Figures and plates
(Figure 1) Sketch-map to show the distribution of Tertiary igneous rocks in the British Isles. The broken lines enclose the areas in the west of Scotland and the north-east of Ireland where Tertiary volcanic rocks are preserved. The dotted line marks the southern limit in England, Wales, and Ireland of basic dykes believed to be of Tertiary age. The situations of the principal plutonic intrusions of Tertiary age are indicated by letters, as follows: K, St. Kilda; S, Skye; R, Rum; A, Ardnamurchan; M, Mull; Ar, Arran; Mo and C, Mourne Ma and Carlingford. The letter X marks the situation of the gabbro and granophyre intrusions of Carrock Fell, possibly of Tertiary age, but only proved to be post-Silurian.
(Figure 2) Section in the southern part of the Isle of Scalpay, showing a faulted area of the basaltic lavas, resting on Jurassic and Cretaceous strata and thrown against the Torridonian. Scale, 3 inches to a mile.
(Figure 3) Section from Talisker Bay through Preshal More and Stockval to the Eynort River, to illustrate the structure of the basalt plateaux. The general dip is westerly, while the important faults usually throw down to the east. It is probable that the base of the volcanic series is in no place very far below sea-level. Scale, 1 inch to a mile.
(Figure 4) Section across the volcanic vent of Kilchrist; showing the volcanic agglomerate breaking through a sharp anticline of Cambrian limestone, and itself invaded by a later intrusion of a peculiar granophyre, full of debris of gabbro, to be described later. Scale, 1½ inch to a mile.
(Figure 5) Contrasted outlines of volcanic agglomerate and granite, as seen from broadford. The low broken hills to the left mark the situation of the Kilchrist vent, and are composed of volcanic agglomerate. The smooth outline of the granite is seen in Beinn Dearg Bheag and Beinn na Caillich, which form part of the Red Hills. In one place on Beinn na Caillich this smooth outline is broken by the outcrop of a large dyke intersecting the granite.
(Figure 6) Cliff-section at Camas Bàn, on the south side of Portree Harbour; about 60 or 70 feet high. This shows the pyroclastic deposit covered by an intrusive sill of dolerite and invaded by two others.
(Figure 7) (S9359) [NG 457 280] × 20. Olivine-Basalt lava, in Allt Dearg Mòr, about 2 miles S.W. of Sligachan: showing pseudomorphs after olivine, composed. of carbonates with a border of iron-oxide.
(Figure 8) Section along Allt Dearg Mòr, near Sligachan: scale, 1½ inch to a mile. The general direction is N.E.–S.W., but the line is made to follow the principal bends of the stream. A number of dykes and a few thin sills are shown, the latter indicating the general dip of the lava-group. A. Amygdaloidal lavas, usually much decayed. B. Hypersthene-Basalt. C. Amygdaloidal Andesite. D. Basalt, very rich in olivine.
(Figure 9) Diagram matie representation of the relations of the trachytic and rhyolitic rocks to one another and to the basalts. The figure is an ideal general section of the group, not drawn to true scale.
(Figure 10) Section along the Sgùrr Dubh ridge, showing the principal laccolitic mass of peridotite and a smaller one to the east, and representing diagrammatically the partial destruction of the peridotite by the gabbro magma in which it became enveloped.
(Figure 11) Dykes or veins of gabbro traversing the peridotite group, in glen south of Allt a' Chaoich, Loch Scavaig: seen in ground-plan.
(Figure 12) Block of banded peridotite, 7 feet long, enclosed in gabbro, in glen south of Allt a' Chaoich, Loch Scavaig: seen in ground-plan. The gabbro is traversed by numerous fissures radiating from the enclosed block.
(Figure 13) (S9228) [NG 480 195] × 20. Dunite, rich in picotite, from the banded peridotites in the glen a little S. of Allt a' Chaoich, Loch Scavaig: consists wholly of fresh olivine and octahedra of picotite.
(Figure 14) (S9234) [NG 458 203] × 20. Peridotite, S.E. slope of Sgùrr Dubh na Dabheinn: chiefly of olivine (showing incipient serpentinisation), with some diallage and felspar (to the right) and opaque octahedra probably of chrome-magnetite.
(Figure 15) Sketch-map to show the shape of the gabbro laccolite of the Cuillins: scale, ½ inch to a mile. The boundary of the laccolite (omitting minor irregularities) is shown by the heavy line; the chief streams and lakes and the coast to the south by lighter lines. The dotted lines are intended to represent approximately the shape of the lower surface of the laccolite, being contour-lines of that surface at intervals of 500 feet, reckoned from sea-level.
(Figure 16) Section through Gars-bheinn, to illustrate alternations of basaltic lavas and gabbro, due to the successive intrusions of the latter rock having followed different bedding-planes in the lavas. Scale, 2 inches to a mile.
(Figure 17) Sketch-map showing the distribution of banded structures in the gabbros of the Cuillins. The strong line indicates the boundary of the gabbro itself: the dotted line encloses the area within which banding is most prevalent. The dip of the banding is shown by arrows. Scale, ½ inch to a mile.
(Figure 18) Section across Druim an Eidhne, to show the relations of the gabbro and granite: scale, 6 inches to a mile. The arrows mark the inclination of the banding in the gabbro at this place.
(Figure 19) Ground-plan of a sheet of fine-grained gabbro and its dyke-feeder, at Eas Mòr, on Allt Coire na Banachdich. Scale, 24 inches to a mile.
(Figure 20) Map showing a part of the Broadford gabbro boss and a small part of the Beinn na Caillich granite, with patches of the Cambrian limestones enclosed in the igneous rock-masses. The area shown lies immediately N.E. of Lochain Beinn na Caillich. For explanation, see text.
