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Note on the geology of the Cuillin Range, Skye
210
NOTE ON THE GEOLOGY OF THE CUILLIN RANGE, SKYE.'*' By WINTOUR F. GWINNELL, B.Sc., F.G.S. (Read July 7th, 1911.)
Cuillins stand in the south of Skye, facing the grand T H Eisland of Rhum, and looking towards that of Eigg, with its
unique ridge of glassy pitchstone. As seen from the sea to the south on approaching Loch Scavaig, the Cuillins form a compact knot of contiguous towering peaks, of which thirteen are said to exceed 3,000 ft., Sgurr Alaisdair, the highest, being 3,275 ft., while Blaven, on a spur to the east, is nearly as high. Compared with the rounded Red Hills of granite, still more to the east, the rugged Cuillins form a striking contrast-in their greater height and steepness, their bare, black or brown surfaces, and their irregular, notched, and often jagged forms. These striking features are due mainly to the nature of the rocks composing them, to the unusual mode of their accumulation, and to their comparatively recent origin, and hence are but little decayed. Standing on the shore of Loch Coruisk, so strangely sunk in the centre of the mountain mass, which measures only nine miles by six, the scene around is yet grander, but more som bre. The lake-level is but slightly raised above sea-level, and its lower end, where the Coruisk stream issues, is within a few score yards of the sea, which is, however, hidden from view by the almost perpendicular rim of rocky peaks rising all around. Though built up in the main of gabbro, almost wholly so around Loch Scavaig, the Cuillins proper and the Blaven spur owe much of their elevation to the thickness of the rocks which underlie the gabbro, including in different parts peridotites and basalts, both, like the gabbro itself, of Tertiary age; also Torridon sandstone and Jurassic rocks. The peridotites, mostly picrites, are mainly composed of olivine, but with more or less pyroxene (both augite and enstatite), spinellids, and sometimes a little anorthite felspar. These form one or more laccolites, 500 to 1,500 ft. in thickness, best developed a little west of Loch Scavaiz. In places the peridotite is traversed by small dykes and veins of the later-formed gabbro. The gabbro occurs as a great sheet or sill partaking of the characters of a laccolite, thinning out all around, while, on the other hand, detached blocks of the peridotite occur as inclusions in the gabbro, e.g. on the shores of Loch Scavaig. Its junction with adjacent older rocks is mostly a gently inclined one, and varies in height from about 2,500 ft. above to an unknown, but * Advance copies of tbis paper were issued to members in connection witb the Long Excursion of 'gIl.
P R OC. G EOI..
Assoc. ,
YOLo
XXII.
PLATE
(rI'l"" "." FIG. I. - IG NE OUS D YKE T RA\ ' ERS lN G J UR ASS IC SEDIME NT S.
XXI X.
RUH d l F . Grt'lllwl!.
R OADS IDE AT S KUL AMUS , NEAR B ROADF ORD, S KYE.
(The dyke divert s th e stream and pro du ces a waterfall. It then forms for some distance th e precipitou s left bank of the stream.) To fa ce p. 11 0 .
I r;'f)/t.I />,)' FIG . 2 -
I\ lI sull F. (;'h'illlldi.
' GRI KE S " (S OL UTIO N CHANNELS) I N MA R:d ORI SE D C AMBRI AN LI ME ST ON E , B E N S UARDAL. K I LCH RI ST . S KYE .
THE GEOLOGY OF THE CUILLIN RANGE, SKYE.
