CHAPTER XLIV

THE BOSSES AND SHEETS OF GABBRO IN THE DISTRICTS OF RUM, ARDNAMURCHAN, MULL, ST. KILDA AND NORTH-EAST IRELAND. HISTORY OF THE GABBRO INTRUSIONS

2. _The Island of Rum_

The mountains of the island of Rum, rising as they do from a wide expanse of open sea, present one of the most prominent and picturesque outlines in the West Highlands (Map VI.). More inaccessible than most of the other parts of the volcanic region, they have been less visited by geologists. They were described by Macculloch as composed of varieties of "augite rock." He noticed in this rock "a tendency to the same obscurely bedded disposition as is observed in other rocks of the trap family," and found at one place that it assumed "a regularly bedded form, being disposed in thin horizontal strata, among which are interposed equally thin beds of a rock resembling basalt in its general characters."[350] Professor Judd repeats Macculloch's observation, that "the great masses of gabbro in Rum often exhibit that pseudo-stratification so often observed in igneous rocks." He regards these masses, like those of Skye and Mull, as representing the core of a volcano from which the superficial discharges have been entirely removed, and he gives a section of the island in which the gabbro is represented as an amorphous boss sending veins into a surrounding mass of granite.[351] In a subsequent paper he gave an excellent detailed account of the mineralogical composition of some of the remarkably varied and beautiful basic rocks constituting the hills of Rum, but added no further information regarding the geological structure of the island.[352]

[Footnote 350: _Western Islands_, i. p. 486.]

[Footnote 351: _Quart. Journ. Geol. Soc._ xxx. p. 253.]

[Footnote 352: _Op. cit._ xli. (1885) p. 354. See also his paper in vol. xlii. of the same Journal.]

Even from a distance of eight or ten miles, the hills of Rum are seen to be obviously built up of successive nearly horizontal tiers of rock. As the summer tourist is carried past the island, in that wonderful moving panorama revealed to him by the "swift steamer" of modern days, these great dark cones remind him of colossal pyramids, and as the ever-varying lights and shadows reveal more prominently the alternate nearly level bars of crag and stripes of slope, the resemblance to architectural forms stamps these hills with an individuality which strikes his imagination and fixes itself in his memory. If choice or chance should give him a nearer view of the scene, he would not fail to notice that it is among the northern hills of the island that the bedded character is so conspicuous, and that it ceases to be prominent in the southern heights, though here and there, as in the upper part of Scuir na Gillean, it may in certain lights be detected even from a distance. Crossing over from Eigg, he would recognize each of the features represented in the sketch reproduced in Fig. 339. Along the shore, red sandstones rise in naked cliffs, from the top of which the ground slopes upward in brown moors to the bare rocky declivities. A deep valley (Glen Dibidil) is seen to run into the heart of the hills, between the bedded group to the north and the structureless group to the south. If the weather is favourable, some eight or more prominent parallel bars of rock may be counted on the two higher cones to the right. These bars are not quite level, but slope gently from right to left. They remind one of the terraced basalts of the plateaux, but present a massiveness and a breadth of intervening bare talus-slope such as are not usual among those rocks.

Nor is this impression of regularity and bedded arrangement lessened when we actually climb the slopes of the hills. I had for years been familiar with the outlines of Rum as seen from a distance, and had sketched them from every side, but I shall never forget the surprise and pleasure when my first ascent of the cones revealed to me the meaning of these parallel tiers of rock. I found it to be the structure of the Cuillin Hills repeated, but with some minor differences which are of interest, inasmuch as they enlarge our conceptions of the process by which the gabbro-bosses were formed.

The northern half of the island of Rum consists almost entirely of red sandstone, which, as already stated, is a continuation of the same formation (Torridonian) so well developed in the south-east of Skye, Applecross and Loch Torridon, and traceable between the Archæan gneiss and the Cambrian strata up as far as Cape Wrath. The sandstones, though full of false bedding, show quite distinctly their true stratification, which is inclined with singular persistence towards W.N.W., at angles averaging from 15° to 20°. If they are not repeated by folds or faults, they must reach in this island a thickness of some 10,000 feet. Their red or rather pinkish tint seems mainly to arise from the pink felspar so abundant in them, for in many places they really consist of a kind of arkose. Pebbly bands with rounded pieces of quartz are of common occurrence throughout the whole formation. Dykes and veins of basalt are profusely abundant. Sometimes these run with the bedding, and might at a distance be taken for dark layers among the pink sandstones. They often also strike obliquely up the face of the cliffs like ribbons.