(Figure 21) (S8043) [NG 481 232] × 20. Olivine-Gabbro, west bank of Sligachan River, just below Allt Coire Riabhach; showing the ophitic structure. This is the rock analysed (I. above), and consists of labradorite, diallage, olivine with secondary magnetite, and a few small crystals of original magnetite.
(Figure 22) (S5369) [NG 49 22] × 20. "Granulitic Gabbro", Druim an Eidhne; probably a highly metamorphosed basaltic lava. It consists essentially of a granulitic aggregate of labradorite and augite, with little octahedra of magnetite.
(Figure 23) Dykes of banded gabbro cutting the banded peridotite group in glen south of Allt a' Chaoich, Loch Scavaig.
(Figure 25) View from Bealach a' Leitir, looking east and south-east. In the foreground is the south-easterly spur of Sgùrr nan Gillean, terminating in Sgùrr na h-Uarnha; in the distance the Blaven range. Both these are of gabbro, while Strath na Creitheach and the low hills in the middle distance are of granite, underlying the gabbro. The junction is indicated. by the dotted line on the slopes of Garbh-bheinn, Blath-bheinn, and Druim an Eidhne.
(Figure 26) Section across Glen Sligachan from Sgùrr nan Gillean (gabbro) to Marsco (granite), to illustrate the supposed nature of the western boundary of the granite in this part.
(Figure 27) Section along a rather sinuous line through Marsco and Ruadh Stac, to illustrate the supposed manner of intrusion of the granite. Scale, ½ inch to a mile.
(Figure 28) Section through Lochain Beinn na Caillich and towards Broadford, crossing the northern part of the granite boss of Beinn na Caillich and the southern part of the gabbro boss; scale, 2 inches to a mile. F, F are faults, the easterly one bringing on the Lias (Pabbay Shales). L, Cambrian Limestone (Balnakiel group); B, basaltic lavas; Gb, gabbro; Gr, granite. L′, B′, Gb′ are enclosed patches of limestone, basalt, and gabbro; B″, an outlier of the basalt resting on an enclosed patch of limestone; T, small outliers of Torridonian upon the limestone, with a thin sheet of granophyre intruded along the dividing "thrust-plane". S G, dyke of spherulitic granophyre; D, dyke of basalt.
(Figure 29) Map of part of the Beinn an Dubhaich granite mass, showing its relation to the Cambrian limestones: scale, 6 inches to a mile. The area included lies to the south of the high-road and of Loch Kilchrist, the head of which is shown on the northern border of the map. The ground to the north of the high-road is covered by alluvium, concealing the junction of the Cambrian with the volcanic agglomerate of the Kilchrist vent. The limestones have been both dolomitised and metamorphosed, the metamorphism having to a great extent brought about de-dolomitisation, as described below. The broken line marks the division between the Balnakiel and Croisaphuill groups, the former (lower) being the nearer to the granite axis, with an anticlinal arrangement. Only a few of the numerous basic dykes are shown. Of those outside the main granite boundary, some are pre-granitic and cut off by the granite, others are post-granitic and stopped by the granite: the dykes in the enclosed patches of limestone all belong to the former category. No dykes intersect the granite.
(Figure 30) Section from Loch Kilchrist to Glen Boreraig, through the old marble quarries. Scale, 4½ inches to a mile. The line of section, passing between Beinn an Dubhaich and Beinn Suardal, does not cross the widest part of the granitic mass. It shows the anticline of the Cambrian limestones with the granite cutting vertically through it.
(Figure 31) Relations of granite and marble (metamorphosed Cambrian limestone) on the lower slopes of Beinn an Dubhaich. This figure represents a typical view of the junction of the two rocks, idealised only to the extent of omitting the heather and bracken which partly conceal the ground. A pre-granitic dolerite dyke is shown intersecting the marble and sharply cut off by the granite.
(Figure 32) Section along Beinn na Cro. A patch of the basaltic lavas, already invaded by sheets of gabbro, has been enveloped in the granite of the Red Hills, which sends numerous offshoots in the form of tongues and dykes through the enclosed mass. The whole is intersected by later dykes of olivine-basalt. Scale, 4 inches to a mile.
(Figure 33) Section in the south-western part of Scalpay, from Corran a' Chinn Ijachclaraich, showing the relation of the granite to the Torridonian strata. The former is the edge of the large mass building the Red Hills, which here terminates in a number of tapering sheets. Scale, 3 inches to a mile.
(Figure 34) Some rarer minerals of the granophyres. A to D × 30, E to H × 100. A to D are riebeckite crystals from the granophyre of Meall Dearg, illustrating the allotriomorphic habit of the larger and the idiomorphic shape of the smaller crystals (S8856) [NG 495 227]. E and F show an unidentified brown mineral intergrown with green horneblende in a granophyre from Druim na Cleochd (S3198). G and H are twinned crystals of allanite(?) in the granophyre of Allt Fearna, near Broadford. In the former are shown the approximate positions of the axes of strongest absorption for the two individuals.
(Figure 35) Section across the deep gully on the N.W. slope of Marsco. Explanation in the text. K is granite of pegmatoid and gneissic structure veining the gabbro as exposed in the bed of the stream a little lower down. Its connection with the granite to the south is only conjectural.
(Figure 36) Sketch-map to illustrate the relations of the enclosed bodies of gabbro and of the rock here styled "marscoite" on the slopes of Marsco. Scale, 3 inches to a mile.
(Figure 37) Outlines of Glamaig and Beinn Dearg, seen from the road a mile north of Sligachan. The broken north face of Glamaig, on the extreme left, consists largely of metamorphosed basaltic lavas. Beinn Dearg illustrates the characteristic rounded outlines of the granite hills; while, in strong contrast with this, the abrupt knoll of Sròn a' Bhealain is seen towards the right of the sketch.