2I I
probably small, depth below sea-level around and beneath Loch Coruisk. The floor of the gabbro-laccolite is thus somewhat saucershaped, but with its rim inclined to the south; the outline in plan is fairly circular, and with an average diameter of seven miles. The gabbro sometimes fringes out at its margin into sheets separated by wedge-like masses of the older lavas, such masses now being isolated enclosures owing to extensive denudation since the gabbro was intruded. Though often cut by dykes of all sizes, the gabbro appears to have given off few or none. It is of many separate intrusions, which often differ much in colour, structure, and size of grain, but are on the whole in roughly parallel sheets of very unequal thickness. The upper are apparently the older, to judge by the rarity of occurrence of dykes of gabbro in the lower sheets, which are presumably the newer. Entangled and enclosed in the gabbro are very numerous patches of older rocks, especially the earlier eruptions of basal lava and agglomerate. These inclusions are mostly lenticular in form, and lie inclined towards the centre of the laccolite, perhaps owing to being caught between distinct intrusions. Both inclusions and gabbro are penetrated by innumerable dykes, sheets and veins of later-probably not much later-age, but also of basic composition. The roof of the laccolite, composed of the older rocks uplifted by the intruded gabbro, has been almost wholly removed by denudation, but fragments of it may perhaps be represented by a few patches of basaltic lavas with breccias, all highly altered, and pierced by tongues of the gabbro. These patches occur only on a few of the summits of the highest peaks above 3,000 ft. in altitude. As the gabbro certainly extends down to sea-level, as seen around Loch Scavaig, and almost to sea-level all around Loch Coruisk, and, with much probability, beneath the latter, though presumably to no great depth, we get a fair estimate of its magnitude. The thickness must approach 3,000 ft., and may in its central part have reached 3,500 ft., which is approximately onetenth of its average diameter of seven miles. The maximum dimensions are eight and a half miles from east to west and six miles from north to south. The several successive intrusions show sharply marked margins thus distinguishable from mere segregation differences. These latter do occur, producing streaks and patches resembling the " heathen" in some granites. They are usually darker and heavier than the general mass, owing to unusual richness in ironores and pyroxenes. When drawn out into long and narrow lenticles a banded or ribbon-like structure results, which is very conspicuous around Loch Coruisk where the bands are inclined inwards towards the north-east of the centre, the dip being usually PROC. GEOL. Assoc., VOL. XXII, PART 4, I9II.] 18
212
WINTOUR F. GWINNELL ON THE CUILLIN RANGE, SKYE.
from 30° to 70°. This gneissic appearance is not due to metamorphism, as was once thought, but to original flow move· ments. To a similar cause is now ascribed the banded structure of many ancient gneisses, including much of the Lewisian gneiss of the adjacent mainland. As to chemical composition, the gabbro of the Cuillins contains about 48 per cent. of silica; 21-26 per cent. of alumina; 15 per cent. of lime; 5-8 per cent. of magnesia; 5-7 per cent. of ironoxides, while metallic iron particles have been detected. The lime percentage is high, but that of the magnesia low, except where olivine occurs as a marked constituent. As to mineral composition the felspar bulks most, being 66-80 per cent., mostly labradorite, though more acid and more basic varieties also occur, ranging from oligoclase to anorthite. The more acid occur chiefly in the pale-coloured felspathic veins, where orthoclase also may occur. Albite and pericline lamellation may occur, sometimes as secondary effects of stresses. Augite is markedly abundant, ranging from 16 per cent. to double this amount. It is rich in lime (about 20 per cent.) and magnesia (12.16 per cent.), as also in ferrous oxide (9-14 per cent.). This latter adds much to the dark colour and the density of the rock. A tendency to ophitic structure is seen in the more or less complete enclosure of the earlier-formed felspar crystals within the larger and later-formed augites. The diallagic structure, associated with a submetallic lustre on planes of parting, is often very marked. This" schiller" appearance may be due to minute inclusions parallel to the orthopinakoid, and in some cases to other crystal faces, such particles being perhaps introduced later along solution planes. Dana's term of pseudo-hypersthene appears appropriate for some of the larger augites with at least two directions of schiller. Formerly these were set down incorrectly as true hypersthene, though rare cases of this mineral are recorded from Loch Coruisk by Prof. Judd. Other rhombic pyroxenes are rather rare. Olivine is a common constituent, though not a constant one, and is rarely abundant; it may be fresh, or more or less serpentinised. Magnetite is the commonest iron-ore, but ilmenite also occurs, though chromite is not known. The results of the action of moving masses of ice during the Great Ice Age are very general and very striking in the whole of the lower regions, especially around the outlet from Loch Coruisk. They clearly show that most of the vast denudation effects which have given rise to the central hollow in which Loch Coruisk reposes were produced in the interval between perhaps Mid-Tertiary and Pleistocene. Since this latter time very little change has occurred, except by frost-action on the higher ridges and peaks, producing the jagged appearance of most of these.
