400 yards the stream flows through the gorge which has been carved out of the bedded lavas. At the upper end of the ravine the cherts and mudstones appear on the south limb of the fold. In the core of the volcanic zone in this section the fine-grained slaggy diabase-lava appears, followed on both sides by the "diabase-porphyrite". On the south limb of the fold, owing to repetition by folding, a continuous section of the red cherts and mudstones is traceable for a distance of 100 yards, and even beyond this main outcrop and within the succeeding green mudstones and greywackes, three arches of the red mudstones and cherts may be selected.

In a south-west direction towards Brakness Hole [NX 05112 76853] on the shore, much of the evidence is concealed, as the area is mostly under cultivation and only isolated rock-exposures are visible. The lavas in the core of the arch in Shallochbraik Burn are traceable for 800 yards to the S.S.W., and are flanked on the south-east side by red mudstones and cherts. It is apparent, however, that the volcanic rocks are gradually "nosing out" towards the S.S.W., for the folds in that direction are represented in the overlying red cherts, green mudstones, and greywackes. The rapid isoclinal folding is evidently accompanied by faulting, for on the north side of the inlier south of Currarie Port the green mudstones and greywackes are brought into contact with the lavas. The cherts can be traced more or less continuously through the fields to the shore at Brakness Hole north of Glendrishaag, where a fine exposure displays their characteristic ribbed character and highly contorted bedding. At the edge of the sea cliff that overlooks the northern edge of Brackness Hole, a small core of diabase-porphyrite lava protrudes through the herts, and is separated by a few yards of red chert from a second core of Arenig lava, which can be traced up the slope for a distance of 100 yards. From the fine cliff-section at the head of Brakness Hole it is apparent that the strata must be arranged in sharp folds, because the red cherts run out seawards in a tongue-shaped form, in the midst of mudstones and greywackes.

In the cove south of Brakness Hole [NX 05112 76853] three anticlines of the radiolarian cherts crop out in the midst of the green mudstones, flagstones and greywackes.

It is a disappointing feature of this inlier that the graptolitic mudstone which at Portandea overlies the radiolarian cherts and mudstones has not here been detected in a similar position, though a careful search for it has been made. The cherts and red mudstones are followed by the barren green mudstones, flags, and greywackes ("Tappins group"). We shall now adduce certain sections at the margin of the main volcanic area north of Currarie, where the volcanic rocks and cherts are succeeded by blue-black mudstones yielding Glenkiln graptolites.

Shore section between Currarie and Downan Point south of the River Stinchar. — [NX 05561 77990] to [NX 06747 80312] Northwards along the shore from Port Currarie for a distance of half a mile, important evidence may be gathered respecting the relations of the Ballantrae volcanic rocks to sediments of Llandeilo age. Graptolites of Glenkiln type have been obtained at two localities from blue-black mudstones which overlie in one case radiolarian cherts and mudstones, and in the other volcanic agglomerates and lavas.

The promotory on the north side of Port Currarie is formed of green mudstones and sandy greywackes. The separate beds of the latter measure generally about a foot or so in thickness, though in some instances they reach four or five feet. These strata, which weather with the characteristic rounded outlines of the "Tappins group", are repeated by many folds, and occupy the coast line for a distance of 200 yards north of Port Currarie, as far as an outcrop of blue-black fossiliferous mudstones which form a dip slope seawards at an angle of 60°, and fold round an exposure of red mudstones and cherts visible at the base of the cliff. The stratigraphical horizon of this band can only be a few feet above the red mudstones and cherts. The bed is not very accessible, but it furnished the following Glenkiln forms after a short search: Didymograptus, Climacograptus caelatus, Dicellograptus sextans, D. divaricatus, and Dicranograptus sp. There can be little doubt that a more prolonged examination would increase this list.

Northwards the green mudstones and greywackes of the "Tappins group" are truncated by a fault, beyond which the red mudstones and cherts succeed. Here the cliff is for the most part inaccessible, but at its base the slaggy lava is exposed on one or more folds. At a point 300 yards north from Currarie Port a well-marked reversed fault slopes northwards at an angle of 30°, and gives origin toa cliff nearly at right angles to the trend of the coast line. On the north side of this dislocation the red cherts, mudstones, and associated lavas, which are there thrown into a series of sharp folds, have been made to over-ride the red mudstones and cherts on the south side of the thrust plane. A short distance to the north another fault at a high angle gives rise to a similar feature, and affects beds belonging to the same horizons. It trends to the north, and is probably reversed. On the north side of this disruption there is a much larger development of the lavas than on the south side.

At a point 600 yards north from Currarie Port [NX 05813 78511], the sequence from the volcanic rocks to the fossiliferous mudstones with Glenkiln graptolites may be again traced. A small dry watercourse here traverses the strata from the top of the Knockgown cliff at a height of 250 feet to the sea level. Its position on the coast is due west of Currarie Farmhouse. The accompanying horizontal section (Figure 100) shows that between the top of the cliff and the sea-level, the rocks are arranged in a syncline and anticline, the graptolitic mudstones forming the centre of the trough.

On the hill slope at the top of the cliff the green porphyritic lava is well shown, and closely resembles the rock of the same type in the Glen Currarie "inlier" already described. Here, too, it forms a conspicuous feature, and has been quarried for dyke-building. The lava from this quarry shows green porphyritic felspars, often measuring more than a quarter of an inch across, in a fine-grained almost black matrix. Under the microscope No. (S5921) [NX 05851 78513] "the green colour of the porphyritic felspars is found to be due to chlorite. The ground mass is composed of lath-shaped plagioclase, augite, chlorite, and iron-ores, including pyrite. The structure is sub-ophitic. The original rock was probably a porphyritic dolerite. The rock is a typical diabase-porphyrite according to German nomenclature".

Beginning at the top of the cliff and descending the section, we find a slaggy lava (a, in (Figure 100)), in which the materials filling the cavities have largely decomposed out, and the vesicular character of the rock is very apparent on weathered surfaces. This slaggy lava, which is about thirty-eight feet thick, is succeeded by about twenty-nine feet of volcanic agglomerate b), having a green matrix charged with rounded blocks and masses of slaggy lava together with fragments of black shale, black chert, and rare fragments of red chert. These sedimentary materials have probably been derived from the Arenig black shales and cherts. The average size of the blocks varies from one to three inches, but some measure from a foot and a half to two feet across.

Near the top of this agglomerate a lenticular band of slaggy diabase-lava (c) occurs, which measures seventeen feet thick at the bottom of the gully, but thins out before reaching the top of the crag. It is succeeded by a few feet of tuff, and eventually by the red radiolarian cherts and mudstones (d),which are 51 feet thick at the top of the crag, and thin out to a few inches down the slope. These are followed at the top of the scar by a thin layer of volcanic materials and sediments, consisting of mudstones or fine tuff with fragments of red and green chert and occasional blocks of porphyrite, one about three feet across. This band is overlain by green mudstones. The succession at the base of the cliff is somewhat different, for near the point where the felsite dyke crosses the dry watercourse the thin development of cherts is followed by red mudstones (e), the two bands measuring about three feet thick, while these are in turn overlain by a lenticular bed of diabase-lava upwards of thirteen feet thick at the broadest part (f). Next in order come mudstones (g) about ten inches thick; ash and calcareous agglomerate (h), 32 feet; red mudstones, followed by green mudstones yielding fragments of graptolites (a). Yet again we find a thin lenticular band of green tuff, which is succeeded by the important zone of fossiliferous mudstones with Glenkiln fossils (2).

The geographical position of this fossiliferous band is well defined, because the mudstones form the roof and walls of a narrow cave which was excavated by the sea when the land stood at a lower level. The total width of the exposure is about 5½-feet, and as the strata seem to lie in the centre of a syncline the thickness of the mudstones will be about three feet. Weathering spheroidally, and having an irregular fracture, the fossiliferous beds are lithologically quite unlike the Glenkiln Shales of the Moffat region, but they to some extent resemble the graptolitic mudstones that overlie the Stinchar limestone, so minutely described by Professor Lapworth. These dark blue, green, and grey mudstones yield graptolites more frequently in certain layers than in others, which are usually preserved in the solid and often in a perfect state of preservation. Here, again, as at Portandea, the occurrence of Dendrograptus is noteworthy in association with many characteristic zonal Glenkiln graptolites, as given in the annexed list:

Dendrograptus gracilis (Hall.)

Didymograptus superstes (Lapw.)

Didymograptus serratulus? (Halt)

Didymograptus extensus? (Hall.)

Callograptus Salteri? (Hall.)

Dicranograptus minimus (Lapw.)

Dicranograptus ramosus (Hall.)

Dicranograptus divaricatus (Hall.)

Dicranograptus intortus (Laiolw.)

Dicranograptus moffatensis (Carr.)

Dicranograptus patulosus (Lapw.).)

Dicranograptus sextans (Hall.)

Dicranograptus var. flexuosus (Lapw.)

Diplograptus euglyphus (Lapw.)

Diplograptus foliaceus (Murch.)

Climacograptus bicornis (Hall.)

Climacograptus caelatus (Lapw.)

ClimacograptusSchärenbergi (Lapw.)

Acrothele sp.

Acrotreta sp.

Lingula sp.

Discina sp.

Discina or Kutorgina sp.

Siphonotreta micula (M'Coy.)

Discinoid shell.

Sponge.

The westward limb of the syncline presents the following sequence in descending order:; Next the fossiliferous mudstones comes a band of agglomerate (k) about seven feet thick, charged with blocks of vesicular diabase-lava, followed by ten feet of green and grey mudstones (1), 26 feet of volcanic agglomerate (m) with a lenticular bed of lava, seven feet of green and red mudstones and charts (n), nine feet of tuff (o), five feet of green and red mudstones (p), and 23 feet of volcanic agglomerate (q). Here a felsite dyke, about fifteen feet broad, traverses the volcanic rocks, and below its outcrop agglomerate with rounded blocks of diabase-lava is well seen. Westwards, a mass of diabase-lava with pillow-structure forms probably an anticlinal fold, for it is succeeded on the seaward side by volcanic agglomerate similar to that just described, and by an outcrop of red mudstones visible in a little skerry. The dry watercourse which descends the sea-cliff marks a line of fault, probably of no great amount, as the outcrops of the beds are not shifted to any great extent.

From the foregoing descriptive details it is apparent (1) that the blue-black mudstones charged with typical Glenkiln (Upper Llandeilo) organisms rest directly on volcanic rocks; (2) that beneath the mudstones lies an alternating series of tuffs, mudstones, and volcanic agglomerate with lenticular beds of lava, showing that during various pauses in the volcanic eruptions ordinary sedimentary deposits were laid down; (3) that from the nature of the agglomerates the local centre from which they were discharged may be supposed to have probably stood near the present site of these pyroclastic materials; (4) that as the radiolarian cherts have here only a very limited development, it is reasonable to infer that the prolonged period indicated by the mass of chert elsewhere in the uplands is here represented by the alternating series of volcanic rocks and sediments; (5) that in this portion of the Ballantrae region the evidence points to the continuance of volcanic activity into Lower Llandeilo time.