(Figure 38) Section from Sròn a' Bhealain to Allt Daraich and thence to the summit of Glamaig; scale, 4 inches to a mile.
(Figure 39) Section through the summit of Glamaig and westward to the Sligachan estuary; scale, 4 inches to a mile.
(Figure 40) Ground-plan of part of Allt Daraich, near Sligachan. Ga. Coarse granophyre or granophyric granite of normal type. M. Marscoite. X. Xenolithic ("spotted") granophyre. G. Gabbro, enclosed patches. B. Amygdaloidal basalt (lava).
(Figure 41) Granophyre crowded with xenoliths of marscoite and the debris of these, including released crystals of labradorite; specimen drawn of the natural size; from Allt Daraich, near Sligachan.
(Figure 42) Section through Cnoc Càrnach, showing two composite triple sills at different horizons in the Lias. Scale, 18 inches to a mile.
(Figure 43) Section of triple composite sill intruded in the coarse pebbly felspathic sandstones of the Torridonian (Applecross Grits) on the southern shore of Camas na Geadaig, in the N.W. of Scalpay. The middle and principal member of the sill is a spherulitic granophyre; above is basalt, 4 ft thick, with a sharp junction; below is basalt, 2 ft thick, with the appearance of a more gradual transition. The junctions show reactions between the basic and acid rocks of the kind described below.
(Figure 44) Section across Allt an 't-Sithean and through Cnoc an 't-Sithean, about 1¼ mile N.N.W. of Sligachan, to show the relations of the basalt (black) and granophyre (white) in the quintuple composite laccolite. The intrusion occurs in the basaltic lava group, and one of the ordinary dolerite sills is shown lower down. The triple composite dyke which has probably fed the laccolite does not appear in this section, but some later basic dykes of simple habit are shown, and these in some cases have failed to penetrate the thick mass.
(Figure 45) Section across the composite sill of Carn Dearg, near Suishnish. Scale, 6 inches to a mile. G, granophyre of sill; B, lower basalt member; D, supposed dyke-feeder of sill; P, later independent intrusion of rock varying from olivine-gabbro to picrite (see Chapter 22).
(Figure 46) Sketch-map of a small area in the interior of the Isle of Scalpay: explanation in the text.
(Figure 47) Altered phenocrysts and xenocrysts in the basalt of the composite sills; × ca. 10. A (S6735) [NG 655 197]. Labradorite phenocryst in upper basalt of Cnoc Mrnach; the cleavage cracks opened probably by heat due to the succeeding acid intrusion. B (S6735) [NG 655 197]. Labradorite phenocryst in the same rock; showing peripheral fissures with a tendency to concentric arrangement. C (S7071) [NG 632 174]. Labradorite phenocryst in lower basalt of Beinn a' Chaìrn; showing opening of cleavage cracks and glass-inclusions, probably of secondary origin, in interior of crystal. D (S6735) [NG 655 197]. Orthoclase xenocryst in upper basalt of Cnoc Chrnach; turbid owing to secondary glass-inclusions. E (S6732) [NG 660 204]. Oligoclase xenocryst in lower basalt of Creag Bhriste; showing similar turbidity and also rounding of the angles by magmatic corrosion. F (S7069) [NG 625 168]. Oligoclase xenocryst in basalt xenolith enclosed in acid rock of Beinn a' Chaìrn; showing a more advanced stage of corrosion, affecting the interior as well as the border of the crystal. G (S6735) [NG 655 197]. Oligoclase xenocryst in upper basalt of Cnoc Càrnach; showing a very advanced stage of corrosion, with formation of new felspar microlites in the interior of the crystal. H (S7069) [NG 625 168]. Quartz xenocryst in basalt xenolith at Beinn a' Chaìrn; rounded, and with border of granular augite (now decayed). K (S7066) [NG 638 181]. Quartz xenocryst in basalt xenolith at same place; showing more advanced corrosion and a deeper border composed of larger granules. L (S6733) [NG 660 204]. Quartz xenocryst in acidified basalt xenolith at Creag Bhriste; showing more advanced corrosion, but the augite border has been resorbed with increasing acidification of the matrix. M (S6731) [NG 645 247]. Xenocryst of alkali-felspar in lower basalt of Rudh' an Eireannaich; showing the margin honeycombed by secondary inclusions.
(Figure 48) Section of composite sill in the Lias at Rudh' an Eireannaich, Broadford Bay. A, a fault; its fissure occupied by a double basalt dyke. B, a small fault. C, escarpment running out to sea eastward, and forming the actual headland. D and E, thin sills, 1½ ft and 1 ft respectively, referred to below.
(Figure 49) Enlarged section of composite sill of Rudh' an Eireannaich, taken at the low escarpment on the shore, the eastward continuation of C in the preceding figure. The numbers indicate the specific gravities of specimens from different parts of the section.
(Figure 50) Ideal curve of variation of magnesia, PpP1. Here PM and P1M1 represent the percentages of magnesia in a basic and an acid rock; p1m, that in a hybrid rock formed by the admixture of the two; and pm, that in a normal rock having the same silica-percentage as the hybrid. The latter has thus an excess of magnesia.
(Figure 51) Ideal curve of variation of lime. The diagram is lettered to correspond with the preceding, lime being understood instead of magnesia. It is seen that the hybrid rock has a deficiency of lime as compared with the natural rock of the same silica-percentage.
