Tyrell, G.W. 1928. The geology of Arran. Edinburgh, HMSO [for the Geological Survey]
Chapter 12 The Cainozoic igneous rocks
Quartz-dolerite and related craignuritic and felsitic intrusions
A. Introduction
We now deal with a numerous and extensive series of intrusions which consist mainly of a basic or sub-basic rock, quartz-dolerite, and its differentiates in the direction of the intermediate type known as craignurite,<ref>See Note 3 p.141.</ref> and finally felsites of related characters. These rocks are to be found mainly in the southern third of the island, and are there widely distributed as sills intruded into the flat-lying or gently-dipping Triassic strata. Dykes, and other masses of transgressive character, are distinctly rare as compared with sills. These sills constitute the main element in the scenery of southern Arran, as they are more resistant to denudation than the soft Triassic strata into which they are intruded, and stand out as scarps with intervening terraces which recall those of the Carboniferous lavas of the mainland. This scarp and terrace topography is particularly well marked in Glen Ashdale (map,
Glen Ashdale and its headwater, the Allt Dhepin, presents a magnificent section of the quartz-dolerite and related sills, and it is therefore proposed to describe this section first as the typical exposure. The sills to the north and south of Glen Ashdale will then be treated. After this will be described in succession the series of large intrusive masses centred about Tighvein, 3.5 miles west of Whiting Bay, and extending northward through Sguiler, The Ross, Cnoc Dubh, and the Sheans, to the Brodick district; and the intrusions of Auchareoch, Auchenhew, Levencorroch, and Kildonan, which are probably southward extensions of the Tighvein masses. Finally, the intrusions in the valley, and at the head, of the Sliddery Water, and a series of western masses, Ballygown, Beinn Tarsuinn, and Tormusk, situated to the east of Blackwaterfoot, will be dealt with.
The intrusions treated in this chapter are remarkable for the abundance of composite sills, and of the phenomena of xenolithic enclosure and hybridization along interior contacts, which they present. In general three petrographic varieties are distinguishable in the field; (1) a dark grey, or bluish-grey, medium-grained to fine-grained dolerite, passing to basaltic types towards their contacts and in the smaller masses — 'blue basalt' was the appropriate field term utilized for these rocks; (2) a light bluish-grey to ash-grey, fine-grained to compact rock, obviously more acid than the 'blue basalts', — this type is identical in composition with the craignurites of Mull; <ref>The Tertiary and Post-Tertiary Geology of Mull, Loch Aline, and Qban,' Mem. Geol. Surv., 1924, p. 224. In later footnotes referred to as Mull Memoir.</ref> (3) a light grey or yellowish to white rock, granophyric to felsitic in texture, in which minute blebs of quartz can usually be distinguished. These types, while broadly recognizable in the field, are nevertheless connected by many intermediate varieties, and form a continuous petrographic series from dolerite to granophyre, or basalt to felsite (p. 141). In the majority of cases the felsitic rocks are the latest injections, and xenolithic enclosure and hybridization of dark basic fragments included within a matrix of felsitic composition is a common feature. In some cases interior contacts are marked by the xenolithic enclosure of one textural or compositional variety of basaltic or intermediate rock by another.
B. Field-relations
1. The Glen Ashdale–Allt Dhepin Section (Figure 11)
The first and lowermost sill is met with at the large waterfall (p. 4) in Glenashdale Wood, one mile from the sea. The lower contact is well seen in the face of the upper fall, and along the escarpments on both the north and south sides of the gorge. The sill rests on horizontal baked sandstones. The basal rock is a compact, banded, pink and grey quartz-felsite which, 5 feet up, remains substantially the same rock, but becomes a trifle coarser in grain. At 10 to 12 feet above the base the coarse felsite passes rapidly into a fine-grained, vesicular 'blue basalt'; Unfortunately no visible feature marks the change on the weathered surface, and the section becomes very inaccessible at this critical point. The 'blue basalt' becomes coarser as it is traced upward; and at the top of the fall, as well as for some distance above it, the rock is a medium-grained quartz-dolerite, which is intersected by several thin, irregular, basaltic dykes, and by acid segregation veins.
Farther up the stream the grain-size begins to diminish until, a few yards within the western boundary of the wood, there is encountered a band of dense 'blue basalt,' containing numerous, faintly-defined, darker xenoliths. This rock rapidly becomes coarser to the west, and forms a strong ledge and a waterfall just within the wood. One hundred and fifty yards above the wood the sill is again exposed. The rock is here slabby, and is deeply weathered to a buff colour, but the blocks are blue-hearted. It is a rather more felspathic type of quartz-dolerite than that of other parts of the composite sill. A spherulitic felsite, similar to that at the lower contact, follows, but with no traceable passage. The top of the sill is seen at the bend a twelfth of a mile above Glenashdale Wood, passing under 2 feet of sandstone, above which comes a small felsitic protrusion, not more than 4 feet thick, although the actual junction with the sediments is not exposed. The uppermost 2 inches of this intrusion are composed of a black, banded, pitchstone-like rock (S24874)
The Glenashdale sill is thus composite, and consists of four main parts, from above downwards as follows (see Section,
- The upper felsite, with intercalation of sediment
- Felspathic quartz-dolerite or craignurite
- The main quartz-dolerite
- The lower felsite
There is a xenolithic band between 2 and 3 indicating an interior contact. The order of injection still requires some further elucidation. It appears probable, however, that the marginal felsite was first intruded, and that after cooling it was split centrally by the main quartz-dolerite injection. While the latter was still hot it was penetrated by a further protrusion of slightly more acid material, which caught up fragments of the basaltic contact rock, and included them in its marginal parts as xenoliths.