From this section on Currarie cliff northwards [NX 05973 78757] for about two miles to Downan Point [NX 06733 80324] , a continuous exposure of volcanic rocks forms the southern termination of the main volcanic area of Ballantrae, and affords the finest sections of the Lower Silurian lavas of the South of Scotland. In the following paragraphs we shall refer to the general characters of the volcanic materials on this part of the coast line and their relations to overlying sediments.

Not far to the north of the section on Currarie cliff, at the foot of a small stream near the Dove Cove [NX 06012 78759], a synclinal fold shows ashy red mudstones with radiolarian cherts, bounded on both sides by volcanic agglomerates resting on diabase-lava. In the Dove Cove [NX 06051 78881] there is another infold of agglomerate without the ashy mudstones. About 300 yards further north a deep gully is formed by the weathering out of a grey porphyritic felsite dyke, which traverses the volcanic rocks.

Wilson's Burn. — This small stream, which enters the sea about a mile to the north of Currarie Port [NX 06462 79548], displays for a distance of 700 yards [NX 06917 79489] an excellent section of the volcanic rocks, radiolarian cherts, and overlying green mudstones and greywackes (Tappins group).

In the ascent of the burn from the sea level, the slaggy diabase-lavas are well seen on the cliff, but inland for a short distance they are buried under boulder clay. They are visible, however, in the bed of the stream and on the south bank at a distance of 250 yards from the shore, where they come in contact with the chocolate-coloured mudstones and radiolarian cherts.

No agglomerates here occur between the cherts and lavas. These red cherts are immediately succeeded by the green mudstones and greywackes (Tappins group), which extend for a distance of 70 yards, followed by the red mudstones on another anticlinal fold. Eastwards as far as the fork of the stream, a compound synclinal fold extends, composed of green mudstones and greywackes, within which occur two minor folds of the red cherts that underlie the mudstones visible on the north cliff. At the fork, a well-marked anticlinal arch of the red mudstones and cherts is truncated by a fault on the north-west side which brings massive greywacke in contact with the cherts. Above the fork, in the main stream, a broad compound anticline of the red mudstones and cherts is followed by green mudstones and greywackes. Here the strata disappear under a covering of boulder-clay. In this important section the fossiliferous mudstones have not been detected, but their position must be near the base of the green mudstones and greywackes.

Between Dove Cove [NX 06053 78885] and Downan Point [NX 06729 80318] the coast displays a magnificent section of the bedded lavas, which here present remarkably uniform Ethological characters. They are usually fine-grained and non-porphyritic, with numerous small spherical vesicles filled with calcite. A specimen of a fine-grained dark green massive rock taken from the shore below Downan, south of the river Stinchar, shows under the microscope "lath-shaped felspars, pseudomorphs after augite, chlorite, and iron ores. The rock may be termed a diabase, and was originally, in all likelihood, a dolerite".

On the shore near Downan [NX 06729 80318], the remarkable "pillow-structure", so characteristic of the Arenig lavas in the Southern Uplands, is typically displayed (Plate 1), (Plate 2), (Plate 4), (Plate 6). The lava-flows are here made up of a series of oval or pillow-shaped masses, varying considerably in size and lying with their long axes parallel to the bedding planes. Each oval mass has a concentric arrangement (Plate 1), (Plate 5), the outer zones being vesicular, while the central portion is usually compact, though fragments of other lavas are sometimes observable in the heart of the spheroids. The latter vary in size, some being circular and measuring about six inches across, while the largest measure nine feet in length by two feet in height and one foot ten inches in diameter.

From his recent work on "Ancient Volcanoes of Great Britain", in which Sir Archibald Geikie describes and figures the "pillow-structure", so beautifully shown by the Arenig lavas of Ballantrae, the following extract is taken:

"This singular structure has already been referred to as strikingly displayed in a rock at the top of Cader Idris. It is found in dark basic lavas, probably of Arenig age, which will be afterwards referred to as occurring along the southern flanks of the Scottish Highlands and also in the North of Ireland. It has been observed by Mr. Teall among the rocks of the Lizard, and has been described as occurring in Saxony and California. In these different localities it is associated with jaspers and cherts, some of which contain abundant Radiolaria. The same structure has been found among the variolitic diabases of Mont Gentyvre, and likewise in some modern lavas, as in that of Acicastello, already referred to". <ref> Ancient volcanoes of Britain, vol i, p.193.</ref>

The spaces between the spheroids of which each lava-sheet is built up are sometimes filled with calcareous matter (limestone), flinty shale, chert, and jasper (Plate 2), (Plate 4), (Plate 6). The calcareous matter does not seem to have penetrated far down through the successive beds, being confined mainly to the surfaces of the flows. Though no fossils have been found on the part of the coast line south of the Stinchar, yet a little to the north of Bennane Head, north of Ballantrae, flinty shales, probably occupying a similar position in the volcanic series, yielded Arenig graptolites and discinoid shells.

Area between the Valley of the Stinchar and Byne Hill, Girvan

While on the shore to the south of the river Stinchar, a perfect passage may be traced from the Arenig volcanic rocks and radiolarian cherts to sediments yielding graptolites of Glenkiln (Upper Llandeilo) age; on the other hand, throughout the main volcanic area of Ballantrae, extending from the valley of the Stinchar to the Byne Hill, near Girvan, a strong unconformability separates the Arenig volcanic rocks and radiolarian cherts from the Kirkland conglomerate, Stinchar limestone, and Benan conglomerate, which lie at the base of the richly fossiliferous strata of the Girvan succession. In the following description we shall begin with the Arenig volcanic nicks, radiolarian cherts, and other sediments which are clearly older than the conglomeratic series containing the Stinchar limestone. As already indicated, one of the striking features of the Ballantrae volcanic area is the development of intrusive igneous materials, composed almost wholly of ultra-basic and basic igneous rocks; indeed, the area which these masses now occupy at the surface is nearly as large as that covered by the bedded lavas and tuffs. As the evidence seems to point to the conclusion that the intrusive rocks are probably the last products of volcanic activity in Silurian time in that region, we shall first describe the development of bedded lavas, agglomerates, and tuffs, with the overlying sediments, beginning with the coast sections, where the relations of the volcanic rocks to the radiolarian cherts are most clearly defined.

Bennane Head, Shore-Section. — About two miles north of Ballantrae the shore from the mouth of the Bennane Burn [NX 09171 85915], northwards by Bennane Head [NX 09069 86606] to Balcreuchan Port [NX 09906 87617], displays a section of lavas with pyroclastic materials, graptolite shales, and radiolarian cherts which, from a stratigraphical point of view, is by far the most important in the volcanic area of Ballantrae. For here the relation of the cherts to the volcanic series is not only clearly defined, but their age is fixed by the occurrence of a band of graptolite shale which yielded to Professor Lapworth a suite of graptolites of Middle Arenig age. Since this important discovery, other fossiliferous bands have been detected in the course of the revision by the Geological Survey, on other horizons, in the heart of the bedded lavas and tuffs at Balcreuchan Port.

Attention will first be directed to the section on the beach at Bennane Head [NX 09069 86606], where the radiolarian cherts, Middle Arenig graptolite shales, and underlying volcanic rocks are thrown into a series of sharp folds, as shown in the accompanying ground-plan and section (Figure 102), (Figure 103).

Thereafter the section on the old sea-cliff east of the road between Girvan and Ballantrae will be referred to. Though the two sections are parallel and traverse beds along the same line of strike, yet, owing to the pitch of the folds, there is a considerable divergence between them.

At the mouth of the Bennane Burn, near high-tide mark, a breccia, e in (Figure 102), with fragments of serpentine embedded in a red sandstone matrix, here marks the local base of the Permian (or Triassic) sandstones, which extend along the coast to Ballantrae. This breccia is immediately followed northwards by red serpentine, but the actual unconformability between the two rocks is not here visible. About fifty yards to the north of the mouth of the Bennane Burn, and about 150 yards to the south of the cave in the old sealcliff, the continuous rock-section begins.

Here two small knobs of highly crushed basic rock resembling gabbro are separated by a few yards of sandy beach from an intrusive mass of dolerite (1U), the intervening hollow being occupied by serpentine (Σ), which is visible on the seaward side at low tide, and is of a bright red colour, much brecciated and veined with calcareous matter. The adjacent mass of dolerite runs apparently E.S.E. and W.N.W., measures about thirty yards across about high-tide level, and decreases in breadth to sixteen yards seawards.

The radiolarian cherts (C) in contact with the northern edge of the intrusive dolerite, at high-tide mark, form a series of rapid isoclinal folds dipping southwards. From this point northwards for a distance of seventy yards, the shore is occupied mainly by volcanic agglomerate and tuff (1 Ts), in which lie several small outliers of radiolarian chert. This volcanic breccia has a light green matrix, in which are set numerous blocks of vesicular diabase-lava measuring from one to three feet across, together with pieces of red, grey, and black chert. An interesting character of this rock is the abundance of its calcareous matter in the form of veins or lenticles, which have not as yet proved fossiliferous. Another important feature is its inclusion of two outcrops of black shale (1), the more southerly of which lies within a few feet of the northern margin of the intrusive dolerite. Fossils have not been obtained by the Survey from these outcrops.

The northern margin of the volcanic breccia is defined by an outcrop of radiolarian cherts, which, about fifty yards south of the Bennane Cave, form several prominent stacks that show the repetition of the beds by folding and their intercalation with fine tuff and volcanic breccia. Here the cherts, displaying the dark olive-green and grey varieties with abundant radiolaria, form an isoclinal trough dipping southwards at angles from 30° to 40°. The prolongation of this syncline is well seen on the old sea-cliff south of Bennane Cave.

For a distance of about twenty yards north from this outcrop of the radiolarian cherts the rocky beach has been worn down to a comparatively level plane of volcanic agglomerate and tuff underlying the cherts. This volcanic breccia closely resembles that just described. Its important distinction, however, is the occurrence in it of the band of black shale yielding Middle Arenig graptolites, which appears on both limbs of an isoclinal arch. The northmost outcrop, which is traceable for a distance of 31 yards, affords graptolites, whilst the southmost band, which can be followed only at intervals, yields finely-preserved shells in soft black shale. These fossiliferous beds consist of hard black flinty shales with thin partings of soft black shale, which are much disturbed, smashed, and contorted. The following assemblage of fossils was collected from these two outcrops in the course of the revision by the Geological Survey:

Tetragraptus Bigsbyi (Hall.)

Tetragraptus bryonoides (Hall.)

Tetragraptus fruticosus (Hall.)

Tetragraptus quadribrachiatus (Hall.)

Tetragraptus sp.

Dichograptus multiplex (Nich.)

Dichograptus octobranchiatus (Hall.)

Dichograptus reticulatus (Nich.)

Dichograptus sp.

Didymograptus extensus (Hall.)

Didymograptus patulosus (Hall.)

Didymograptus sp.

Phyllograptus typus (Hall.)

Dendrograptus sp.

Callograptus diffusus (Hall.)

Climacograptus confertus (Lapw.)

Acrotreta Nicholsoni (Dav.)

Discina sp.

Lingula sp.