(Figure 52) Sketch-map illustrating the distribution of the basic sills, and also of the multiple basic dykes, in relation to the large plutonic intrusions. Scale, ¼ inch to a mile. (a) The heavy dotted line indicates the area (embracing the plutonic intrusions with a narrow surrounding belt) which is free from sills belonging to the great group. The lighter dotted line marks the limit (in this part the eastern limit) of multiple sills. This depends partly upon the general attenuation of the group in this direction, but partly also upon the progress of erosion, since the multiple sills are developed chiefly in the upper portion of the lava group. (b) The heavy broken line indicates the distribution of the principal multiple basic dykes. They are found within an elongated oval tract, about eleven miles long, centring in the great gabbro laccolite and having its long axis in the general direction of the dykes themselves. This oval tract, however, is divided into two detached areas by the plutonic masses. It is not improbable that better exposures might enable us to join these two areas on the west side of the Cuillins, but on the east side the granite has offered an impenetrable resistance (see Chapter 17).
(Figure 53) Section through Monadh Meadale and Beinn Totaig; showing intrusive sills (indicated by vertical hading) in the basaltic lavas, and showing how these run together to form double and multiple sills. Scale, 4 inches a mile.
(Figure 54) Preshal More, near Talisker, seen from the south-west.
(Figure 55) Section through Druim na Crìche, about 5½ miles S.S.W. of Portree and 5½ miles N.W. of Sligachan; showing composite double sills or laccolites of the Roineval type. Scale: 6 inches to a mile. The prolongation below the surface is partly conjectural.
(Figure 56) Geological map of Roineval, showing the composite double sills. Scale: 4½ inches to a mile.
(Figure 57) Section through Roineval, showing the double sill at the summit and the lower mugearite sill; Mugearlte dyke-feeders. also conjectural relation of the sills to Scale: 4½ inches to a mile.
(Figure 58) Sketch-map illustrating the distribution of certain groups of acid intrusions in relation to the granite of the Red Hills. Scale, ¼ inch to a mile. (a.) The line (of small circles connected by dashes) in the gabbro area indicates the western limit of granite and granophyre veins, in so far at least as they are locally abundant and noticeable, in the gabbro of the Cuillins. It may probably be taken as showing, with rough approximation, the concealed extension of the granite beneath the gabbro laccolite. (b.) The roughly semicircular belt, enclosed by a line of dots and dashes, marks the distribution of the peculiar composite (basic and acid) intrusions of the symmetrical kind, which we have distinguished as the Cnoc Càrnach type. (c.) The heavy broken line indicates the area of distribution of the minor acid intrusions in general. It is an irregular oval, about 24 miles long, centring in the granite of the Red Hills, and having its long axis in a direction nearly agreeing with that of the dykes. (d.) The short heavy line at A marks the position of the peculiar felsite of the Alaisdair Stone-shoot.
(Figure 59) Section across circus north of Rudha Chinn Mhòir, in the N.W. of Scalpay. T, Torridonian sandstone; C, Triassic conglomerate; G, granophyre sill; G′, granophyre dyke; S, basalt sill; B, B, basalt dykes.
(Figure 60) Sections across circus north of Rudha Chinn Mhòir, in the N.W. of Scalpay. T, Torridonian sandstone; C, Triassic conglomerate; G, granophyre sill; G′, granophyre dyke; S, basalt sill; B, B, basalt dykes.
(Figure 61) [SS71]. Thin slice from one of the rhyolitic dykes, apophyses from the granite, intersecting the gabbro of Druim an Eidhne; magnified four diameters. To the left are seen a number of coalescing spherulites. These have grown in one place round a group of felspar crystals, in other places round micropegmatite phenocrysts, which have in part the outlines of felspar crystals but are crowded with inclusions of quartz in micrographic intergrowth. The general mass of the rock has a strongly marked flow-structure, and in the right-hand half of the figure the fluxion-lines are seen to diverge in a fashion which indicates discontinuous flowing movement in the mass.
(Figure 62) Ground-plan of a small area in the lower part of Tairneilear, showing dykes terminating abruptly against volcanic agglomerate.
(Figure 63) Sketch-map illustrating the bearings of the basic dykes in different parts of Skye. The letters C and R mark the situations of the Cuillins and the Red Hills respectively.
(Figure 64) Ground-plan on shore west of Broadford Bay; to show the abrupt breaking off and lateral shifting of a dyke.
(Figure 65) Ground-plan on shore west of Broadford Bay; to show lateral shifting of a dyke, the two portions of which run out into veins.
(Figure 66) Ground-plan on shore west of Broadford Bay; to show lateral shifting of a dyke with veins tending to connect the two detached portions.
(Figure 67) Ground-plan on shore west of Broadford Bay; to show a number of associated dykes sharply deviated for a short distance.
(Figure 68) (S6421) [NX 12 89] × 30. Diabase or coarse dolerite, dyke ½ mile E.S.E. of summit of Ben Aslak and 2 miles S.W. of Kylerhea. Typical ophitic structure. The felspar crystals in this and numerous similar rocks show between crossed nicols a strong zonary banding, which does not disappear with the albite-lamellation, and is therefore due to the marginal portion being of different composition from the interior.
(Figure 69) Sheaths and cores on surface of a basaltic sheet, rather more than ⅓ mile S.W. of Cnoc a' Chàise Mòr, near Knock. Scale, ¼ of natural size. (C. T. C.)
(Figure 70) Sheaths and cores, the latter with perlitic forms, on surface of a basaltic sheet, same locality as the preceding. Scale, 1/10 of natural size. (C. T. C.)
(Figure 71) Margin of small basalt dyke, N.W. of Ben Lee, near Sligachan; × 10. This figure is merely diagrammatic. A. Interior of dyke, very dark in the slice, but crowded with minute felspar microlites aud enclosing locally groups of little felspar crystals. B. Narrow spherulitic band, birefringent but without any good "black cross". C. Dark spherical bodies of concretionary nature, without action on polarised light. D. Single band of small spherulites like G, and narrow seam of black glass. E. Pleochroic spherulites, described in text, p. 344. F. Band full of obscure spherical bodies giving no reaction between crossed nicols . G. Minute spherulites, coalescing into bands. H. Black glass forming actual edge of dyke.