The next sill in upward succession first appears at the foot of the Baoileig Burn, and may therefore be referred to as the Baoileig Sill. Its base can be examined in the gorge of the Allt Dhepin extending from the confluence of the Baoileig Burn to a waterfall 200 yards farther west. At the former locality the rock is finely columnar, and rests on horizontal red sandstones and marls. The upper contact is seen at the top of the waterfall. The rock is fine grained throughout, becoming very dense at the contacts, and is in general light grey or blue-grey in colour. It carries sporadic, slender, white, phenocrysts of felspar. The composition is that of a felsite of craignuritic affinities. About the middle of the sill is a band with flow-elongated vesicles. The thickness is estimated at 40 feet.
For 150 yards the bed of the burn is now occupied by horizontal red sandstones and marls. Then begins the outcrop of the great Garbad quartz-dolerite sill (so-called from Cnoc na Garbad to the south), which occupies the burn and its banks for nearly a mile. The Garbad mass contains numerous, thin, lenticular intercalations of sediments; or, in other words, from the focal mass of Cnoc na Garbad, there are thrown off numerous subsidiary offshoots which thin out among the sediments to the east and west.
At the waterfall in the Allt Dhepin (100 yards E.S.E. of the 39 feet O.D.) is seen the easternmost of these offshoots. At the fall it transgresses sharply across the sandstones, which are much disturbed and faulted. This sill clearly thins out to the east amongst the sediments; to the west and north-west it appears to join up with the main mass of the Garbad sill. It can be traced up to the waterfall near 539 feet O.D., where a small patch of sediments is brought in by a fault throwing 6 feet to the north-west.
Above this point there is a more extensive intercalation of sediments in the bed of the burn, with cliffs of 'blue basalt' on either side. The main central part of the Garbad mass is reached in a remarkable dyke-gorge S.E. of 634 feet O.D., east of Cnoc an Fheidh (873 feet O. D.). The rock is here a coarse quartz-dolerite (S24459)
From this point onward the Allt Dhepin section is penetrated by an extraordinary number of dykes which run mostly north-west and north-north-west
A section in a tributary flowing east and entering the Allt Dhepin to the north of Cnoc an Fheidh illustrates the multiple character of the Garbad mass
A small fault running north-west to south-east at the bend in the burn below 786 feet O.D. at length brings in a sill (the Allt Dhepin sill) on a higher horizon than the Garbad mass. This sill is well exposed at the large waterfall near 786 feet O.D., where it is underlain by red sandstones and marls. It consists of an ash-grey felsite of craignuritic affinities; a rock collected from the top of the fall has been analysed ((S24458)
The Allt Dhepin section thus reveals at least five large sills and sill-complexes composed of rocks ranging from quartz-dolerite to felsite of craignuritic type as follows:
Approximate height of base. Feet | Estimated maximum thickness. Feet | |
5. Uppermost sill — craignuritic felsite. (Loch na Leirg sill) | 1010 | 100 |
4. Allt Dhepin sill — craignuritic felsite | 770 | 210 |
3. Garbad sill — quartz-dolerite | 500 | 250 |
2. Baoileig sill — felsite and quartz-dolerite of
craignuritic affinities |
420 | 40 |
1. Glenashdale sill — quartz-dolerite and felsite
composite sill |
190 | 210 |
As the strata are nearly horizontal it is comparatively easy to estimate the maximum thicknesses of the respective sills. Still higher in the succession are the intrusions which form part of the Tighvein complex which is described later (p. 133); and in the coastal regions of Whiting Bay and at Largybeg there are one or two sills on a lower horizon than the Glenashdale sill (p. 132).
2. Sections North of Glen Ashdale
A series of five tributaries to the Glenashdale Burn provides the means of tracing the main sills to the north. These burns are the Allt Garbh which drains Loch na Leirg; and then, to the south-west, a series of four unnamed burns which have been designated A, B, C, and D, respectively, from north-east to southwest (see
The Glenashdale sill forms a massive outcrop following the northern boundary of Glenashdale Wood. There is a fine glaciated exposure where Burn B crosses the moorland track on the north side of the wood. The sill also forms a range of cliffs crossing Burn A. At this point the sill bifurcates; a northern branch, continuously thinning, causes a small waterfall in the Allt Garbh, below Creag Bhan, and then fades out among the sediments. The branching here may be taken as confirmation of the view that the sill is multiple. The main branch, however, crosses the Allt Garbh at the northern corner of Glenashdale Wood (see map,
One or two 'blue basalt' sills, on lower horizons than the Glenashdale sill, are exposed in the Cal Burn at North Kiscadale, Whiting Bay.