Obolella or Obolus sp.

Kutorgina, or Obolella sp.

Discinoid shells.

Sponge rods.

Professor Lapworth recorded the following forms from this band, which are not given in the above list, viz.:; Didymograptus bifidus (Hall), Caryocaris Wrighti (Salter), and Dictyonema. Regarding the assemblage obtained by him from these exposures, Professor Lapworth states that "it is almost unnecessary to point out that we have here a Scottish representative of the well-marked fauna of the middle zones of the Arenig–Quebec or first Ordovician fauna of South Britain, Northern Europe, and Eastern America. Not only is the general facies of the fauna of the well-known Arenig, Paint Levis or Skiddaw type, but the Bennane forms occur always in the same association".<ref> "On Ballantrae Rocks of South Scotland and their place in the Upland Sequence". — Geol. Mag., Dec. III., vol. vi., No. 1, p. 20, January 1889,</ref>

It is significant that though the agglomerate forming the core of the arch between the two outcrops of the Arenig black shales otherwise resembles that overlying the same shales, no fragments of chert or black shales were detected in it. The thickness of the agglomerate that overlies the Arenig black shale and is succeeded by the radiolarian chert varies from eight to twenty feet. The agglomerate is succeeded northwards by a fine exposure of radiolarian cherts and mudstones, which, occupying the shore for a distance of upwards of 100 yards, are repeated by a series of sharp isoclinal folds (Plate 7). Near the southern margin of this belt of cherts thin bands of tuff are intercalated with the chert (Plate 8). As this broad outcrop of Arenig cherts and mudstones occupies a compound synclinal fold, the volcanic breccia and Middle Arenig black shales reappear on the north side of the trough. They are there pierced by a mass of intrusive dolerite (1 U), which extends northwards along the coast for 200 yards. At the northern margin of the cherts, the tuff that occurs between them and the intrusive dolerite is fine-grained, and only a few feet thick. Both the black flinty shale and the tuff are welded to the intrusive igneous rock.

At this point on the shore, the strike of the strata changes and now runs nearly due north for over 200 yards. Just above high-water mark the Arenig black shales can be traced at intervals, keeping parallel to the margin of the intrusive dolerite, and bounded on the east by the agglomerates of Bennane Head. Here they have yielded graptolites and linguloid and discinoid shells. Although the Arenig black shales are truncated by the intrusive dolerite, graptolites can be collected from them within junction.

On the old sea-cliff near the Bennane Head Cave, a section east of the main road runs parallel to that on the beach, and exposes mainly the radiolarian cherts. The chief folds that occur on the beach can be recognised on the cliff, which here reaches a height of 250 feet. The absence of the coarse volcanic agglomerate which underlies the cherts is due to the pitch of the folds being inclined towards the sea. Nevertheless, owing to the sharp plications of the strata and to the denudation of the beds at different levels, the underlying volcanic rocks are exposed in the cores of some of the folds.

About fifty yards to the north of the cave, in the angle of the receding cliff, the Arenig black shales and tuffs appear, with a southerly dip at a high angle. Here the intercalation of pyroclastic material with radiolarian chert is admirably seen in several alternations of these rocks. As each band of tuff contains blocks of the chert, it is evident that, during pauses in the volcanic activity, the radiolarian ooze consolidated on the sea-floor as chert, and was disrupted by the successive explosions. Proceeding southwards, the observer traverses highly folded radiolarian, cherts (Plate 9) till, on reaching the south end of the cliff, he finds the prolongation of the second broad band of agglomerate to the south of the cave. The band of Arenig black shales occurs on both sides of this detrital mass. Its southern outcrop, where the cliff bends to the east, is rapidly folded, and the centre of one of tbfe folds is occupied by chert. From this point eastwards, the volcanic agglomerate forms the higher part of the cliff for about 100 yards, while serpentine is in situ at the base, the latter being in all likelihood a continuation of the mass visible on the shore (Figure 101) and (Figure 102). The boundary of the serpentine is shifted by a small fault, which likewise changes the strike of the beds, the trend on the east side of the dislocation being nearly east and west. East of this fault the serpentine forms a small boss, bounded on the north side by radiolarian cherts, on the east by Arenig lava, and on the south by the alluvium of the Bennane Burn. Another knob appears a few yards further east, bounded on both sides by Arenig lava. These isolated exposures of serpentine are believed to be parts of one and the same mass. From the fault eastwards along the cliff to the Bennane Burn — a distance of 100 yards — the radiolarian cherts are continuously laid bare.

Bennane Burn Section. — [NX 09256 85942] The first visible rock above the mouth of this stream consists of a band of thin bedded lava, showing typical pillow-structure, and enclosing pieces of chert with radiolaria. It is followed by a thin seam of dark green or black shale, succeeded by dark green and red rediolarian cherts. Here the interstratification of bands of coarse and fine tuff with the radiolarian cherts is well shown at a small waterfall in the main stream, and still more clearly in a small side stream to the west of the line of section. At this latter locality, also, bands of nodular tuff occur between the top of the lavas and the overlying cherts, which are not seen in the main burn.<ref>This stream section is figured and described by Sir A. Geckle in the "Ancient Volcanoes of Great Britain", vol. i., p. 198.</ref>

A specimen of one of these fine tuffs No. (S5932), [NX 09467 86006] composed of a compact hornstone-like rock mottled with green and red tints, taken by the Director-General from a point near the small waterfall in Bennane Burn, shows extremely interesting microscopic characters. Mr. Teall says that it contains "a few minute (0.1 mm.) angular fragments of felspar in a somewhat turbid mass, giving cryptocrystalline polarisation under crossed nicols. When viewed with a high power and a contracted aperture, so as to bring out the slight differences in refractive index of the different constituents, the main mass of this rock shows the structure of a very fine volcanic ash.<ref>See "Untersuehungen fiber die Lenneporpbyre", by O. Mugge: Neues Jahrb Beilage Band VIII. (1893), p. 648.</ref> Fibrous particles and others showing cusp-like projections between contiguous concavities may be distinctly seen. Both in form and size there is a resemblance between these structures and the glassy particles of the Krakatoa ash which fell on the Norham Castle' at a distance of 57 miles from the point of eruption".

Above the waterfall, the Bennane Burn shows a section of red, green, and grey chert with thin partings of mudstone or fine ashy material, for a distance of 50 yards, the whole series being much puckered. These strata are succeeded by green greywackes, composed largely of derivative volcanic material.

Coast-Section from Bennane Head to Balereuchan Port. — Resuming the shore-section at a point about 150 yards north of Bennane Cave [NX 09131 86303], we find the volcanic agglomerate which underlies the Arenig black shale on the cliff and in the road cutting, the beds being nearly vertical (Plate 10). At a bend in the road, about 300 yards north of Bennane Cave, a good example of a reversed fault with a nearly horizontal plane is followed by a second reversed fault a few feet higher up. On the shore below the road, the vertical bedding of the agglomerate and tuff is well displayed, the parallel bands being etched by the action of the waves, when alternations of fine and coarse material are developed (Plate 10). The included blocks, composed of fine-grained slaggy lava, are, as a rule, more rounded than those in the Knockdolian agglomerate to be referred to presently. The matrix consists of comminuted fragments of volcanic materials, having a purple or green tint.

About 700 yards north of Bennane Cave, beneath the high road where it winds round Port Vad [NX 09280 86907], the junction of the agglomerate with the underlying lava is seen on the cliff at low tide. The thickness of the agglomerate that underlies the Bennane Head black shale, exposed on the shore, between Bennane Cave and Port Vad is 700 feet. Here, at the base of the cliff, by means of a reversed fault the agglomerate is driven over a mass of pillowy, fine-grained diabase, and on the face of the cliff the junction of the lava with the agglomerate is visible. The pyroclastic material on which the lava reposes is a green, highly basic tuff, not unlike that of Mainshill, near Ballantrae, to be described in the sequel. The matrix has a green tint, is fine-grained, decomposes readily, and is abundantly charged with blocks of fine-grained slaggy lava. This tuff, which is about 45 yards in breadth, rests on a second mass of lava, and occupies the whole bay at Port Vad. Here the pillow-structure so characteristic of these lavas is well displayed, both on a stack in the centre of the bay and also on a cliff section that forms the northern promontory. At this latter point, as in the Downan section above described, the long axes of the pillow-shaped masses are arranged along the bedding planes, which dip seawards at an angle of about 65°.

A specimen of the lava taken from this bay No. (S5922) [NX 09280 86907] shows under the microscope that the "rock is too much altered for precise determination. The ground-mass contains acicular microlites of felspar. The amygdules had usually a narrow zone of chlorite on the outside; the interior is sometimes occupied by a granular aggregate of felspar (?). The rock is much veined, and the veins, which traverse both ground-mass and phenocrysts, are apparently composed of felspar and calcite".

From the northern promontory the pillowy lavas are succeeded for a distance of about 100 yards by beautiful porphyritic lavas, with large phenocrysts of felspar (diabase-porphyrite). Their strike being nearly north and south, and their inclination seawards, they are succeeded by agglomerate, which is visible in a gully about 120 yards north from Port Vad, but, owing to the angle of dip, is exposed on the cliff for only a short distance, and then passes out to sea. Northwards for 200 yards the coastline is formed of fine-grained diabase-lava, succeeded by about ten feet of fine tuff, the bottom bands of which fill up the uneven surface of the underlying lava-flow. The tuffs are followed by diabase-lava, showing pillow-structure, which occupies the shore for a distance of fifteen yards.

We have now reached an interesting point in the section, for here, in a gully running nearly north and south along the strike of the beds, distant about 90 yards from the mouth of a small burn which joins the sea south of Balcreuchan Port, a thin band of fossiliferous red mudstone occurs between beds of pillow lava. From the manner in which this band folds round the rugose surface of the lava and fills up the cracks and cavities below it, we may justly infer that the material must have been deposited between successive volcanic discharges (Plate 11). The strata here are nearly vertical, and their dip, if they have any, is seawards. The band of red ashy mudstone is extremely hard and splintery, and yields fossils sparingly, but the following assemblage of Middle Arenig fossils was obtained:

Tetragraptus bryonoides (Hall.)

Tetragraptus Headi (Hall.)

Trigortograptus ensiformis (Hall.)

Dichograptus sp.

Climacograptus confertus ? (Lapw.)

Caryocaris Wrighti (Salt.)

Siculae of graptolites.

The underlying lava is highly porphyritic, and the porphyritic crystals on its upper surface project into the red ashy mudstone, suggesting that the underlying mass had not long consolidated before the fine tufaceous mud was laid down.

A few yards to the east of this fossiliferous locality, green, fine-grained, vesicular lava shows typical pillow-structure. Two specimens of it were collected; one from the margin of one of the spheroids, the second from the core of the same "pillow".

The former No. (S6415) [NX 09429 87309] under the microscope is found to be composed "of long slender microlites of felspar, giving straight or nearly straight extinction, chlorite, granules of epidote, and possibly also of unaltered augite, a few small specks and plates of iron-ore, and an ill-defined brown substance, which probably represents interstitial matter. The vesicles are filled mainly with calcite, but they sometimes contain also portions of the interstitial matter". The microscopic examination of the specimen from the centre of the pillow-shaped mass No. (S6416) proved that "the rock is not homogeneous. Portions show true igneous structure, being composed of felspar-microlites, chlorite, &c. Other portions are composed of vesicular lapilli, and possess the structure of a tuff".