(Figure 72) Sketch-map to illustrate the distribution and inclination of the inclined basic sheets of the Cuillins. The strong line marks the outline of the gabbro area: the dotted lines enclose the areas within which the inclined sheets are found, and the arrows (with figures) indicate the dips of the sheets.
(Figure 73) Section across the gabbro area to show the vertical distribution of the inclined sheets. The strong line shows the base of the gabbro laccolite: the short lines represent the inclined sheets, and are drawn at approximately the true inclinations. The letters refer to localities, as follows: GB, Glen Brittle; SB, Sgùrr na Banachdich; C, Coruisk; DR, Druim nan Ramh; DE, Druim an Eidhne; SC, Strath na Creitheach; B, Blath-bheinn; LS, Loch Slapin.
(Figure 74) Section to illustrate the shifting of an inclined asic sheet, cutting the gabbro, near the outfall of Allt a’ Chaoich, Loch Scavaig.
(Figure 75) Sketch-Map to illustrate the distribution of the peridotites, older and younger. Scale, ¼ inch to a mile. The older plutonic laccolites of the south-western Cuillins (with one in the Isle of Soay) are marked in black. The large crescentic area enclosed by a dotted boundary embraces the younger peridotite dykes of the Cuillins and the Strathaird peninsula. The only peridotite dykes outside this area are a group on the coast of Loch Brittle at B, but peridotite sills occur in Soay as indicated. The boss of An Sgùman is situated at the point marked S, and the intrusions of Glamaig and Carn Dearg at G and D, on the prolongations of the two horns of the crescent.
(Figure 76) Sketch-Map to show the distribution of some trachytic and other dykes. (CL) The broken line encloses the oval area of distribution of the Drynoch group of trachytes. (b.) The line made up of dots and dashes marks the limits of distribution in Skye of the trachytic and allied dykes of Sleat and the Broadford district. (c.) The small circles connected by straight lines indicate the known localities of acid pitchstone dykes. (d ) The small oval enclosed by the dotted line shows the area affected by the Coirechatachan type of dykes, probably altered pitchstones. It falls in the middle of the narrow strip of country including the occurrences under (c).
(Figure 77) Diagram to show the varying inclination of the "rodding" in the dykes of the Broadford and Sleat districts. Explanation in the text. Del. C. T. C. a. Trachyte. Allt Mòr, about two miles S.E. of Drochaid Lusa. In amygdules also. b. Trachyte. Burn about a mile W. of the top of An Sgulan, Kinloch. c. Trachyte. Allt Cùl Airidh Lagain, nearly three-quarters of a mile above the road. d. Trachyte. Allt Réidhe Ghlais, nearly three-quarters of a mile above the road. In amygdules also. e. Trachyte. Rather more than half a mile S.E. of Broadford Bridge. f. Trachyte. Allt Cùl Airidh Lagain. Nearly a third of a mile below the road. In amygdules also. g. Trachyte. Allt a Choin, about 200 yards slightly E. of N. of Kinloch. In amygdules also. h. Trachyte. Near the foot of Allt Lochan Sgeir, near Kinloch. In amygdules also. i. Felsite. Burn E. of Cnoc na Cubhaige, Broadford. j. Felsite. Burn about half a mile slightly E. of S. of Cnoc na Cubhaige. k. Trachyte. About two-thirds of a mile E.N.E. of Ben Suardal. 1. Trachyte. Coast, about 200 yards E.N.E. of Ardnameacan, Loch na Dal. In amygdules also. m. Acid Pitchstone. Allt Duisdale, nearly 1500 yards above the road. In amygdules also. n. Dyke of doubtful character, with oligoclase, hornblende, and biotite (S6855) [NG 672 083]. Rather more than half a mile S. of Cnoc a' Chaìse Mòr, Knock. In amygdules also. o. Basaltic: S.G. 2.87. Rudha Dubh Ard, near Ord. p. Basaltic. Coast about 330 yards N.E. of Inver Aulavaig. q. Trachyte. Coast, nearly half a mile S.S.E. of Ostlig House. In amygdules also. r. Trachyte. Nearly a mile E.S.E. of Meall Buidhe (S.W. of Armadale). In amygdules also.
(Figure 78) Dyke with "rodded" structure, near the river and foot-path, E. of Coire-chatachan, near Broadford. The figure shows the southerly face of the dyke as exposed, w-ith the rodding, the inclination of which to the horizon changes in a length of five yards from 8° to 52°.
(Figure 79) Terraced hills on the west side of GIen Varragill; outline view, looking northward to the Storr which is seen in the distance (right). The terraces are caused by the very numerous intrusive sills intercalated in the dasaltic lavas.
(Figure 80) Outlines of hills on the west side of Glen Brittle, seen from near Bealach a' Mhaìm. The terraced appearance is due to the intrusive sills, and the long straight gullies are determined by the weathering of dykes.
(Figure 81) View of Blath-bheinn, looking west from near Kilchrist. Above the drift-covered Jurassic rocks of the foreground, An Carnach, formed by strong intrusive sills in the basalt group, illustrates the plateau type of scenery. Behind this rise Slat-bheinn and An Stac, in which the sills rapidly die out, and the indurated basalt lavas themselves constitute marked features, the bedding being sharply turned up towards the mountains. Blath-bheinn and Clach Glas in the background are characteristic gabbro mountains. (Del. C. B. Phillip.)
(Figure 82) The "Inaccessible Pinnacle" of Sgùrr Dearg, seen from near the summit-cairn. In the background the outlines of Sgùrr Sgùmain (right) and Sgùrr a' Coir' an Lochaiu (left).