The Baoileig sill can likewise be traced across Burns C, B, and A, and crosses the Allt Garbh at the lower end of the gorge known as Creag Bhan. Its course between the Glenashdale Burn and Burn D, however, is rather uncertain. In Burn C it encloses a lenticle of sediments, and near Burn B its outcrop is broken by a small north-west to south-east fault. North of the Allt Garbh it throws off at least two branches which die out amongst the sediments.
The Garbad sill can be traced from the Allt Dhepin north-westwards across the moor by means of a series of scattered exposures. Good sections of it are exposed on Cnoc Mòr, and in a gorge at the head of Burn B (
Between the outcrops of the Baoileig and Garbad sills there appears on both sides of Glen Ashdale a series of disconnected masses of spherulitic felsite or granophyre. These are all thick and massive intrusions which have the habit of ending off very abruptly. One is seen in the Creag Bhan section
Very little is known as to the extent of the Allt Dhepin sill on the eastern side of the valley. It extends as far south as the gully near 739 feet O.D.
3. Sections South of Glen Ashdale
The various sills cause well-defined scarp and terrace topography on both sides of Glen Ashdale, but particularly so on the south side
The Baoileig sill forms an equally well-marked escarpment and terrace which, on the north flank of Torr na Baoileig, makes a narrow ledge just below the steep escarpment marking the outcrop of the Baoileig felsite. The Baoileig sill is cut off and offset by the Torr an Loisgte fault, and continues as a small ledge and scarp around the base of Torr an Loisgte, where it turns south and passes above the Giants' Graves. A good section is exposed in the southern headwater of the Allt Crompucaidh, where the sill is 15 to 20 feet in thickness, and is dipping along with the adjacent sediments at 30° to the W.S.W.
Between the Baoileig sill and the scarp of the Dippin crinanite there occur two masses of felsite precisely similar to those occurring on the north side of Glen Ashdale along the same horizon. The Torr na Baoileig mass consists of a coarse spherulitic felsite or granophyre, and is well exposed in the Baoileig Burn. To the east it ends off very abruptly about half a mile west of Torr an Loisgte. The second mass forms the hill of Torr an Loisgte, which is a prominent scenic feature in the view from Whiting Bay to the south. The rock is a spherulitic felsite which weathers with a minutely-nodular surface. On the west side the intrusion appears to be bounded by the Torr an Loisgte fault; on the east it rises as a short abrupt scarp above the Baoileig sill (see map,
The great Garbad sill which, on the south side of the Allt Dhepin, spreads around Loch Garbad as centre, is a very thick mass of coarse quartz-dolerite. It is well exposed in many places, notably around Loch Garbad, along its eastern scarp, and at Cnoc na Garbad (959 feet O.D.). At Loch Garbad the rock carries many xenoliths of fine-grained 'blue basalt', and it may therefore be conjectured that there is an interior contact at this locality.
The most notable feature of the Garbad sill is the extraordinary straight scarp running N.N.W. to S.S.E., which bounds it on the eastern side (see One-inch Geological Map, Arran, 1910). This scarp runs from the Allt Dhepin to Cnoc na Garbad, a distance of one mile. It is in line with the upper part of the Allt Dhepin, and it coincides in direction with the majority of the dykes of this region, and with many small faults. The scarp of the Dippin crinanite appears to be cut off abruptly against it (see map,
On the north side of Cnoc na Comhairle (913 feet O.D.), a flat-topped hill three-quarters of a mile south-east of Torr na Baoileig, three sills of 'blue basalt' may be traced above the scarp of the Dippin crinanite. The lowermost is thin and impersistent, and shows a peculiar vitreous facies at a point a quarter of a mile S.S.W. of Torr an Loisgte; the second is rather thicker, but fades out rapidly both to the east and west; the third and highest is much more persistent, extending from the Garbad escarpment round the summit of Cnoc na Comhairle to Torr na Cloiche. The actual summit of the hill is composed of sandstone.
On the south side of Glen Ashdale, as on the north, one or two 'blue basalt' sills are found at horizons lower than that of the Glenashdale sill. One such, for example, is found in the section across Largybeg Point (
On a still lower horizon is the sill which forms Creag Dubh, the southern horn of Whiting Bay. The lower contact of this sill is exposed at low tide, and is seen to rest on sandstone dipping W.S.W. at 20°. The thickness of the mass is estimated at 30 feet. The rock consists of an acid quartz-dolerite. On one visit a glomeroporphyritic segregation of yellow felspar, 6 inches in diameter, and elliptical in shape, was seen on the seaward side of the crag.