A few yards west of the mouth of the burn which falls into the sea south of Balcreuchan Port, a zone of chocolate or red ashy sediment is associated with fine tuff. Here, on a cliff formed by these sedimentary beds and fine tuffs, the fossils were obtained from a thin dark ashy seam, which are given in the annexed list:

Tetragraptus bryonoides (Hall.)

Tetragraptus sp.

Climacograptus sp.

Caryocaris Wrighti (Salt.)

Radiolaria.

Acrotreta sp.

Obolella sagittalis? (Salt.)

Discinoid shells.

This fossiliferous band is shown by these fossils to be of Middle or Lower Arenig age. It is interstratified with diabase-lava and coarse volcanic agglomerate, the latter being visible at the mouth of the burn just referred to. This breccia is followed by lavas showing typical "pillow-structure", which extend as far as the middle of the bay of Balcreuchan Port. Here red mudstones and cherts, lying probably in a synclinal fold, and in all likelihood on the horizon of the Arenig cherts of Bennane Head, are admirably seen on a cliff on the north side of the cove, where they are interstratified with agglomerates. They are also visible in a small promontory in the middle of the bay.

The volcanic origin of the bedded lavas and agglomerates in Balcreuchan Port was clearly recognised by Professor Bonney, whose description is so interesting and accurate that we reproduce it here. "We descended from the high road to the water's edge at Balcreuchan Port, a romantic cove enclosed by precipitous cliffs. On the shore, rather on the southern side, is imperfectly exposed a little of a rather decomposed dark-coloured serpentine, apparently intrusive in a rock consisting of angular fragments of felstone, in an earthy-looking decomposed ground mass. Their condition and the nature of the ground — strewn here with boulders and debris — makes it difficult to form a decided opinion about this rock, but I have little doubt that it is really a coarse porphyrite tuff or volcanic agglomerate. Overlying this is a great mass of hard reddish-coloured rock — here fairly compact — of which the whole cliff appears to be composed. Towards the base, the rock has a rather slaggy aspect, and is slightly vesicular. It generally exhibits an irregular jointed structure, the most developed planes being rudely parallel to the base; is occasionally slightly columnar, and often rudely spheroidal, this structure being in one part very marked. Sometimes, also, curved sub-spheroidal joints may be seen. The spheroids are often traversed by radial cracks. Where parts of the shells have fallen away, and fragments have been left projecting from the rotten, dusty, or pasty material which has accumulated from decomposition, the rock has a look of breccia. This, however, in the ordinary sense of the word, I am convinced it is not. By clambering along the screes to the south, we found the rock become decidedly porphyritic, the crystals of the plagioclase felspar often being about a third of an inch long. Good specimens of the rock were difficult to obtain, as it was very tough and traversed by minute concealed joints. From the evidence in the field alone. I should not hesitate to consider this a true igneous rock, and probably a lava-flow".<ref>Quart. Jour. Geol. Soc., vol. xxxiv., p. 780.</ref>

From Balcreuchan Port [NX 09898 87565] north-eastwards to Burnfoot [NX 10887 88203] , the coastline cuts obliquely across the strike of the beds. For a space of 300 yards the section traverses the western limb of the syncline occupied by the red cherts and mudstones, the rocks being composed of lava with pillow-structure dipping towards the southeast. Beyond the centre of the syncline, which is here occupied by agglomerate, fine-grained diabase lavas with pillow-structure extend as far as Games Loup [NX 104 880] near Balcreuchan Port. North of that point, the coast-line is defined by a wall of rock formed by hard and brecciated lava; while on the seaward side a hollow runs parallel to this cliff, and beyond the hollow, serpentine is exposed in stacks that rise above the shingle of the beach.

From the foregoing details of the typical Bennane Head section, the following conclusions may be drawn:

1. The highest visible beds consist of red, green, and grey radiolarian cherts and overlying green greywackes and shales, the cherts being interstratified with tuffs and volcanic breccias. (Thickness of cherts with tuffs, 70 feet.)
2. Underneath the radiolarian cherts occur coarse agglomerate and tuff, which contain a thin band of black flinty shale charged with Middle Arenig graptolites. (Thickness, 720 feet.)
3. These agglomerates and tuffs pass downwards into vesicular diabase-lavas and "diabase-porphylite" lavas, occasionally associated with breccia and tuff, and containing thin fossiliferous seams that yield Middle Arenig graptolites. (Minimum thickness, 700 feet.)
4. If the correlation of the red mudstones and cherts in Balcreuchan Port with those at Bennane Head be correct, then it is clear that the volcanic series in this shore section must form a synclinal fold.
5. The nature of the pyroclastic materials at Bennane Head suggests the probability of a local centre of eruption having been situated near that locality.

Pinbain Shore-Section. — [NX 13755 91568] Another important shore-section occurs at Pinbain, about a mile north of Lendalfoot, where the lavas and tuffs are associated with fossiliferous strata of Arenig age ((Figure 104) ground plan, (Figure 105) section). About fifty yards south of the point where the Pinbain Burn enters the sea [NX 13717 91362], the junction between serpentine and black shales is visible, the two rocks being separated by a thin dyke of basic rock about 2½ feet broad. The microscopic examination of this rock No. (S6431) shows that "the dark part appears to be composed of minute augites and altered felspars, but owing to the fineness of the grain and to subsequent alteration it is difficult to be certain. The yellowish-green colour appears to be due to the development of granular epidote. The original rock was probably a basalt". The hardened black shales have here yielded a specimen of Climacograptus confertus in fair preservation. Immediately to the north of these shales on the sandy beach, black, green, and grey shales, interstratified with agglomerate, occupy the coast for a distance of ten yards. The black shale is shattery and breaks into thin flakes. The agglomerate contains blocks of diabase-lava, fragments of black shale, and oval masses of limestone, measuring in one case 4ft. by 3ft. Lenticular masses of lava likewise occur in this banded series of sediments and volcanic breccias.

Immediately to the north, lavas appear which show pillow-structure, but in places are much brecciated, though on the beach the outlines of the pillows are Mill preserved (1B). On the south and west sides the brecciated lava is separated from the sedimentary series by a layer of agglomerate from two to seven feet thick, containing blocks of porphyrite and small fragments of black shale. On the west or seaward side, the black shales are traversed by two small faults (see ground-plan, (Figure 104)). The mass of lava, which occupies the coast for a space of twenty yards, forms stacks rising above the sea level near high-water mark.

Not far to the north of the mouth of Pinbain Burn, the volcanic agglomerate again appears, containing blocks of diabase-lava, black shale from six inches to a foot across, black chert, and limestone (1 T s). West of the mouth of Pinbain Burn, and to the west of this mass of agglomerate, small knobs of black shale are visible on the beach near low-tide level, while another exposure occurs about twenty yards further north, which is covered during high water. A second mass of agglomerate crops out near high-water mark, almost due east of the last skerry of black shales.

The various outcrops of black shale just referred to near the mouth of Pinbain Burn, from the occurrence of Climacograptus confertus in one of their seams, are correlated with the Middle Arenig black shales of Bennane Head. Further, it is probable that the masses of agglomerate north of the mouth of Pinbain Burn may be younger than these sediments, as they contain pieces of black shale and black chert.

About 31 yards north of the northmost exposure of black shales gabbro (1 U) is visible on the sandy beach, and a few yards further north serpentine (E) is found in contact with green porphyritic lavas (1 B, see ground plan, (Figure 104)).

Returning now to the Pinbain Burn, and ascending this stream from the high road, we find black shales — evidently the prolongations of those visible on the beach — dipping towards the south-east at high angles, and containing oval masses of limestone. Here they occur in the midst of agglomerate, and are succeeded northwards by alternations of tuff and diabase-lava, till the rocks disappear under a deep covering of boulder clay.

To the north of the Pinbain Burn a fragment of the 25-feet beach ((Figure 104)) is bounded east of the road by a prominent cliff, and forms two recesses with a projecting spur in the centre. Agglomerate is exposed on the old sea-cliff as far as the more northerly recess, where the narrow band of serpentine visible on the shore forms the central part of the back-ground, bounded on the south side by agglomerate and on the north by green porphyritic diabase-lava. Two stacks project out of this raised beach, one composed of serpentine and the other of volcanic agglomerate. The latter is traversed by a basalt dyke (B) which runs up the north face of the pinnacle.

At the point where the northern margin of the old sea-cliff abuts on the road, dark blue and grey ashy shales, flags, and fine tuffs dip to the S.S.E. at high angles. On the south side they are faulted against the green porphyritic diabase-lava just referred to, and on the north side they are succeeded by volcanic agglomerate, containing bombs of volcanic rocks of various types; one variety consisting of a pink fine-grained felsitic rock, apparently more acid than any of the common diabase-lavas. In the centre of this sedimentary and volcanic series, where the beds consist of hard ashy ribs from two to three inches thick, alternating with shaly mudstones, graptolites have been found in thin blue shales which are remarkably fresh and have not been much disturbed. The following fossils were here collected:

Tetragraptus Bigsbyi (Hall.)

Tetragraptus quadribrachiatus (Hall.)

Tetragraptus bryonoides (Hall.)

Phyllograptus typus (Hall.)

Dendrograptus sp.

Dichograptus sp.

Climacograptus sp.

Caryocaris Wrighti (Salt.)

Discinoid shell.

This assemblage closely resembles that obtained from the Middle Arenig black shales at Bennane Head, though the bands containing them are believed to occupy a slightly lower stratigraphical position.

Proceeding now to the description of the shore-section northwards from the last-mentioned fossiliferous locality to Kennedy's Pass [NX 14821 93138], which marks the northern limit of the volcanic series on the coast, about a mile south-west of. Ardwell Bay, we find that the coarse agglomerate beneath the fossiliferous shales is followed by brown fine-grained tuff and sedimentary bands, not unlike those yielding the graptolites at the roadside. Northwards, a band of coarse tuff, like that in Stockinray Bay, to be described presently, is associated with lenticular bands of lava, the beds dipping seawards at high angles. To the north of this tuff, the shore is occupied by a beautiful porphyritic lava ("diabase-porphyrite") with large porphyritic felspars, measuring in some instances 9–10ths of an inch in length. The rock can be traced from the shore up the western slope of Pinbain Hill and over the crest of that eminence, where it varies somewhat in character; becoming in places rather fine-grained and not so markedly porphyritic. On the shore, at low tide, however, it displays the pillow-structure so characteristic of the more fine-grained diabase-lavas. Below high water mark it is pierced by a dyke of basic rock, which separates it from a remarkable type of agglomerate which will now be described.