NOTE ON THE GEOLOGY OF THE CUILLIN RANGE, SKYE.'*' By WINTOUR F. GWINNELL, B.Sc., F.G.S. (Read July 7th, 1911.)
Cuillins stand in the south of Skye, facing the grand T H Eisland of Rhum, and looking towards that of Eigg, with its
unique ridge of glassy pitchstone. As seen from the sea to the south on approaching Loch Scavaig, the Cuillins form a compact knot of contiguous towering peaks, of which thirteen are said to exceed 3,000 ft., Sgurr Alaisdair, the highest, being 3,275 ft., while Blaven, on a spur to the east, is nearly as high. Compared with the rounded Red Hills of granite, still more to the east, the rugged Cuillins form a striking contrast-in their greater height and steepness, their bare, black or brown surfaces, and their irregular, notched, and often jagged forms. These striking features are due mainly to the nature of the rocks composing them, to the unusual mode of their accumulation, and to their comparatively recent origin, and hence are but little decayed. Standing on the shore of Loch Coruisk, so strangely sunk in the centre of the mountain mass, which measures only nine miles by six, the scene around is yet grander, but more som bre. The lake-level is but slightly raised above sea-level, and its lower end, where the Coruisk stream issues, is within a few score yards of the sea, which is, however, hidden from view by the almost perpendicular rim of rocky peaks rising all around. Though built up in the main of gabbro, almost wholly so around Loch Scavaig, the Cuillins proper and the Blaven spur owe much of their elevation to the thickness of the rocks which underlie the gabbro, including in different parts peridotites and basalts, both, like the gabbro itself, of Tertiary age; also Torridon sandstone and Jurassic rocks. The peridotites, mostly picrites, are mainly composed of olivine, but with more or less pyroxene (both augite and enstatite), spinellids, and sometimes a little anorthite felspar. These form one or more laccolites, 500 to 1,500 ft. in thickness, best developed a little west of Loch Scavaiz. In places the peridotite is traversed by small dykes and veins of the later-formed gabbro. The gabbro occurs as a great sheet or sill partaking of the characters of a laccolite, thinning out all around, while, on the other hand, detached blocks of the peridotite occur as inclusions in the gabbro, e.g. on the shores of Loch Scavaig. Its junction with adjacent older rocks is mostly a gently inclined one, and varies in height from about 2,500 ft. above to an unknown, but * Advance copies of tbis paper were issued to members in connection witb the Long Excursion of 'gIl.
P R OC. G EOI..
Assoc. ,
YOLo
XXII.
PLATE
(rI'l"" "." FIG. I. - IG NE OUS D YKE T RA\ ' ERS lN G J UR ASS IC SEDIME NT S.
XXI X.
RUH d l F . Grt'lllwl!.
R OADS IDE AT S KUL AMUS , NEAR B ROADF ORD, S KYE.
(The dyke divert s th e stream and pro du ces a waterfall. It then forms for some distance th e precipitou s left bank of the stream.) To fa ce p. 11 0 .
I r;'f)/t.I />,)' FIG . 2 -
I\ lI sull F. (;'h'illlldi.
' GRI KE S " (S OL UTIO N CHANNELS) I N MA R:d ORI SE D C AMBRI AN LI ME ST ON E , B E N S UARDAL. K I LCH RI ST . S KYE .
THE GEOLOGY OF THE CUILLIN RANGE, SKYE.