4. Tighvein, Sguiler, The Ross, Cnoc Dubh, The Sheans, and the Brodick area
Tighvein — The masses of igneous rock occuping these localities form a complex and connected series of intrusions extending northward from Tighvein (1497 feet O.D.), situated about 3 miles west of Whiting Bay, to the neighbourhood of Brodick. On the One-inch Geological Map of Arran (1910) the exposures on Tighvein are mapped in the form of an ellipse with its long axis in a north to south direction, which suggested the possibility of a ring-dyke complex in this neighbourhood. In spite of the fact that the exposures are poor and disconnected, owing to the deep covering of peat on the Tighvein plateau, the writer is inclined to believe that the intrusions are mainly concordant in character, and are sills injected on a higher horizon than those of the Glen Ashdale and Allt Dhepin region. The writer has only so far studied the eastern half of the Tighvein complex, and it may be that the sections on the west and south will provide better evidence as to the disposition of the intrusive masses.
A good section on the south-east side of Tighvein is provided at the head of the Allt nan Clach, one of the headwaters of the Kilmory Burn. A traverse of the little gorge which extends from the Kilmory to Lamlash track to the summit of the plateau gives the section shown in
From this point to the summit of Tighvein the ground is covered with peat, on the surface of which are strewn numerous blocks of microgranite. This rock occupies the area practically to the summit; but the top of Tighvein itself, and of an isolated knoll to the south-west, consists again of augite-diorite. This rock is cut by numerous irregular veins of an acid rock. A ridge near 1490 feet O.D. provides the best exposure of the microgranite. This rock appears to form the summit of the plateau, and falls to the east in a fairly well defined scarp on to the terrace of the lower augite-diorite. No contacts are seen, and it is therefore difficult to decide whether we are dealing with sills or broad ring dykes. The spreading of the outcrops of the augite-diorite to the north and to the south-west, however, seems to indicate that these masses should be interpreted as sills. The upper part of the section
A little western headwater of the Allt Dhepin, descending from 1490 feet O.D. east of the summit of Tighvein, provides a section on the eastern face of the hill
Between this point and Urie Loch there are many large blocks and an exposure of a coarse, rotted, gabbroid augite-diorite. The boundary of this rock, marked by small crags, sweeps round the head of the Allt Dhepin immediately south and south-east of Urie Loch; and in a crag on the eastern side of the valley the coarse gabbroid rock contains xenoliths of a dark, fine-grained, ' blue basalt,' and the whole exposure is traversed by irregular veins of granophyric rock.
Xenolithic rocks of the same character, with but slight variations in texture, are to be seen in the exposures around Urie Loch. A little intrusive boss of spherulitic felsite, probably a devitrified pitchstone (p. 215), breaks through the augite-diorite on the south shore of the loch; and a dyke-like mass of the same character borders the loch close by. A dyke of fine-grained quartz-dolerite with angular xenoliths of still more fine-grained basalt, occurs on the north-east shore of the loch.
Sguiler — The xenolithic augite-diorite may be traced at intervals through the peat in the region between Urie Loch and the summit of Sguiler (1332 feet O.D.), which overlooks Monamore Glen. The base of the sill shows an extraordinary development of xenolithic enclosure, the fine-grained basaltic rock having apparently been shattered, and then intimately interpenetrated by a more acid rock. This phenomenon is discussed at greater length in connection with the Sheans (p. 136). On the peat-covered terrace immediately to the south of the summit of Sguiler numerous fragments of felsite are to be seen, which are interpreted as belonging to a sill injected just below the base of the Sguiler mass. From Sguiler the boundary of the basic sill runs south-east to Creag na h'Ennie, and thence to east of Urie Loch.
The Ross — The Ross is the prominent hill (992 feet) which rises on the north side of Monamore Glen, between the Monamore and Benlister valleys. The summit of The Ross consists of a coarse gabbroid rock. On the south side of the hill, the base of the sill, consisting of fine-grained dolerite near the texture of ' blue basalt,' is seen. It is injected by fine-grained veins and thin sills of basalt, with very dense, almost tachylytic, margins. These injections range from a fraction of an inch to about a foot in thickness. Near the western end of the outcrop on the south side of The Ross, intercalations of sandstone, baked to hard white quartzite, make their appearance, and cause the marked banding on the face of the cliff as seen from the Ross Road. This banding is parallel to the general easterly dip of the sill. At the western end of the sill there is also a marked primary banding in light and dark, coarser and finer, more acid and more basic, bands.
The nature of the sill of The Ross is doubtful. The Survey possesses no specimens, and those in the Glasgow University collection are much decomposed. The thin sections are puzzling. There is very little free quartz, and all the ferro-magnesian minerals have gone over to chlorite. Without fresh material it is hard to say whether this mass is really a continuation of the Monamore crinanite (as it is mapped on the One-inch Geological Map), or a member of the quartz-dolerite suite.