In Stockinray Bay [NX 14061 91854], about 600 yards north of the mouth of Pinbain Burn, at low tide level, a conglomerate rock with a greenish matrix contains blocks of various volcanic materials, and differs to some extent from the usual pyroclastic materials connected with the lavas. It may be a small patch of the Benan conglomerate which occurs further north in Kennedy's Pass (postea p. 498).<ref>The Benan conglomerate is associated with the Stinchar limestone (Upper Llandeilo), and must be distingtished from the volcanic Agglomerate of Bennane Head of Arenig age.</ref> On the landward side of this conglomerate a mass of agglomerate occupies a considerable part of the bay. The matrix contains crystals of plagioclase set in a black, compact ground-mass, with a somewhat resinous lustre. Under the microscope, a specimen No. (S6425) shows that "the matrix is absolutely opaque, except at the extreme edge, where it appears to be of deep brown colour". Mr. Teall describes the rock as a porphyritic tachylite. Rounded blocks of porphyritic lava from six inches to a foot across, appear in the agglomerate (Plate 12). From its general characters, it is probable that the rock may be due to the explosion of a mass of pyroxenic lava. The rocks in this bay are traversed by dykes of porphyrite and dolerite, the former varying much in width and having a tortuous course.

The remarkable agglomerate of Stockinray Bay is succeeded northwards by a fine development of porphyritic lavas, with large porphyritic felspars, from half an inch to an inch in length (Plate 13). They form prominent cliffs, which afford excellent opportunities for studying the lithological characters of the rock. In places the felspar crystals are oriented in the direction of the flow of the lava. A specimen No. (S6419) taken from the shore at this locality is thus described by Mr. Teall: "Phenocrysts of altered greenish felspar in the form of thin tablets, often measuring half an inch across, occur in a dark fine-grained greenish matrix. The microscopic examination shows that the ground mass of the rock is formed of short laths of felspar, chlorite, grains of unaltered augite, and plates of ilmenite. The rock contains also irregular patches of considerable size, entirely filled with aggregates of chlorite. It is a diabase-porphyrite, according to German authors. Felspar concentrates at 2.64 (oligoclase)". These lavas occupy the shore for a distance of about 250 yards, and at their northern limit an irregular layer of jasper and about a foot of green tuff are followed by a mass of basic agglomerate. From this point northwards to the mouth of Kilranny Burn [NX 14547 92686], a distance of 700 yards, the shore displays an alternation of fine-grained vesicular and porphyritic lavas (diabase and diabase-porphyrite), and is traversed by six basalt dykes, some of which give off thin seams or veins.

The fossiliferous strata (Benan Conglomerate, &c.) visible on the beach north of Kilranny Burn that drains the northern slope of Pinbain Hill, will be described in the sequel at present it may be sufficient to state that, in the bay below Kennedy's Pass, the fault that brings the Benan Conglomerate in contact with the volcanic series is well seen, its course being marked by a small hollow'that runs north and south. Here, a basalt dyke about six feet broad rises along the line of fault, and shows a marked linear arrangement of its constituents parallel to its walls. The road section from Kennedy's Pass southwards for 400 yards traverses the Arenig lavas, where they are much shattered and disturbed by the adjacent fault. The thickness of the volcanic series exposed on the shore between Pinbain Burn and Kennedy's Pass is estimated at 1400 feet.

In the foregoing pages we have adduced evidence to prove that in the centre (Bennane Head) and near the northern margin (Pinbain) of the main volcanic area of Ballantrae, volcanic rocks and overlying sediments occur, which, on palaeontological grounds, can be shown to occupy similar stratigraphical horizons in the Arenig division of the Silurian system. We shall now proceed to show that near the southern limit of the main area, in the valley of the Stinchar, the Arenig cherts and underlying black shales likewise appear at the surface in association with coarse volcanic agglomerate, as at Bennane Head.

Mains Hill, Ballantrae, and Balnowlart. — [NX 09279 82864], [NX 10265 83154] Along the valley of the Stinchar, from near the bridge across the river at Ballantrae to Balnowlart, a distance of about a mile and a half, the radiolarian cherts and red mudstones can be traced at intervals in association with tuff and agglomerate. They are exposed at several places on the road, where they are interstratified with fine tuff, and they can be seen on the steep north bank of the river, traversed by dolerite dykes. The best section of them is to be found in a road cutting, at the back of Balnowlart Farm-steading, where the red cherts and mudstones are interbedded with green tuff, and dip to the N.N.E. at an angle of 45°. Certain dark purple seams might here yield fossils; indeed, a fragment of a graptolite was obtained, though it proved to be indeterminable. In the sequel we shall refer to the occurrence of the radiolarian cherts and black shales on another fold, about a mile further up the Stinchar valley (Figure 116).

This outcrop of radiolarian cherts is flanked on the north side by a peculiar type of agglomerate, which, extending from Mains Hill near Ballantrae to Balnowlart, with an average breadth of 300 yards, shows in places rude bedding, the planes being inclined at high angles to the N.N.E. It may be best studied in numerous quarries on Mains Hill, within half a mile of Ballantrae. The matrix is green, and contains numerous small lapilli of fine vesicular lava, together with blocks and masses of diabase-lava which contain porphyritic augite and plagioclase. A striking feature of the more basic parts of the mass, and of the enclosed blocks, is the abundance of crystals of augite. In this respect the rock bears a close resemblance to the augite-andesite tuff of Bail Hill, Sanquhar.<ref>See the "Ancient Volcanoes of Great Britain", vol. i., p. 195.</ref> A specimen taken from the agglomerate exposed in one of the quarries shows under the microscope No. (S6426) that "it is composed of fragments of igneous rocks, which vary somewhat in structure. The greater part of the section is composed of a rock containing large phenocrysts of augite, which is yellowish green in thin section, and phenocrysts of altered plagioclase set in a matrix of microlitic felspar, augite (?), and iron ores. The rock contains vesicles filled with carbonate". So large are some of the masses of diabase-porphyrite in this agglomerate that some of them may really be portions of lava-flows. The microscopic examination of a specimen No. (S5933), from one of these included masses shows "large idiomorphic crystals and grains of augite, measuring a quarter of an inch across, in a dull dark matrix. The ground mass is composed more or less of decomposed plagioclase in the form of short laths; grains of augite and magnetite; chlorite and other decomposition products are present". Like the agglomerate on Bail Hill, Sanquhar, that at Mains Hill points to the explosion of a tolerably basic mass of pyroxenie lava, and from the fact that agglomerate of this peculiar type is restricted to the neighbourhood of Ballantrae it is probable that it may have originated from an independent volcanic centre.

To the north of Mains Hill the augite-andesite tuff is succeeded by lava, protruding at intervals through the drift to within half a mile of Bennane Burn. From the appearance of the red mudstones and cherts in a field about a quarter of a mile S.S.E. of Croseclays, it may be inferred that the volcanic rocks are here repeated ley folds. The thickness of the agglomerate of Mains Hill is 800 feet.

The radiolarian cherts can be followed along the strike of the outcrop at Balnowlart about a mile still further to the northeast on the south side of Salachan Hill [NX 12920 84586], as will be shown in the sequel. Though the cherts at Balnowlart are inclined to the N.N.E., and appear there to plunge underneath the Mains Hill agglomerate, it is evident that an inversion of the beds has taken place, for about a mile to the east of Ballantrae, on the banks of the river and by the side of the road, the cherts are repeated by sharp folds, dipping sometimes to the north and sometimes to the south.

Knockdolian Hill, Ballantrae. — [NX 11320 84793] The development of agglomerate, breccia, and tuff round independent centres, which is one of the characteristics of the Arenig volcanic rocks of Ayrshire, is strikingly exemplified on the picturesque conical hill of Knockdolian (869 feet), which rises above the northern side of the valley of the Stinchar, two miles from Ballantrae. The slopes of the hill are comparatively rocky, only patches of vegetation occurring here and there. Excellent opportunities are thus afforded for studying the structure of the ground. The river Stinchar — the general trend of which is south-westerly — on reaching Knockdolian Castle, is deflected towards the south round the eastern base of the hill, and thereafter resumes its normal course to the sea (Figure 107).

The geological structure of Knockdolian presents some interesting features.<ref>See view of Knockdolian Hill from' the east, "Ancient Volcanoes of Britain", vol, i., p. 194.</ref> The hill is composed of bedded lavas and overlying breccias and agglomerates, which are arranged in the form of a great elliptical dome, the long axis of which runs E.N.E. and W.S.W. The core of the arch is formed of bedded lavas, which are confined to a small part of the eastern declivity south of Knockdolian Castle. The lowest bed of the volcanic series here visible consists of a beautiful porphyritic lava, with a dark green base and light green porphyritic plagioclase crystals, closely resembling the Currarie "diabase-porphyrite" already described. It is the most prominent member of the lavaform series, and is traceable for a distance of 400 yards, being followed by fine-grained vesicular diabase-lava like that on the shore at Downan (1B, (Figure 107)).

Overlying the bedded lavas comes a great succession of agglomerates and breccias, arranged in concentric layers which curve round the dome at angles varying from 50° to 60°, and form the greater part of the hill. The characteristic feature of the breccias is the absence of any fine-grained matrix, the rock being wholly composed of angular fragments of vesicular fine-grained lava. Though the average size of the included blocks over much of the hill is tolerably uniform, ranging from a quarter of an inch to two and three inches across, yet the bedding can be determined by- the alternating layers of coarser and finer composition. The coarsest materials are found on the western declivity, immediately to the north of Macherwhat Farmhouse [NX 11268 84388], where some of the included blocks measure four feet across. Viewing the agglomerate as a whole, we may observe that the materials become coarser as we pass westwards to the area north of Macherwhat, while the finest materials occur near Finnart Holm [NX 12337 84839], at the base of the eastern slope (1 Ts, (Figure 107)).

On the southern declivity, below the crest of the hill, several narrow bands of fine-grained diabase run more or less parallel to the bedding of the agglomerate. These may be intrusive, but this point is uncertain; at anyrate, several thin veins of similar material, not more than two inches thick, traverse the breccias. The microscopic examination of one of these black compact rocks associated with the agglomerate at the roadside north-east of Macherwhat No. (S6418) shows that "felspars, some of which are large enough to be termed microscopic phenocrysts, and magnetite, are the only definitely recognisable minerals. There is also a brown substance. The grain of the rock is so fine that its composition cannot be definitely determined by the microscope".

The peculiar type of angular volcanic breccia, so characteristic of Knockdolian Hill, is likewise found on the south-east side of the valley on Salachan Hill [NX 12920 84586], as will be shown in the sequel. From the evidence now brought forward the agglomerates and breccias of Knockdolian may be conjectured to have been the products of a cinder cone, the materials of which were to some extent assorted under water. From the gradation westwards in the coarseness of the materials, it is probable that the focus of eruption lay somewhere beneath what is now the western slope of the hill. The minimum thickness of the volcanic rocks of Knockdolian Hill is estimated at 800 feet (lavas, 300 feet, agglomerates and breccias 500 feet, the latter increasing in thickness westwards). Along the northern slope of Knockdolian, the lavas and agglomerates are abruptly truncated by a fault which brings intrusive igneous rocks of ultra-basic and basic types (serpentine, gabbro, &c.) in contact with them (Figure 107).