(Figure 83) Outlines of the Alaisdair group of mountains, seen from Sgùrr Dearg. The highest point is Sgùrr Alaisdair (3275 feet), separated from its neighbour, SgùrrTearlach, by the great stone-shoot. To the left is Sew Choinnich, and to the right the broken ridge of Sgùrr Sginnain with the outline of Sgùrr nan Eag behind,
Plates
(Plate 1) Exposed surface of volcanic agglomerate, Druim an Eidhne.
(Plate 2) Banded structure in the peridotite group, An Garbh-choire.
(Plate 3) Brecciated appearance, due to xenolithic structure, in the peridotite group, An Garbh-choire.
(Plate 4) Veined structure in the peridotite group, An Garhh-choire.
(Plate 5) Strongly banded structure and felspathic veins in gabbro, Druim an Eidhne.
(Plate 6) Banded structure in gabbro, Druim an Eidhne.
(Plate 7) Crushed granite, from the shore between Allt Fearna and Strollamus Lodge, 2½ miles N.W. of Broadford. Natural size. The lower figure is from a typical specimen, while the upper one shows an earlier stage in the process of brecciation,
(Plate 8) Marsco, from the North-west.
(Plate 9) Part of the northern face of Preshal More, near Talisker, showing curvature of columns.
(Plate 10) Columnar sill of dolerite forming the cliff at Rudha Buidhe, near Braes.
(Plate 11) Weathered surface of acid dyke, Druim an Eidhne, showing crowded spherulites.
(Plate 12) Weathered surface of acid dyke, Druim an Eidhnei slowing tortuous flow-structure,
(Plate 13) (Frontispiece) View up Loch Coruisk, showing part of the Cuillin range. The mountains seen are as follows, from left to right: Sgùrr Dubh Bheag in the foreground; the top of Sgùrr Dearg (with the "Inaccessible Pinnacle") appearing over Sgùrr a' Coir' an Lochain (with vertical cleft); the broken ridge of Sgùrr na Banachdich; the double summit of Sgùrr a' Ghreadaidh in the centre; the four peaks of Sgùrr a' Mhadaidh; Bidein Druim nan Ramh (partly seen); and the Druim nan Ramh ridge bounding the valley on the right.
(Plate 14) View of Clach Glas from Garbh Bheinn.
(Plate 15) View from Coire na Creiche.
(Plate 16) View of Sgùrr nan Gillean.
(Plate 17) Fig. 1. × 20. Olivine-basalt lava, above schoolhouse, Braes, S. of Portree: showing olivine replaced by a mineral comparable with iddingsite. See p. 34. Fig 2. (S6772) [NG 520 363] × 20. Olivine-basalt lava, Rudha Buidhe, near Braes, S. of Portree: showing another type of pseudomorph after olivine. See p. 34. Fig 3. × 40. Microstructures of the basic lavas. A. (S8185) [NG 42 28] "Granulitic" structure in olivine-basalt, near bridge over Allt Fionnfhuachd, Drynoch; the rock analysed. See pp. 31, 36. B. (S9246) [NG 47 29] Ophitic structure in hypersthene-basalt, lower part of Allt Dearg Mòr, near Sligachan. A bastite pseudomorph after hypersthene appears in the lower left-hand corner. See pp. 36, 38. C. Ocellar structure in basalt at base of group, S. of Sgùrr nan Each: a type rich in augite and without olivine. See p. 37. D. (S9366) [NG 614 273] Microlitic structure in augite-andesite, S. coast of Scalpay: the augite is mostly chloritised. See p. 37. Fig. 4. (S7460) [NG 537 196] × 10. Metamorphosed amygdule in basalt, close to granite on E. side of Blath-bheinn; showing a crystalline aggregate of new plagioclase felspar, partly with radiate grouping, replacing zeolites. See p. 51. Fig. 5. (S2700) [NG 587 240] × 10. Metamorphosed amygdule in basalt, near granite, Creagan Dubha, N. of Beinn Dearg Mhòr (of Strath): showing a granular crystalline aggregate of new felspar, derived from zeolites, with a border of epidote grains. See pp. 51, 52.
(Plate 18) Fig 1. (S8731) [NG 437 191] × 20. Highly metamorphosed amygdaloidal basalt, near gabbro, N.E. of An Sgùman. The rock is completely reconstituted, and presents the appearance of some so-called pyroxene-granulites. The figure shows part of an amygdule, now consisting of alternate zones of augite and felspar. See pp. 52, 53. Fig. 2. (S9238) [NG 470 200] × 20. A northite-olivine rock (troctolite) in the peridotite group, lower part of An Garbh-choire: consisting simply of olivine and anorthite, the latter often traversed by numerous fine fissures, which radiate from the olivine grains. See p. 73. Fig. 3. (S8705) [NG 456 219] × 20. Enstatite-anorthite-rock (norite) in the peridotite group, N. of Sgùrr a' Coir' an Lochain, near Coruisk. The chief constituents are enstatite and anorthite, the latter interstitial to the former. At the bottom of the figure is a crystal, half of enstatite, half of diallage, and immediately to the left of this a crystal-grain of olivine. See p. 74. Fig. 4. (S7462) [NG 520 214] × 100. Augite of gabbro on the W. slope of Blath-bheinn: showing the basal striation, accentuated by a fine "schiller" structure, and combined with the orthopinacoidal twin to give the "herring-bone" arrangement. See p. 109. Fig. 5. × 10. Augite of gabbro at head of Loch Scavaig: showing the prismatic cleavage and diallage - structure. The latter, parallel to the orthopinacoid, is developed only in the marginal portion of the crystal. See p. 109. Fig. 6. (S7849) [NG 49 22] × 10. Foliated gabbro in the banded part of the group, Druim an Eidhne: showing a parallel orientation of the component crystals. See p. 119.