A typical 'blue basalt' dyke, 12 feet wide, and running in a north-north-west direction, cuts the base of the Ross sill. This rock is a quartz-basalt obviously belonging to the oversaturated suite. It weathers with a whitish crust, and is mapped as a felsite on the One-inch Geological Map (19I0). It may perhaps be regarded as evidence for the view that the sill of The Ross really represents the Monamore crinanite.
Cnoc Dubh — (1003 feet O.D.) is the summit of the plateau north of the Benlister Burn. It is occupied by an ill-defined and poorly-exposed doleritic intrusion, of which the small detached masses to the east and north-west (including the Sheans) appear to be outliers (see One-inch Geological Map). The only specimen collected from this mass (S25048)
The Sheans — The Sheans consist of three abrupt rocky hills rising on the edge of the plateau to the south and south-east of Glen Dubh, about 2¾ miles south-west of Brodick Pier. The south-eastern hill shows a compact grey basaltic rock at the foot of the slope. On its summit, however, there is a porphyritic dolerite, intersected by fine-grained acid veins which weather as whitish raised reticulations on the surfaces of the exposures. The central Shean is separated from the above by a deep peat-filled depression. This hill has a long protrusion to the south. At the south-eastern end of the main knoll soft red sandstone, in a horizontal attitude, occurs at the base of the slope, clearly passing beneath the dolerite, and establishing the sill character of the intrusion. The rock is of the same general character as that of the south-eastern hill, but is only sparingly porphyritic. It shows the most extraordinary brecciation, the separated angular or partially-rounded fragments of dark-coloured and rather compact basalt being enveloped in a more acid type which weathers light grey or white. The fragments are mainly between 2 and 6 inches in greatest diameter, and are generally somewhat rounded as if solutional effects had taken place between the xenoliths and the matrix. In freshly-broken material the lines of junction between fragments and matrix are often hard to distinguish, although they are brought out most spectacularly by weathering. The best place to collect specimens is slightly below the summit of the hill on the western side. Occasionally the light acid material forms stratiform bands in the dark rock.
It seems perfectly clear from the exposures that the dark basalt was shattered when solid, and that the acid material then filled up the intervening spaces, and was hot enough to produce some solutional and hybridization effects. The whole thickness of the tabular mass, estimated at 50 feet, appears to have been involved in this shattering and veining process; and it appears possible to trace increasing grain-size in the xenoliths from the base of the hill to the top, showing that the brecciation occurred within a normally-cooled intrusive mass.
Although there is a peaty depression between the central and northern hills of the Sheans, the exposures show that the sill is really continuous between the two hills. On the north side of the northern hill the same xenolithic rock is seen as on the central hill, but there is apparently a greater quantity of acid matrix. The base of the intrusion is not actually seen, but it is marked by the occurrence of a narrow terrace of hard, white, carious sandstone.
Brodick area — A mile east-north-east of the Sheans, and about 2 miles south-south-west of Brodick, there are many small sills exposed in the headwaters of the Lag a' Bheith. These are mainly of felsitic composition. Their thickness rarely exceeds 20 feet. The best exposure is in a small road-metal quarry situated where the old moor road crosses the easternmost headwater, about a cmarter of a mile west-south-west of the Stone Circle near the fourth milestone on the Brodick–Lamlash road. This is in a fresh-looking, light blue-grey, craignuritic felsite (S25059)
The composite sill described by Dr. A. Scott<ref>A Composite Sill in Glen Cloy, Arran, Trans. Geol. Soc. Glasgow, vol.xv., part ii., 1915, pp. 140–150.</ref> from Glen Cloy, Brodick, belongs to the series under description. The greater part of it is a dark-grey felsitic rock which has a spherulitic appearance. Towards the top of the sill this rock is overlain by a black, flinty, pitchstone material, with a sharp junction between the two parts. The total thickness of the mass is estimated at 12 feet.
An interesting section of a small composite sill belonging to this suite is seen in the Rosa Water at Rosaburn Bridge, near the Smithy, Brodick. The section is as shown in
5. Auchareoch, Auchenhew, Levencorroch, and the Kildonan area
The Auchareoch, Auchenhew, and Levencorroch masses appear to represent the horizons of the Tighvein complex in a southward direction. The Kildonan intrusions, however, are injected on a horizon below that of the Dippin crinanite, and are therefore on the same stratigraphic level as the Baoileig and Glenashdale sills of the Whiting Bay district.
Little is known as to the field-relations of the three large masses above mentioned. They all consist of a coarse quartz-dolerite. The Auchareoch sill is well exposed in tributaries of the Kilmory Water near the farm of Auchareoch, but its surface and its boundaries are largely obscured by peat. Auchenhew and Levencorroch Hills are capped by what appears to be the same sill of quartz-dolerite, the outcrop of which has been severed into two parts by the deep gorge of the Levencorroch Burn. The Auchenhew mass is represented by three specimens in the Survey collection (S6885)
An interesting section of a small 6-foot sill of 'blue basalt' on a lower horizon than the Auchenhew–Levencorroch mass is exposed in the Levencorroch Burn at the small waterfall called Easa Cumhang.<ref>A sketch of this occurrence is given in J. W. Gregory and G. W. Tyrrell, Excursion to Arran, Proc. Geol. Assoc., vol. xxxv., part iv., 1924, p. 418, but the Auchenhew mass is wrongly called crinanite.</ref> The sill is intersected by a 2-foot basalt dyke. It can be traced eastward to the path on the south side of Auchenhew Hill, but apparently does not reach the burn to the east.