The horizon of the volcanic rocks of Knockdolian is defined by the occurrence of black shales and black cherts on the banks of the Stinchar, at the south-eastern base of the hill, at the west and east terminations of the alluvial flat named Finnart Holm. Though grantolites have not been obtained from these exposures, the bands are evidently on the horizon of the Middle Arenig black shales at Bennane Head (1 (Figure 107)).

Inland volcanic area south of Stinchar. — The contemporaneous volcanic rocks on the shore between Dove Cove [NX 06060 78885] and Downan Point [NX 06740 80334], which have been already described, stretch inland along the south side of the Stinchar valley, where they form a narrow belt about half a mile broad, extending from the coast-line at Downan to Little Dangart [NX 17360 86335] , a distance of eight miles. Although they consist mainly of bedded lavas, some intrusive igneous rocks occur among them between Salachan [NX 12433 84401] and Craigneil [NX 14418 85280], to the east of Knockdolian.

In order to understand clearly the geological structure of the volcanic area on the south side of the valley of the Stinchar, it must be borne in mind that there are here two groups of sediments, separated from each other by a marked unconformability, doubtless representing a great interval of time. The older group, comprising red, grey, and black radiolarian cherts and black shales, are conformable with the Arenig lavas; while the younger group, consisting of conglomerates, mudstones, and fossiliferous limestones (Craigneil), belong to the horizon of the Stinchar Limestone series and Benan Conglomerate. Both groups have been affected by a common system of folding, and hence much care is necessary to discriminate between them.

The southern boundary of the volcanic area forms a slightly sinuous line extending from Meadow Park near the shore, by Garphar, Knockdhu, to Little Dangart. Throughout nearly the whole of this distance, it coincides with a fault which brings the overlying strata of Upper Llandeilo age in contact with the volcanic series.

At the south-western limit of the area, near Meadow Park [NX 07365 79906], the relations of the bedded lavas to the Arenig cherts and overlying sediments, as already described, are shown, for a passage upwards from the lavas and cherts can there be traced into the green mudstones and greywackes (Tannins group). For about a mile to the north-east the red mudstones and cherts are visible at intervals in small knobs in the fields, and also in a section where a stream crosses the high road about a quarter of a mile to the north of Burnside [NX 07898 80319], which is the last visible outcrop of them. Thence north-eastwards to the Kilphin Burn [NX 10625 80614], at various spots the lavas are almost in immediate contact with green mudstones and pebbly greywackes without the intervening red mudstones and cherts. It is probable, therefore, that the fault is here of no great amount, being merely sufficient to hide the radiolarian cherts. North-eastwards to the Laggan Burn the rocks are buried under drift, and from the latter point to the Water of Tig, especially near Garphar, the volcanic series is almost in immediate contact with the green mudstones and greywackes. At one locality, in the road at Garphar Farmhouse [NX 11465 83065], the strike of the greywackes and shales is oblique to that of the lavas. In Knockdhu Burn [NX 13500 84434] the fault is actually visible: jointed ferruginous greywackes are there brought into contact with shattered lavas, the strata being much disturbed and broken on both sides of the dislocation (Figure 107). Thence to Craigneil [NX 14662 85097] and Strifeland Burn [NX 15249 85271] that joins the Stinchar south-east of Colmonell, the evidence is concealed, but in that stream the fault is again seen, bringing greywackes with green and red mudstones against the lavas. From the Strifeland Burn to Little Dangart the junction line is obscured by drift.

From the shore at Downan [NX 06739 80341] to Heronsford [NX 11993 83601], on the Water of Tig, the bedded lavas, as seen in the fields and in burns that drain into the Stinchar, consist of vesicular diabase with pillow-structure, of the same general type as those on the shore already described (Figure 106). From Heronsford east to Craigneil Castle [NX 14718 85320] the geological structure is somewhat complicated, owing to the infolding of a narrow belt of fossiliferous limestones, mudstones, and conglomerates (Stinchar Limestone and Benan Conglomerate) which are in immediate contact with the volcanic series and radiolarian cherts.

To the south-east of Knockdolian Castle, the Salachan Hill [NX 12942 84780] (Figure 107) rises to a height of over 300 feet from the alluvial terraces on the south bank of the river, where the Arenig cherts and black shales are associated with lavas and volcanic agglomerates. The relations of the strata are represented in (Figure 107). At the bend in the Stinchar opposite Finnart Holm [NX 12536 85124], black cherts are exposed on the east bank of the river, probably a prolongation of the outcron abort half st mile to the south-west, at the southern base of Knockdolian Hill, These cherts are followed southwards by volcanic agglomerates containing fragments of chert while the north-west slope of Salachan Hill [NX 12758 85083] presents volcanic breccia and agglomerate of the same type as those of Knockdolian Hill, being made up of angular and sub-angular fragments of fine-grained diabase. About the level of the 200-feet contour line, the agglomerate is succeeded by fine-grained vesicular diabase-lava, probably occurring in a fold. Southwards the lava is succeeded, at a level of 300 feet, by more volcanic breccia of the Knockdolian type, with, however, an ashy matrix. Advancing southwards across the hill we find breccia of a more basic type, till in the fields on the southern slope two outcrops of vesicular lava with pillow structure occur, separated from each other by volcanic breccia with fragments of black chert. At the more southerly exposure, in a small quarry in a field, the rock, though highly decomposed, shows the characteristic pillow-structure in small oval-shaped masses. Here the evidence is partly concealed under drift, but the exposures in the adjoining fields indicate that the lavas and volcanic breccias are here associated with cherts and black shales. The red cherts of this locality (about a third of a mile E.N.E. of Salachan Farmhouse [NN 00035 51620]) are coated and veined with hematite to such an extent as to have led to the belief in the existence here of a rich vein of iron ore.

South-eastwards on the high-road leading to Colmonell, the radiolarian cherts are to be seen on the south side of a small wood, close to a mass of gabbro. They are slightly granulitised, probably owing to contact metamorphism produced by the adjacent eruptive rock. On the south side of the gabbro mass a belt of serpentine is well seen in quarries at the roadside. Southwards gabbro again appears, followed by serpentine at a bend in the burn of Knockdhu. Southwards, in that stream beyond a blank in the section, the Arenig lavas appear, followed by conglomerate, which, from the included pebbles, is evidently on the horizon of the Benan conglomerate. Within a short distance the lavas reappear, followed unconformably by conglomerate, green mudstones, and grits. The blocks of limestone in the conglomerate have yielded specimens of Girvanella. Here the burn flows through a rocky gorge, at the head of which, a fault brings the unconformable fossiliferous mudstones and grits in conjunction with the radiolarian cherts and mudstones of Arenig age to the south. Higher up the stream the lavas reappear, till at a point about 500 yards above Knockdhu Bridge the fault occurs that marks the southern limit of the volcanic area.

Though in this section but a small development of the strata rests unconformably on the volcanic rocks and radiolarian cherts, yet they can be traced for some distance to the south-west and north-east, as will be shown: in the sequel.

The narrow strip of Arenig volcanic rocks visible in-the Craigneil and Strifeland Burns, between two faults; is composed mainly of diabase-lavas with pillow-structure. These rocks are well seen also on the rising ground immediately to the south of the fossiliferous Stinchar Limestone group at Craigneil Farmhouse. About midway between the two faults which bound the narrow volcanic band in the Strifeland Burn, a bed of tuff may probably lie near the top of the volcanic series.

Colmonell to Aldons. — [NX 14856 85990], [NX 19702 89374] A narrow belt of volcanic rocks lying between two faults stretches from the village of Colmonell along the northern slope of the Stinchar valley to Aldons, where it crosses the river in the direction of Pinmore Mains [NX 21040 90046] (Sheet 8 of Survey Map). Along the southern margin of this volcanic band, conglomerates, sandstones, and mudstones (Kirkland and Benan series) are brought into conjunction with the lavas; while the fault on the north, side brings intrusive igneous rocks, Arenig lavas, and the Aldons (Stinchar) Limestone and conglomerate successively in contact with the volcanic rocks.

Throughout this area, the volcanic rocks consist chiefly of lavas showing pillow-structure, which are well displayed in the glen near Poundland and in the ravine west of Daljarrock [NX 18806 87907]. An interesting development of agglomerate and breccia may be seen on, the Craig Hill [NX 16596 87419] and Prieston Hill [NX 16034 87237], about a mile northeast of Colmonell village, where these pyroclastic materials are associated with black shales and radiolarian cherts. This agglomerate, which contains blocks of volcanic rocks, together with pieces of chert and black shale, set in a green matrix, seems to lie in a series of compound isoclinal folds; dipping to the southeast, the best exposure being on the southern slope of Craig Hill. The rock is underlain by fine-grained diabase-lava, showing pillow-structure. At several places along the junction-line between the agglomerate and lavas, a thin band of black shale, in places much corrugated, makes its appearance. It is seen near the ruins of a shepherd's house about 400 yards N.N.W. of Craig Farm [NX 16748 87093], and in greater development about 100 yards further north, near a stone fence. The radiolarian cherts and red mudstones, some of which are nodular and others more massive, form four isolated patches on the south slope of Prieston Hill, about the level of the 500-feet contour line [NX 16049 87067], tha largest outlier measuring about 400 yards in length.

From the lithological characters and order of arrangement of the volcanic rocks and associated sediments on the Craig and Prieston Hills, there can be little doubt that we have here representatives of the sequence already described as occurring at Bennane Head. No fossils have yet been obtained from these bands, but it is highly probable that a prolonged search would result in the discovery of Middle Arenig graptolites in the black shales.

Further to the north-east, on Glessal Hill [NX 18095 87887] and Bargain Hill [NX 19032 88380], lenticular masses of agglomerate occur, which are not found in the intervening gorges, and may therefore be outliers of more widespread deposits. The old limestone quarry at Daljarrock [NX 18939 87791] seems to have been opened in a vein rock along the line of fault that separates the conglomerates and sandstones of the Kirkland series (Upper Llandeilo) from the lavas. A specimen No. (S6437) of dark massive green rock from this quarry  showed no definite structure under the microscope, save that it was originally an igneous rock with lath-shaped felspars.

Aldons Hill. — To the north of the area described in the foregoing paragraphs, a patch of ground about a square mile in extent is occupied chiefly by bedded lavas. It comprises the Aldons Hill [NX 19124 90282], with parts of the Breaker Hill [NX 17734 89050] and Fell Hill [NX 18401 90172]. The lavas consist mainly of the vesicular diabase type, showing pillow-structure, but occasional flows are composed of diabase-porphyrite. The northern slopes of the Aldons and Fell Hills, overlooking Chapelcroft, are marked by a band of fine-grained flinty rock, that weathers white. To the south of Chapelcroft, where it occurs in knolls east of the burn, this rock is very compact like chert, and when traced about a quarter of a mile to the west, becomes vesicular. Microscopic examination does not furnish any evidence as to its origin, but it may probably be one of the fine-grained lava-form rocks. A lenticular mass of coarse agglomerate occurs on knolls between the Breaker Hill and Bargain Hill. The lavas of the small tract now referred to are bounded on the east and north sides by the Benan conglomerate, which there rests unconformably on the volcanic series, and on the west side by one of the largest intrusive masses of serpentine.