(Plate 19) Fig. 1. Graphic structures in granophyres; × 40, crossed nicols. A. Roadside E. of Strollamus Bridge: showing micrographic intergrowth of striated oligoclase and quartz, and in the upper part of orthoclase and quartz, in connection with an orthoclase phenocryst. See p. 161. B. Glas-Bheinn Bheag, near margin of intrusion: showing part of a single spherulite with the gradation from a cryptographic structure near the centre of growth to a visibly micrographic towards the periphery. See p. 281. Fig. 2. (S2667) [NG 50 24] × 40. Granophyric granite, Marsco: showing a rude micrographic growth round a grain of quartz. See p. 162.
(Plate 20) Fig. 1. (S8962) [NG 500 258] × 20. Gabbro partially fused and injected by the granite magma, gully on the N.W. face of Marsco. The minerals shown are felspar, partially destroyed augite, greenish brown hornblende, magnetite, apatite, and some interstitial quartz. Of the original constituents of the gabbro there remain relics of augite and some of the large crystals of labradorite. See p. 182. Fig. 2. (S5344) [NG 490 230] × 30. Phenocryst of micropegmatite in spherulitic dyke, Druim an Eidhne. It has served as the starting-place for subsequent spherulitic growths. See p. 284. Fig. 3. (S8694) [NG 513 249] × 40. Granite modified by absorption of gabbro material, S.E. ridge of Marsco. The figure shows aggregates composed of greenish brown hornblende with little scales of biotite, larger flakes of biotite enclosing apatite, and irregular grains of magnetite. The rest is of quartz, oligoclase, and orthoclase. See p. 184. Fig. 4. (S8188) [NG 418 351] × 20. Porphyritic Olivine-Dolerite, dyke on Roineval, two miles N of Drynoch: showing a felspar phenocryst enlarged by a later growth with crystalline continuity. See p. 329. Fig. 5. (S8733) [NG 50 30] × 50, crossed nicols. Spherulite in pitchstone, W. face of Glamaig: showing a concentric shell structure. See p. 404.
(Plate 21) Fig. 1. (S7551) [NG 508 289] × 30. Xenolith of marscoite from the "spotted" granophyre of Allt Daraich, near Sligachan. The figure shows one of the large labradorite crystals, much fissured, in a ground-mass of hornblende, oligoclase, orthoclase, quartz, magnetite, and apatite. There has been some impregnation by the surrounding acid magma. See p. 195. Fig. 2. (S7858) [NG 505 258] × 100. Augite crystals replaced by fibrous green hornblende and granules of magnetite, in marscoite from the gully on the N.W. face of Marsco. See p. 186. Fig. 3. (S9982) [NG 444 122] × 100. Vitrified Torridonian grit, in contact with a dolerite sill, S. coast of Soay. Some relics of quartz-grains remain in a corroded shape. The rest is a clear colourless glass enclosing minute crystals of cordierite, magnetite, and a pyroxenic mineral. See p. 246. Fig. 4. (S9371) [NG 645 249] × 30. Corroded xenocryst of oligoclase in small sill above the composite sill of Rudh' an Eireannaich, near Broadford. The crystal, except at its centre, is greatly affected by secondary inclusions. In one place corrosion has eaten away the crystal, forming an inlet occupied by the ground-mass with its small felspar crystals. See p. 229.
(Plate 22) Fig. I. (S3200) [NG 621 170] × 20. Spherulitic felsite, above Boreraig: showing dense radiate spherulites, sometimes grown round quartz crystals, with interspaces having a granular structure. See p. 281. Fig. 2. (S8951) [NG 724 263] × 20. Dolerite, dyke at point E. of the mouth of Allt na Nighinn, 2 miles W. of Kyleakin. At the top of the figure is one of the shapeless later felspar crystals, enclosing numerous needles of apatite. See p. 322. Fig. 3, (S5389) [NG 48 22] × 40. Part of a large composite spherulite from an acid dyke cutting the gabbro of Druim an Eidhne. The centre of the spherulite is outside the figure, above and to the right. There are minor centres of radiation, which serves as the apices of conical growths directed outward, the axes of the cones conforming with the principal radiate arrangement. One such cone, cut along its axis, occupies the central part of the figure; while below, to the right, are others cut at some distance from their axes so as to present parabolic sections. The clear crystals are quartz, the opaque ones pyrites. See p. 286. Fig. 4. (S7862) [NG 515 261] × 20. Porphyritic Olivine-Dolerite, dyke cutting the granite of Ciche na Beinne Deirge, 3 miles S.E. of Sligachan. This represents the Beinn Dearg type of dykes, and is the rock analysed. See p. 326. Fig. 5. (S6711) [NG 623 205] × 20. Porphyritic Basalt, dyke cutting Cambrian limestone ¼ mile S. by E. of Suardal, about 2 miles S.S.W. of Broadford. This rock illustrates one variety of the Suardal group of dykes, containing phenocrysts of labradorite and grains of olivine in a ground-mass of smaller felspars, abundant magnetite, and finely granular augite. See p. 327.
(Plate 23) Fig. 1. A. (occupying three quadrants) (S8849) [NG 691 233] × 40. Tachylyte, dyke ⅓ mile S. of Loch Ashik, near Broadford: showing the central part of the dyke, which consists of a clear colourless glass crowded with rods of magnetite in parallel groups. The small clear spaces are amygdules. See p. 349. B. (lower left-hand quadrant) (S8846) [NG 492 337] × 200. Basalt, largely glassy, with skeleton crystallites of felspar, central part of a thin dyke with tachylytic selvages, N. slope of Meall Odhar Beag, near Sligachan. See p. 345. Fig. 2. × 40. Spherulitic or variolitic basalt, forming a sheet at Camas Daraich, Point of Sleat. See p. 347. A. (occupying three quadrants) (S7845) [NM 566 996]. Prevalent type, showing only part of one of the large spherulites. B. (lower left-hand quadrant) (S7846) [NM 566 996]. Marginal modification, largely vitreous, with olivine in the form of skeleton crystallites.