A sill on a somewhat lower horizon is exposed in a fine section at the waterfall of Eas Mòr in the Allt Mòr burn running on the eastern side of Auchenhew Hill. The gorge below Eas Mòr heads in an amphitheatral feature caused by the outcrop of a craignuritic felsite sill, 20 to 30 feet thick, forming the waterfall. The sill rests on nearly horizontal Triassic marls, and on the eastern side of the gorge shows a sedimentary intercalation. Beneath the fall the strata are bent and broken by one or two small faults. Another sill of the same material appears at a small fall one-third of a mile higher up the burn, which is here called the Loch Burn.
The cliff at the back of the 10-ft. raised beach below Kildonan Castle is composed of a sill of craignuritic felsite 15 to 20 feet thick. This is well exposed in the quarry at Lloyd's Signal Station, and there rests on sediments which are seen on the seaward side of the quarry. The rock is a typical 'grey basalt' generally with scattered slender porphyritic felspars (S6358)
To the west the sill extends much farther than is shown on the One-inch Geological Map (1910). An exposure is seen in the cliff behind the Schoolhouse at Kildonan, half a mile west of Kildonan Castle. To the east the sill extends along the raised-beach cliff up to the houses at Port a' Leacach, where it suddenly ends off. At this point, however, another sill appears on the foreshore, of exactly the same petrographical character. Its upper surface, with patches of sediments still adhering to it, forms a flat spread on the foreshore for about a quarter of a mile to the northeast of Port a' Leacach. This sill appears to have suffered monoclinal bending at places, and patches and sheets of sediments are included within it. Its relation to the Kildonan Castle intrusion is doubtful. It may be the same mass dropped down to the east along a north-west fault, or it may form a separate sill injected on a slightly lower horizon.
One or two intrusions of quartz-dolerite (S6413)
The island of Pladda, one mile south of Kildonan, is mainly occupied by a well-marked sill which, judging from the Survey specimen (S6397)
6. Region west of the Sliddery Valley
A quarter of a mile north-east of the ninth milestone on the Ross Road, and 3 miles W.S.W. of Lamlash, a small quarry gives a section in a thin sill which shows an extraordinarily intimate interpenetration of a dark compact basalt by spherulitic felsite
The moor to the north and west of the quarry is strewn with rounded blocks of medium to coarse-grained quartz-dolerite, and there are numerous outcrops of the same rock farther to the west along the Ross Road. This is presumably the elongated sill which is shown on the One-inch Geological Map as extending for about 3 miles on the eastern side of the Sliddery Water.
Nothing is known of the field-relations of the Glenscorrodale mass on the western side of the Sliddery Water. It is represented by two specimens (S6401)
The complicated ring-shaped outcrop 22 miles east of Blackwaterfoot, which includes Cnoc Ballygown (735 feet O.D.), Beinn Tarsuinn (965 feet O.D.), and Cnoc an Loch (1107 feet O.D.), belongs to a composite sill including coarse quartz-dolerite (S6375)
The head of the Allt Cul na h'Eilde provides a section in the northern part of the Beinn Tarsuinn mass. The sill rests on practically horizontal Triassic marls and sandstones. Its base consists of spherulitic felsite, which passes into rocks of craignuritic composition higher up the burn. The summit of Cnoc Ballygown consists of a coarse quartz-dolerite of Garbad type, poorly exposed and much decomposed. Lower down on the western slope the rock be comes distinctly more acid.
It is probable that many of the felsites of the Sliddery district, as, for example, at Bennecarrigan, Glenrie, and in the Clauchan Glen, are related to the quartz-dolerite–craignuritic-felsite suite under description.
In a quarry at Pien Bridge, Shiskine, there is a section in a thin composite sill, the exterior member of which is quartz-basalt, and the interior member a felsite.
C. Petrography
The rocks to be described in this section are derived from an oversaturated magma, and in nearly all their varied manifestations show traces of the presence of free silica. Their range of composition varies from basic to acid; their textures from glassy to coarsely holocrystalline. For descriptive purposes we propose to divide them into three groups according to composition, as basic, intermediate, and acid. The basic types are fine-grained quartz-gabbros or quartz-dolerites; the intermediate rocks are augite-diorites and rocks of craignuritic composition; the acid rocks are felsites of distinctive characters belonging to the acid end of the craignurite series.
The nomenclature of these rocks presents a perplexing problem. An attempt has been made in the following descriptions to apply the nomenclature of the similar rocks of Mull.<ref>Mull Memoir, 1924, p. 227.</ref> The chief difficulty has been found in regard to the intermediate types which have been termed craignurite in Mull.