Central Volcanic Area between Bennane Head and Lochton. — The central area of bedded volcanic rocks now to be described forms a narrow belt varying from half a mile to a mile in width, and is bounded on the north and south by the two parallel intrusive masses of serpentine. It stretches inland from Bennane Head [NX 09126 86572], by Knockormal Hill [NX 13230 88070]  and Carleton [NX 12810 89233], to Lochton [NX 17677 92841], in the valley of the Lendal Water, a distance of a little more than six miles. The radiolarian cherts, the Arenig black shales and underlying agglomerates, the diabase and diabase-porphyrite lavas, which are so typically developed on the shore between Bennane Head and Balcreuchan Port, are all met with in this central area, where they are repeated by various folds. It is true that, owing to cleavage and lines of movement traversing the strata, especially the black shales, their stratigraphical horizon has not been definitely jproved by fossils within this district, but the general sequence of rocks, taken in connection with the fact that the cherts, black shales, agglomerates, and lavas of the inland area occupy the same series of compound folds as those at Bennane Head, points to the conclusion that the rocks are of the same age.

The tract immediately to the east of Bennane Head, and along the course of Bennane Burn, is largely covered with drift, but agglomerate occurs in the fields north-east of Balcreuchan Farmhouse, near Troax, and again at South Ballaird, near the source of the Bennane Burn, where it is associated with vesicular diabase-lava. In the field to the east of South Ballaird Farmhouse [NX 12240 87373], along the course of a footpath, and in adjacent quarries, black shales, grey shales, and cherts are reduplicated by folds. On a small hill at the north-east corner of this field, agglomerate is exposed at the surface, followed by black shales at the base of the slope and in the heart of the mass to the E.N.E. of South Ballaird Farmhouse, where they appear to lie in a fold of the agglomerate. Again, on the western slope of the closely adjacent Lochton Hill [NX 13159 87540], a mile and a half south of Lendalfoot, black shales are repeated by sharp folds like the bands to the west. The volcanic breccia is traceable northwards from Lochton Hill for a few hundred yards to Knockormala Hill [NX 13212 88081], and east wards to the Moak Hill [NX 13853 87907], where it appears to rest on diabase-lavas with pillow-structure. The junction-line between the lavas and the agglomerate is a complicated one, inasmuch as the two groups of rock form a series of insoculating folds. In like manner, where the agglomerate becomes fine-grained on the south slope of Lochton Hill, the isoclinal folding of the tuff and the black shales is evident.

The hollow to the east of Knockormal Hill is covered with drift, but on Knockdaw Hill [NX 16203 88910], to the S.S.W. of Millenderdale, and in the burns draining the north-west slope of that eminence, there is a fine development of lavas, composed mainly of vesicular diabase, and in places of diabase-porphyrite. Thin lenticular masses of blue limestone in one of the burns south of Knockdaw Castle [NX 15102 89262] resemble the sedimentary limestone that fills the interstices between the spheroids of lava on Downan shore, south of Ballantrae. The relation of the lavas to the intrusive gabbro and serpentine along the southern slope of Knockdaw Hill will be described in the sequel (see p. 471).

The northern portion of the central area of volcanic rocks presents several features of special interest. Skirting the shore to the south of Lendalfoot, the lower portion of the western declivity of Carleton Hill [NX 12758 89327] is occupied by serpentine, while the remainder of the ridge is formed of a complex of diabase-lavas and tuffs with black shales, epidiorite, hornblende-schist, and flaser-epidote rocks, the whole being traversed by a later series of dolerite dykes, older than the Benan. Conglomerate and Stinchar Limestone, and a few basalt dykes, probably of Tertiary age. The lithological characters, exact limits, and relations of some of the members of this complicated area might well furnish materials for more detailed investigation.

On the south-east slope of the hill, about the level of the 500-feet contour line, a band of black shale can be traced for some distance, which, over part of the outcrop, is so cleaved that fossils would be difficult to obtain from it. At the southern limit of Balsalloch Hill [NX 12359 88791], however, a continuation southwards of Carleton Hill, the beds are not deformed, and might be expected to yield graptolites, but a short search has so far not been successful. The soft black shales are there associated with black cherts or flinty shales, like the Middle Arenig band at Bennane Head, and rest on tuff and diabase-lava, but along their line of outcrop, on the south-east slope, masses of gabbro occur close to them. The diabase-lava is traceable along the south-east slope of Carleton Hill, but along the top and western slope the rocks have undergone considerable change. One of the recognisable lavas of the porphyritic type from this hill No. (S5923) shows, in the hand specimens, "conspicuous phenocrysts of white felspar in a compact dark green matrix", and under the microscope large and small phenocrysts of felspar and small phenocrysts of augite in a brown matrix, which is not isotropic".

Along the western slope of Carleton Hill, above the margin of the serpentine, a remarkable series of platy rocks of peculiar lithological characters dips at high angles to the north-west, and may be studied in many excellent exposures. They are traversed by numerous dolerite dykes running in a westerly direction. They have a dark purple and green tint, are very fissile, and break under the hammer into small angular fragments it one point, high up the slope south of Carleton Fishery [NX 12448 89245], they contain a thin lenticular band of blue crystalline limestone. Generally they are fine-grained; indeed, in some instances the constituents cannot be definitely determined under the microscope, but nearly all the rocks show marked flaser structure. They have been traced along the border of the serpentine area from the west slope of Carleton Hill, across the Lendal Water by Laigh Knockclauch [NX 17268 91812] to Lochton [NX 17683 92835], a mile and a half south of Byne Hill, where the rocks have been subjected to considerable dynamic movement. Some of these platy rocks contain epidote in considerable quantity, interleaved in green bands of hornblende-schist.

A specimen taken from the north-west face of Carleton Hill (Np. 6501) shows under the microscope "that the dark portion of the rock possesses marked parallel structure due to shearing, but is of so fine a grain that the minerals cannot be definitely determined. The light yellowish-green Lands contain epidote, a pale brown garnet, and possibly malacolite". Another specimen interleaved in the platy rocks from the same place No. (S7783) [NX 12743 89326] displays "a foliated aggregate of pale green hornblende, showing merited signs of pressure fluxion. Some felspar was probably present in the original rock, but it is not now definitely recognisable". A third specimen from the same series No. (S7784) exhibits "'eyes' of felspar in a fine-grained matrix of hornblende-schist. Mr. Teall is inclined "to regard this rock as a porphyritic dolerite or diabase modified by dynamic action. The hornblende is green, not brown". Still another microscopic section, taken from an exposure in the Lendal Water behind the gamekeeper's cottage No. (S6507), "contains streaks and flaser rich in epidote", similar to No. (S6501). What the precise origin of these flaser-epidote rocks may have been is at present uncertain, but this much is clear, that the planes of schistosity must have been developed in them and in the associated hornblende-schist before the intrusion of the dolerite dykes, which are older than the Stinchar Limestone and the Benan. Conglomerate (Upper Llandeilo). In the sequel we shall show that the south-eastern margin of the serpentine mass between Carleton Hill and Lochton has been a line of movement, and that these platy rocks may have had their schistose planes impressed on them by that movement.

In the hollow between Carleton Hill and the intrusive mass of gabbro on Knockormal Hill [NX 12759 88691], the volcanic rocks consist mainly of bedded lavas with occasional bands of agglomerate. About a third of a mile to the south of Carleton Castle (in ruins), in a road cutting by the side of Little Carleton Burn, black shales with a south-easterly dip, rest on tuff, containing fragments of black shales; while underneath the tuff lies some compact diabase-lava. It is highly probable that this outcrop of black shales may be an exposure of the Middle Arenig band of Bennane Head. This suggestion seems to be confirmed by the fact that in the small burn that joins the Lendal Water at Cundry Mill [NX 14403 90032], to the east of Carleton Mains, red mudstones and radiolarian cherts appear in association with agglomerate. These rocks are met with about 400 yards up the stream, while near the foot of the same burn lavas are associated with the red cherts and mudstones.

Near the farm-steadings of Knockdaw [NX 15079 89614] and Laigh Knockdaw [NX 15206 89803], south of the Lendal Water, Arenig lava, both of the porphyritic and diabase types, is exposed in the burn sections and in knolls in the fields. These outcrops are of importance, since they help to fix the position of the fault that brings in the younger fossiliferous rocks of the Girvan series to the east. Fine-grained lavas are also visible in quarries behind Barclewan Farmhouse [NX 15479 90181], on the north bank of the Lendal Water. A beautiful example of the porphyritic type is to be found on the hill-top above Pennyland Wood [NX 15749 90651], about 500 yards to the north of Barclewan, where large porphyritic felspars, sometimes an inch in length, project on weathered surfaces. The lavas are here traversed by basalt dykes running in a north-west and south-east direction. About half a mile to the west of this hill-top, debris of red mudstones and cherts is ploughed up in the fields, on the strike of the outcrop in the small burn that falls into the Lendal Water south of Cundry Mill, above referred to.

Along the south-east margin of the serpentine area, in the direction of Knockbrake [NX 27646 64851], Laigh Knockclauch [NX 17273 91812], and Lochton [NX 17688 92849], several sections in streams which drain the hill slope west of the Lendal Water, display lavas, tuffs, black shales, and flaser rocks. About 300 yards west of the ruins of Knockbrake, diabase-lava, black shale, and mudstone occur on knolls to the south of the serpentine. Again, in a small burn west of Laigh Knockclauch, the rocks bordering the serpentine are laid bare for a distance of 200 yards. At the mouth of this streamlet, a vein of serpentine is exposed, followed by black shales yielding fragments of Tetragraptus and Climacograptus, and by porphyritic diabase-lava. Further up stream the rocks display under the microscope Nos. (S7800), (S7801) the structure of intrusive rocks rather than that of lavas, the augite being quite fresh. Near the margin of the serpentine, compact platy rocks are met with, a specimen of which No. (S7803), when examined microscopically, is found to be a fine-grained hornblende-schist containing fallen epidote.

Similar evidence of flaser-structure occurs on the hill slope about 300 yards further north, and a quarter of a mile south-west of Loch Lochton [NX 17057 92154], where a lenticular strip of garnetiferous hornblende-schist lies near the margin of the serpentine, but is apparently surrounded by that rock. A specimen taken from this exposure, at a point about 600 yards north-west of Laigh Knockclauch No. (S7804), shows "brown hornblende, malacolite, and felspar". Another specimen, collected from the same band of hornblende-schist, a few yards to the west No. (S7805), exhibits "streaks and flaser of epidote in a very fine-grained matrix show-, fug pressure fluxion. The matrix is similar to that of No. (S7802). It is a rock which has been subjected to considerable dynamic action, and the timer of epidote may possibly represent porphyritic crystals of felspar which have been epidotised".

In the Lochton Burn serpentine is seen for about 300 yards above Laigh Knockclauch, followed by an intrusive basic mass like gabbro. Next come black shales, visible at a waterfall, succeeded by alternations of lavaform rocks and flaser-rocks. Along the road that skirts the west side of Loch Lochton, several outcrops of tuff or agglomerate with intercalations f black shales are met with, one in a small quarry due west of the south end of the loch. The tuff, agglomerate, and black shale are much shattered, altered, and injected with igneous material.