(Plate 24) Fig. 1. (S8850) [NG 708 183] × 150. Tachylyte, dyke ¾ m. N.E. of Kinloch, Sleat district: a quasi-spherulitic rock. See pp. 349, 350. A. Outer portion of one of the large spherulitic bodies, showing parallel rods of augite, with some magnetite, embedded in a colourless glass. B. Central part of a spherulite, showing abundant felspar, as well as augite and magnetite, with a smaller proportion of glassy base. Fig. 2. (S6794) [NG 617 233] × 20. Pitchstone, dyke in Allt a' Choire, above Coirechatachan, near Broadford: showing groups of crystallites, each surrounded by a clear ring. The turbid appearance of the rest of the glassy mass is due to a crowd of more minute crystallitic growths. See p. 407. Fig. 3. × 20. Pitchstone, E. slope of Glas Bheinn Mhòr; showing perlitic cracks surrounding phenocrysts of quartz: also groups of crystallites, each surrounded by a clear ring. See p. 405. Fig. 4. (S7479) [NG 523 201] × 30. Xenolith of quartzite from a basic dyke, S. end of Blath-bheinn. The figure shows the quartzite corroded by the basic magma and an inlet occupied by radiating fibres of new-formed felspar, probably oligoclase. See p. 352.
(Plate 25) Fig. 1. (S9372) [NG 628 285] × 30. Porphyritic Dolerite, dyke 400 yards N.W. by N. of Scalpay House. The lower part of the figure shows part of one of the circular felspathic areas which represent vesicles filled by the oozing in of the residual magma. See p. 331. Fig. 2; × 20. Xenoliths in basic dykes. A. (S7483) [NG 526 194]. Edge of ga,bbro xenolith in dyke in Abhuinn nan Leac, Strathaird; showing the earliest stage of breaking up by the formation of numerous fissures; also the development of secondary inclusions in both felspar and augite. See pp. 355, 361. B. (S6716) [NG 633 208]. Detached quartz-grain from granite xenolith in dyke on ridge N. of Ben Suardal, near Broadford; showing the earliest stage of breaking up by the formation of fissures, which here tend to run parallel to the outline; also incipient corrosion. See p. 356. FIG, 3. × 20. Basic dyke carrying granite xenoliths, on shore N.E. of Corry Lodge, Broadford. See p. 357.
A. (S6719) [NG 644 243]. The normal dolerite, where free from foreign material. B. (S6720) [NG 644 243]. Portion enclosing abundant debris of granite, of which two detached quartz grains are shown, eack with its corrosion-border of granular augite. The matrix, partly obscured by alteration, is of fine texture and of much less basic composition than the normal dolerite. Fig. 4. (S8723) [NG 436 182]. Picrite, An Sgùman. A. (occupying three quadrants); × 30. Showing olivine, augite, anorthite, etc. The dendritic inclusions of magnetite in the olivine are conspicuous in the large crystal in the lower left-hand quadrant: in the crystal at the top of the figure they are cut at right angles to their plane, and so appear like rods. See p. 381. B. (lower right-hand quadrant); × 110. Showing the dendritic inclusions more highly magnified. See pp. 68, 69, 381.
(Plate 26) Fig. 1. (S9980) [NG 427 141] × 32. Porphyritic Picrite, sill S. of Leac nan Faoileann, I. of Soay. The porphyritic elements are olivine and picotite. These recur also in a second generation, but the bulk of the ground-mass is of slender rods of felspar with sub-parallel arrangement and interstitial augite, the structure recalling that of some variolitic basalts. See p. 380. Fig. 2. (S7370) [NG 585 105] × 20. Ceratophyre (?), large dyke W. of Loch Gauscavaig, near Tarskavaig, Sleat: showing an aggregate of crystals of alkali-felspar with small crystals of augite and grains of magnetite. See p. 397. Fig. 3, (S6851) [NG 610 045] × 20. Ceratophyre (?), dyke nearly ⅔ mile N. by W. of Cnoc an Sgùmain, Armadale: showing phenocrysts of crypto-perthite in a ground-mass composed of an aggregate of crystals of alkali-felspar penetrated by very numerous needles of augite. See p. 397. Fig. 4. (S3201) [NG 615 165] × 30. Glassy Augite-Andesite, dyke on shore of Loch Eishort, W. of Boreraig: showing phenocrysts of felspar, augite, and magnetite in a ground-mass composed of clear glass crowded with crystallitic growths, largely in the form of minute rectangular gratings. See p. 401.
(Plate 27) Fig. 1 × 40, crossed nicols. Micro-structure of the trachytic dykes of the Broadford and Sleat districts, showing the tendency to sheaf-like groupings of the felspars. See p. 394. A. (S6749) [NG 646 231]. Dyke S.E. of Broadford. B. (S5082) [NG 721 234]. Dyke in Allt Mòr, about 2 miles S.E. of Drochaid Lusa and 5 miles E. of Broadford. Fig. 2. (S5424) [NG 729 152] × 400. Augite-Andesite, dyke ⅓ mile S.W. of Rudha Guail, N.E. of Isle Ornsay. The structure is intersertal, and the figure shows also a part of a vesicle occupied by glass crowded with crystallites. See p. 401.
(Unnumbered table) Sequence and in some degree the mutual relations of the chief groups of igneous rocks of the regional and local series in Skye.
(Title page) The Tertiary igneous rocks of Skye. Title page.
(Front cover) The Tertiary igneous rocks of Skye.