Craignurites are rocks in which the matrix has a characteristically immature type of crystallization, with skeletal crystals of felspars and pyroxenes in an acid crypto- or micro-crystalline base. The larger felspar and pyroxene crystals, too, possess an elongate habit.<ref>Mull Memoir, 1924. </ref> Hence the main features of craignurite, as defined in the Mull Memoir, appear to be the immature type of crystallization, and the general columnar, acicular, and skeletal development of the minerals.
The compositional range covered by the term craignurite is very wide. It embraces rocks with silica percentages between 55 and 70, the normal and characteristic type, however, containing about 65 per cent. The group, as a whole, should perhaps be referred to as the craignurite series; the normal intermediate type as craignurite in the narrow sense; the basic end-member as craignurite-dolerite; and the acid end-member as craignurite-felsite. In Arran, however, rocks which are undoubtedly of craignuritic composition rarely show the characteristic texture and mode of crystallization of the craignurite series. The matrix and mesostasis of many of the rocks certainly show an immature type of crystallization, but it is more of a felsitic and spherulitic character than acicular and skeletal.<ref>Some of the slices were submitted to Dr. H. H. Thomas and Mr. E. B. Bailey, both of whom are of the opinion that the term craignurite should be applied very sparingly to the Arran rocks, as the characteristic texture is quite rare.</ref> Many of the Arran rocks closely resemble the rocks of the Cnoc Carnach group of composite sills in Skye.<ref>A. Harker, The Tertiary Igneous Rocks of Skye, Mem. Geol. Surv., 1904, p. 215.</ref>
The basic members of the suite, represented by the analysed rock (S24459)
The mesostasis is a brownish, turbid, felspathic substance with undulose extinction, in which an occasional irregular area of quartz is set, and which is riddled by needles of apatite. In isolated areas it becomes distinctly spherulitic. Its distribution is rather patchy; and the slice (S24459)
A specimen from the Glenashdale sill, from below Torr an Loisgte (S24867)
One of the rocks from Auchenhew Hill (S6887)
Another Auchenhew rock (S6886)
Types of intermediate or craignuritic composition are represented in a coarse-grained form, of which a good example is found in the Tighvein complex, near the shepherd's cairn on the northeast side of Urie Loch (S25055)
A rock from Auchenhew Hill (S6885)
A rock of much the same character, but much finer in grain, and more felspathic than that of Urie Loch, forms the thick veins and patches of light rock which intersect the augite-diorite at the summit of Tighvein (S6409)
Rocks of intermediate composition, but definitely craignuritic in texture, are rare in Arran, and are very poorly represented in the Survey collection. A rock from near the base of the Glenashdale sill, at the top of the great waterfall in Glen Ashdale, appears to be a craignurite near the basic end of the series (S6382)
Many of the rocks belong to the acid end of the craignurite series. These types are distinguished in the field by their characteristic compact texture, light bluish-grey colour, and by the presence of small, sporadic phenocrysts of white felspar, which are sometimes aggregated into little stellate groups (S25057)
A rock of these characters which has been analysed
Of almost identical characters is the acid rock which forms the basal member of the Glenashdale sill (S6383)
'A specimen from the lower part [of the sill] is a grey, fine-textured rock, without evident porphyritic crystals, and gives the specific gravity 2.68. In a thin slice (S6358)
The rock of the massive dyke at the north end of the Largybeg Point section
The rock of the small quarry in the Lag a' Bheith (p. 137) is somewhat richer in chloritized pyroxene and iron-ores than the type rock of the Allt Dhepin sill (S25059)
A cryptocrystalline, spherulitic type (S25060)
Allied to the craignuritic felsites are certain other acid intrusions in the southern half of Arran, as, for example, the spherulitic felsite or granophyre which is inserted at intervals between the Baoileig and Garbad sills in the Glen Ashdale region; other large acid sills, as, for example, those of the Sliddery valley; and the microgranite which is involved in the Tighvein complex. The spherulitic felsite of Glen Ashdale is represented by a specimen from below Cnoc Mòr on the north side of the glen (S24873)
The large acid intrusion which forms part of the Tighvein complex is treated here, as it is almost certainly closely associated with the augite-diorites of that centre. It may be regarded as the most fully crystallized representative of the partial magma which gave rise to the acid craignurites and craignurite-felsites. It is represented by three slices in the Survey collection (S6384)
Xenolithic and hybrid rocks
In the Mull Memoir<ref>Trans. Geol. Soc. Glasgow, vol. xv., part ii., 1915, p355.</ref> it is shown that the late acid residues of the intermediate and subacid magmas have reacted with earlier minerals, and that these effects are particularly marked when the residual liquor has migrated within the limits of the individual intrusion. Many of the mineralogical phenomena thus produced are entirely similar to those which are due to hybridization, i.e. to the commingling of magmas, or to the assimilation of foreign material within a magma. Since the validity of the reaction principle seems to be unquestionable,<ref>N. L. Bowen, The Reaction Relation in Petrogenesis, Journ. Geol., vol. xxx., 1922, pp. 177–198.</ref> it is clear that assimilation and hybridization effects should only be inferred when there is good evidence of magmatic mingling or the dissolution of xenoliths. Some of the rocks described above have been regarded as acidified basalts and dolerites, or basified felsites and granophyres, but there is no reason to doubt but that many of them are perfectly normal products of differentiation, assuming that reaction between the early-crystallized minerals and the late acid residua has been in progress.