2I I
probably small, depth below sea-level around and beneath Loch Coruisk. The floor of the gabbro-laccolite is thus somewhat saucershaped, but with its rim inclined to the south; the outline in plan is fairly circular, and with an average diameter of seven miles. The gabbro sometimes fringes out at its margin into sheets separated by wedge-like masses of the older lavas, such masses now being isolated enclosures owing to extensive denudation since the gabbro was intruded. Though often cut by dykes of all sizes, the gabbro appears to have given off few or none. It is of many separate intrusions, which often differ much in colour, structure, and size of grain, but are on the whole in roughly parallel sheets of very unequal thickness. The upper are apparently the older, to judge by the rarity of occurrence of dykes of gabbro in the lower sheets, which are presumably the newer. Entangled and enclosed in the gabbro are very numerous patches of older rocks, especially the earlier eruptions of basal lava and agglomerate. These inclusions are mostly lenticular in form, and lie inclined towards the centre of the laccolite, perhaps owing to being caught between distinct intrusions. Both inclusions and gabbro are penetrated by innumerable dykes, sheets and veins of later-probably not much later-age, but also of basic composition. The roof of the laccolite, composed of the older rocks uplifted by the intruded gabbro, has been almost wholly removed by denudation, but fragments of it may perhaps be represented by a few patches of basaltic lavas with breccias, all highly altered, and pierced by tongues of the gabbro. These patches occur only on a few of the summits of the highest peaks above 3,000 ft. in altitude. As the gabbro certainly extends down to sea-level, as seen around Loch Scavaig, and almost to sea-level all around Loch Coruisk, and, with much probability, beneath the latter, though presumably to no great depth, we get a fair estimate of its magnitude. The thickness must approach 3,000 ft., and may in its central part have reached 3,500 ft., which is approximately onetenth of its average diameter of seven miles. The maximum dimensions are eight and a half miles from east to west and six miles from north to south. The several successive intrusions show sharply marked margins thus distinguishable from mere segregation differences. These latter do occur, producing streaks and patches resembling the " heathen" in some granites. They are usually darker and heavier than the general mass, owing to unusual richness in ironores and pyroxenes. When drawn out into long and narrow lenticles a banded or ribbon-like structure results, which is very conspicuous around Loch Coruisk where the bands are inclined inwards towards the north-east of the centre, the dip being usually PROC. GEOL. Assoc., VOL. XXII, PART 4, I9II.] 18
212
WINTOUR F. GWINNELL ON THE CUILLIN RANGE, SKYE.
from 30° to 70°. This gneissic appearance is not due to metamorphism, as was once thought, but to original flow move· ments. To a similar cause is now ascribed the banded structure of many ancient gneisses, including much of the Lewisian gneiss of the adjacent mainland. As to chemical composition, the gabbro of the Cuillins contains about 48 per cent. of silica; 21-26 per cent. of alumina; 15 per cent. of lime; 5-8 per cent. of magnesia; 5-7 per cent. of ironoxides, while metallic iron particles have been detected. The lime percentage is high, but that of the magnesia low, except where olivine occurs as a marked constituent. As to mineral composition the felspar bulks most, being 66-80 per cent., mostly labradorite, though more acid and more basic varieties also occur, ranging from oligoclase to anorthite. The more acid occur chiefly in the pale-coloured felspathic veins, where orthoclase also may occur. Albite and pericline lamellation may occur, sometimes as secondary effects of stresses. Augite is markedly abundant, ranging from 16 per cent. to double this amount. It is rich in lime (about 20 per cent.) and magnesia (12.16 per cent.), as also in ferrous oxide (9-14 per cent.). This latter adds much to the dark colour and the density of the rock. A tendency to ophitic structure is seen in the more or less complete enclosure of the earlier-formed felspar crystals within the larger and later-formed augites. The diallagic structure, associated with a submetallic lustre on planes of parting, is often very marked. This" schiller" appearance may be due to minute inclusions parallel to the orthopinakoid, and in some cases to other crystal faces, such particles being perhaps introduced later along solution planes. Dana's term of pseudo-hypersthene appears appropriate for some of the larger augites with at least two directions of schiller. Formerly these were set down incorrectly as true hypersthene, though rare cases of this mineral are recorded from Loch Coruisk by Prof. Judd. Other rhombic pyroxenes are rather rare. Olivine is a common constituent, though not a constant one, and is rarely abundant; it may be fresh, or more or less serpentinised. Magnetite is the commonest iron-ore, but ilmenite also occurs, though chromite is not known. The results of the action of moving masses of ice during the Great Ice Age are very general and very striking in the whole of the lower regions, especially around the outlet from Loch Coruisk. They clearly show that most of the vast denudation effects which have given rise to the central hollow in which Loch Coruisk reposes were produced in the interval between perhaps Mid-Tertiary and Pleistocene. Since this latter time very little change has occurred, except by frost-action on the higher ridges and peaks, producing the jagged appearance of most of these.