Similar evidence is obtained in the small burn that passes Lochton Cottage and joins the Bynehill Burn. Behind the ruins of the shepherd's house at Lochton, tuffs and black shales are visible, followed by shattered and altered lavas, which are succeeded in turn, as we pass north-westwards, by flaser-rocks, highly sheared, and in contact with the serpentine, the whole section being only about 100 yards in breadth. Between Lochton [NX 17660 92901] and Balkeachy [NX 18574 93438], porphyritic lavas are visible on knolls near the stream, and highly sheared rocks are seen at a sharp bend in the burn itself. The physical relations of these rocks, however, are entirely obscured by drift and alluvium. The position of the boundary fault along the south-east margin of the serpentine is indicated by shattered exposures of that rock in the Bynehill Burn, about 400 yards west of Balkeachy Farm steading [NX 18209 93387]. Here an old shaft was sunk in search of copper ore.

From the foregoing evidence it is clear that, along a line extending from Carleton Hill to Lochton, south of Byne Hill, black shales occur in association with agglomerate, tuff, and lava, while near the margin of the serpentine a series of highly sheared materials, including flaser-epidote rocks, has undergone much deformation. Though the black shales of Lochton and Carleton Hill have not as yet yielded graptolites, they are provisionally correlated with the Middle Arenig band at Bennane Head, owing to the occurrence of Tetragraptus in black shales at Laigh Knockclauch (p. 458).

Northern inland volcanic area, including Pinbain Hill. — The remaining district of the main volcanic area of Ballantrae which now falls to be described includes a small triangular tract, lying to the east of the shore-section already described between the mouth of the Pinbain Burn [NX 13768 91428] and Kennedy's Pass [NX 14818 93146]. The bedded lavas and tuffs visible on that part of the shore when traced inland for a short distance, are seen to be bounded on the north by the Benan Conglomerate, and on the south and east by ultra-basic and basic intrusive rocks.

The beautiful diabase-porphyrite lava visible on the shore south of Stockinray Bay [NX 14070 91853] forms the greater portion of Pinbain Hill [NX 14845 92056], and is prolonged eastwards into Kilranny Hill [NX 15472 92296], where it forms the highest part of the ridge. Along the south-east aide of Pinbain Hill it lies immediately in contact with the most northerly mass of serpentine, but north-eastwards towards Kilranny Hill the strike of the beds turns more to the north, and a gradually thickening wedge of agglomerates, tuffs, and ashy sediments comes in. These rocks are identical in character with those found on the Pinbain shore to the north of the serpentine. The pyroclastic materials and ashy sediments dip at high angles to the south-east, and become more fine-grained in that direction near their point of junction with the intrusive rocks; indeed, some of the finer bands closely resemble the ashy shales which yield Middle Arenig graptolites in the road-cutting on the Pinbain shore (p. 444), and there can be little doubt that they occupy a similar horizon. These beds are also visible near the head of a burn that drains the north-west slope of the Grey Hill, and falls into the sea near the fourth milestone from Girvan.

The west side of Pinbain Hill affords a good section across the strike of the beds in a burn that drains its slopes, and also those of Kilranny Hill. In the burn, above the point where it is crossed by the old road, the agglomerate, so well developed in Stockinray Bay, is exposed, while the beautiful porphyritic lava (diabase-porphyrite) which forms the northern headland of that bay is also seen in the same burn at the old road. The red cherts occur to the north-east of this section.

Small inliers of Arenig volcanic rocks near the main volcanic area of Ballantrae. — To the east of the main volcanic area of Ballantrae, several patches of Arenig volcanic rocks form "inliers" among the younger unconformable fossiliferous strata of Silurian age. Of these, the most important is met with on Daldowie Hill, on the south side of the Water of Assel, where the lavas, tuffs, intrusive gabbro, and associated cherts form a patch about a mile long and a third of a mile broad (Sheet 8). From the disposition of the overlying fossiliferous sediments of the Stinchar Limestone group, it would appear that the rocks of this "inlier" form a compound anticline, arranged in the following descending order: (1) Cherts and jasper containing radiolaria with calcareous knots; (2) basic lavas showing pillow-structure; (3) fine tuffs or ashy sediments; (4) volcanic agglomerate.

The volcanic agglomerate or coarse breccias in a narrow strip towards the north-east margin of the area, are followed southwards by a band of finer tuff or ashy shales, and these by a broad zone of diabase-lava, in which the radiolarian chert and jasper occur. The volcanic rocks are pierced by dykes and bosses of gabbro, which locally harden and alter the agglomerate and breccia. Blocks of gabbro occur in the adjacent conglomerates associated with the Stinchar Limestone group, but they have not been detected in the volcanic agglomerates.

In several places on Daldowie Hill [NX 21897 92797] and Shalloch Hill [NX 21902 93222], ribs and bands of hard purple chert and jasper occur, which contain radiolaria. Pieces of these cherts are common in the adjacent conglomerates of the Stinchar group. At one spot (marked "Whinstone Quarry" on the six-inch map [NX 21845 93372]), at the west end of Shalloch Hill, a peculiar rock, quarried for dyke-building, consists of red cherts and jasper, brecciated in place and recemented by red calcite. The crystalline limestone at this place contains oval masses and knots of a rock resembling serpentine or other ultra-basic material, but microscopic examination throws no light on its orgin.

A few yards to the south of the inlier above-mentioned, a small patch of volcanic tuff, charged with blocks, of diabase-lava, is visible near the head of the Minuntion Burn, associated with the Benan Conglomerate and Stinchar Limestone.

In the south quarry nearest the Lane Toll at Craigwells [NX 25691 95422], 2½ miles north-east of Daldowie Hill, decomposing gabbro rises from underneath the Stinchar Limestone group; further to the south-west a small, mass of ultrabasic rock, is likewise exposed near the limestone.

West of the Girvan and Portpatrick railway, small bosses of Arenig lavas, with intrusive gabbro, project from the surface in the fields to the north of Meikle Letterpin [NX 19806 91617]. Small exposures of the Benan Conglomerate are found near the lavas, but the actual junction of the two rocks is not seen.

Another small inlier of agglomerate is visible on Glendrissock Hill [NX 18988 94201], about half a mile to the south-east of the farmhouse of that name. On the south side of the hill black shales and cherts occur, not far to the north of the outcrop of the Stinchar Limestone between Pinmacher and Laggan, which may be in the horizon of the band at Bennane Head that yields the Middle Arenig fauna.

Craighead (Sheets 8 and 14 of the Survey Map). — The largest and most important inlier of the volcanic series occurs at Craighead, on the north side of the Girvan valley. Between Killochan Station and Craighead limestone quarries, it measures about a mile in length, and between the Girvan railway and the top of Craighead Hill [NS 22470 01142], about half a mile in breadth. This area seems to be wholly composed of vesicular diabase-lava with pillow-structure, which is well seen in the railway cutting at Killochan Station [NS 22187 00622], in the cutting in the wood near the Craighead limestone quarries [NS 23280 01297], in knolls in the wood north-east of Woodhead Farmhouse, on the top of Craighead Hill, and in small rocky knolls in the cultivated ground to the north-west of that hill (1 B, (Figure 109)).

On the south-east side the volcanic area is truncated by a fault which brings down the Carboniferous rocks, and at the eastern corner the Arenig lavas are unconformably overlain by the Craighead Limestone series, which will be described in the sequel. On the north-west side, however, the lavas are succeeded by red radiolarian cherts and green mudstones, visible in the road leading to Barneil Farmhouse and in the fields and wood about a quarter of a mile east of Woodhead [NS 22018 00955]. The charts are vertical or highly inclined, and are immediately followed by green mudstones, shales, and greywackes resembling the "Tappins group" of the Stinchar valley. The radiolarian cherts cannot be traced round the northern margin of the inlier towards Craighead; indeed, judging from the surface-features in that area, the lavas seem to he abruptly truncated by a fault, but the overlying strata north of Craighead Hill are buried under drift.

In a quarry in the small wood at Woodhead Farm steading [NS 21612 01024], red cherts, black shales, and black cherts appear in association with volcanic agglomerate, and a few yards to the east of the wood there is an exposure of basic igneous rock. Though only Climacograptushas been got from these black shales, vet their lithological characters and their occurrence with agglomerate suggest the probability that they lie on the horizon of the Middle Arenig Shales at Bennane Head. The small outcrops of volcanic rocks evidently belong to a separate inlier from that of Craighead (1, 1 Ts, (Figure 109)). About a mile to the north-east of the Craighead limestone quarries, at Blaweary, still another inlier of diabase-lava is traceable for about 200 yards, in association with the limestone of the Craighead series.

Inliers South of the Stinchar Valley near Barr (Sheet 8). — [NX 27739 94148] On the south side of the valley of the Stinchar, in the neighbourhood of Barr, various arches of Arenig lavas and radiolarian cherts occur as inliers in the green mudstones, shales, and greywackes of the Tappins group, followed in normal sequence by the greywackes, shales, and grits of the Southern Uplands. Indeed, the sections now under consideration are identical with those on the shore at Brakness Hole and Shallochbraik, south of Currarie (p. 424), which show a passage upwards from the lavas and cherts to the overlying mudstones, shales and greywackes.

A glance at the published map (Sheet 8) will show the various lenticular bands of Arenig chert with cores of lava which have been traced at intervals from the head of Muck Water northeast by the Howe of Laggan to Aldinna [NX 35320 95155], in the valley of the Stinchar, a distance of about seven miles. These outcrops are mostly isoclinal arches, and resemble many similar lenticular folds of the lavas and cherts along the northern margin of the tableland between Loch Doon and the Lammermuir Hills. The lavas are traceable by means of small bosses across the moor from the head of the Muck Water towards the Gregg Water, re-appearing in a small tributary of the latter stream, on the col of the Howe of Laggan, and on the slope of the Pinbreck Hill, which is about a mile from Aldinna, in the valley of the Stinchar. They consist of fine-grained green diabase with small rounded vesicles resembling the vesicular diabase type at Downan. In each case they are associated with red radiolarian cherts which pass upwards into green mudstones, shales, and greywackes. The section that best shows the relation of the lavas to the cherts may be seen in a small tributary of the Gregg Water, about 200 yards to the south-west of Darley shepherd's house[NX 29551 91455]. Near the foot of this stream a fine exposure of slaggy diabase-lava forms the core of the arch, surrounded by red cherts with radiolaria and green mudstones, the beds being isoclinally folded to the south-east, The relation of the Arenig cherts to the green mudstones, shales, and greywacke bands of the "Tappins group", is admirably seen in a small tributary of the Stinchar, on the north side of the valley and to the east of Tappins Hill. At the foot of this stream, the radiolarian cherts dip towards the south-east, while up stream the barren beds of the "Tappins group" are well displayed.

About a mile and a half to the north of the strike of the series of arches just referred to, and within half a mile of the river Stinchar at Dularg, a small exposure of Arenig lava is associated with red cherts on the hill to the east of Traboyack Farmhouse [NX 26187 92084].