Nevertheless there are many undoubted examples of partial assimilation and hybridization in Arran, in which fragments, generally of basic composition, are enveloped in acid magmas, with reactional or solutional effects. The Sheans and the intrusion at the summit of Sguiler furnish the best examples in the Survey collection of xenolithic rocks within the group described in this chapter; but there are other fine examples between the gabbro and granophyre of the Central Ring Complex (p. 177). The rocks of the Sheans are represented by two slices (S24384)
The light acid rock in which the xenoliths are enveloped differs from the xenoliths in its somewhat coarser grain, the greater relative abundance of felspar, and in the presence of alkali-felspar which forms a scanty mesostasis along with quartz, and which also forms a few phenocrysts mainly of perthitic soda-orthoclase. The boundary between xenoliths and matrix is quite sharp, and there has been little or no reaction between the two. Little fragments of the xenoliths have, however, been spalled off, and can be found in process of disintegration. The slice (S24384)
The rock of Cnoc Dubh (S25048)
A slice from the sill in the Rosa Water, Brodick (S7538)
D. Chemical composition
Analyses of quartz-dolerite or basic craignurite of the Garbad sill, and of craignuritic felsite from the Allt Dhepin sill, both taken from the type section of the Allt Dhepin (p. 126) have been made for this Memoir by Mr. E. G. Radley (
4 | E | F | 5 | 6 | G | H | |||
SiO2 | 54.00 | 52.16 | 55.82 | 71.58 | 69.26 | 70.70 | 71.30 | ||
A12O3 | 13.09 | 11.95 | 11.47 | 12.20 | 11.60 | 1178 | 11.24 | ||
Fe2O3 | 3.53 | 4.86 | 3.68 | 1.51 | 1.31 | 1.32 | 1.80 | ||
FeO | 8.45 | 9.92 | 7.66 | 1.77 | 2.57 | 3.45 | 2.84 | ||
MgO | 3.49 | 3.77 | 4.08 | 0.50 | 1.10 | 0.53 | 0.61 | ||
CaO | 5.55 | 7.14 | 7.88 | 1.98 | 2.61 | 1.30 | 1.56 | ||
Na2O | 3.27 | 2.36 | 2.53 | 2.83 | 2.08 | 2.48 | 3.44 | ||
K2O | 1.80 | 1.74 | 2.00 | 3.86 | 3.88 | 4.71 | 4.66 | ||
H2O> 103° | 1.71 | 1.95 | 1.88 | 0.76 | 1.67 | 1.14 | 1.04 | ||
H2O< 105° | 1.26 | 0.56 | 0.66 | 1.1 | 1.61 | 0.50 | 0.39 | ||
TiO2 | 2.83 | 3.25 | 1.62 | 0.44 | 0.45 | 1.27 | 0.58 | ||
P2O3 | 0.31 | 0.24 | 0.23 | 0.13 | 0.10 | 0.26 | 0.22 | ||
MnO | 0.37 | 0.18 | 0.40 | 0.31 | 0.45 | 0.07 | 0.31 | ||
CO2 | 0.25 | 0.18 | 0.08 | 1.07 | 1.76 | 0.51 | — | ||
S | — | 0.18 | — | — | — | 0.08 | — | ||
FeS2 | 0.14 | — | 0.09 | nt. fd. | nt. fd. | — | nt. fd. | ||
(Ni, Co)O | nt. fd. | — | 0.04 | nt. fd. | nt. fd. | — | nt. fd. | ||
BaO | 0.02 | — | 0.03 | nt. fd. | nt. fd. | — | 0.07 | ||
Li2O | tr. | — | tr. | nt. fd. | nt. fd. | — | ? tr. | ||
100.07 | 100.44 | 100.18 | 100.04 | 100.45 | 100.10 | 100.06 |
4. (S24459)
(S18467)
(S16800)
5. (S24458)
6. (S24453)
Q. (S18464)
H. (S16803)
The closest Mull analogues of the Arran quartz-dolerites are the quartz-dolerite of Talaidh type from Cruachan Dearg (S18467)
The rock from the Allt Dhepin sill, which was selected as typical of the acid end-members of the craignurite series, turns out to be a thoroughly acid type with 38 per cent. quartz as computed from the norm of the analysis (5,
% Salic Constituents | % Quartz | |
Anal. 5 | 89.2 | 38.0 |
Anal. 6 | 83.6 | 38.5 |
Anal. G | 87.7 | 35.2 |
Anal. H | 85.8 | 27.8 |
G.W.T.