But, notwithstanding their apparent continuity, there can be no doubt that these sandstones have suffered from those powerful terrestrial disturbances which have affected all the older rocks of the North-West Highlands. On the west side, where they plunge steeply into the sea, they have undergone a change into fine laminated rocks, which might at first be mistaken for shales, but which owe their fissility to shearing movements. Along their southern border, from a point on the east coast near Bagh-na-h-Uamha, south of Loch Scresort, to the head of Kilmory Glen, they are abruptly truncated against a group of dark, flaggy and fissile schists and fine quartzites or grits, which in some places are black and massive like basalt, and in others are associated with coarse grey gneiss. That some of these rocks are portions of the Lewisian series can hardly be doubted, and their structure and relations are probably repetitions of those between the Lewisian gneiss and Torridon sandstone of Sleat in Skye. I found also on the northern slopes of Glen Dibidil a patch of much altered grey and white limestone or marble, which reminded me of the Cambrian limestone of Skye. The red sandstones in a more or less altered condition are prolonged to the south-east promontory of the island.

In passing over the zone of these more ancient rocks, we find them to present increasing signs of alteration as they are traced up the slopes towards the great central mass of erupted material. The pink sandstones gradually lose their characteristic tint, and grow much harder and more compact, while the veins and dykes of basalt and sheets of dolerite intersecting them increase in number. The zone of black compact quartzite, which lies to the south of the sandstones, and which at one point reminds us of basalt, at another of the flinty slate of the schistose series, likewise displays increasing induration. Its bedding, not always to be detected, is often vertical and crumpled. But the most remarkable point in its structure is the intercalation in it of bands of breccia. These vary from less than an inch to several yards in diameter; they run mostly with the bedding, but occasionally across it. The stones in them are fragments of the surrounding rock embedded in a matrix of the same material, but also with pieces of a somewhat coarser grit or quartzite. A band of coarse breccia forms the southern limit of this zone along the northern base of Barkeval and Allival. In general character it resembles the thinner seams of the same material just referred to. The matrix so closely agrees with the black flinty quartzite, that but for the included stones it could hardly be distinguished; so greatly has the mass been indurated that the stones seem to shade off into the rest of the rock. But here and there its true brecciated nature is conspicuously revealed by prominent blocks of hardened sandstone. This band of breccia must in some places be 150 or 200 feet broad. It has no distinct bedding, but seems to lie as a highly inclined bed dipping into the hill. It may possibly be a crush-breccia belonging to a period earlier than the volcanic eruptions. It is at once succeeded by a black flinty felsite like that of Mull. The groundmass of this rock, so thickly powdered with magnetite grains as to be almost opaque under the microscope, displays good flow-structure round the turbid crystals of orthoclase and the clear granules of quartz. Further up the hill, the rock becomes lighter in colour and less flinty in texture--a change which is found to arise from more complete devitrification, the groundmass having become a crystalline granular aggregate of quartz and felspar with scattered porphyritic crystals of these minerals (microgranite). In some places, the felsite incloses fragments of other rocks. A specimen of this kind, taken from the head of Coire Dubh, shows under the microscope a brown micro-felsitic groundmass, with crystals of felspar and augite, inclosing a piece of basalt, composed of fine laths of plagioclase, abundant magnetite and a smaller proportion of granules of augite.

This band of felsite and microgranite may be traced continuously from Loch Gainmich along the base of Barkeval and Allival, and similar rocks appear at intervals on the same line round the eastern base of the hills. Immediately above this belt of felsitic protrusions comes the great body of gabbro. It will be observed that here, as in Skye, the base of the gabbro mass presents a horizon on which injections of acid rocks have been particularly abundant. Whether the breccias be regarded as the result of earlier rock-crushings, or as due to volcanic explosions during the Tertiary period, they are evidently older than the eruption of the gabbros. In that respect they may be compared with the agglomerates through which the youngest eruptive bosses of Skye have made their way; but their component materials have been derived from the surrounding platform of ancient rocks, and not from subterranean lavas.

For my present purpose, however, the chief point of importance is the structure of the gabbro mass that springs from that platform into the great conical hills of Rum. The accompanying sketch (Fig. 340) will convey a better idea of this structure than a mere description. At the base, immediately above the felsite just referred to, bedded dolerites make their appearance, much intersected with veins from the siliceous rock. Veins and dykes of basalt also cut all the rocks here, the newest being those which run in a north-west direction. The lowest sheets of dolerite are succeeded by overlying sills of coarser dolerites, gabbros, troctolites, etc., which are as regular in their thickness and continuity as the ordinary basalts of the plateaux. The band of light-coloured troctolite, in particular (Fig. 341), about 20 to 30 feet thick, which has been already referred to for its remarkable laminar structure, can be followed for some distance along the base of the hill as a marked projecting escarpment. This rock at once arrests attention by its platy or fissile structure, parallel to the bedding-surfaces of the sheet. Indeed hand-specimens of it, as I have said, might readily pass for pieces of schistose limestone, especially if taken from the upper part. It consists of successive layers, which on the weathered surface divide it into beds almost as regular as those of a flagstone, each bed being further separated into laminæ marked off by the darker and lighter tints of their mineral constituents. The darker layers consist of olivine, and the lighter of plagioclase. This segregation here and there takes the form of rounded masses, where the minerals are more indefinitely gathered together. The affinity of the rock with intrusive sheets is further displayed by the occurrence of abundant nut-like aggregates of pale green olivine. Examined under the microscope, flow-structure is admirably seen, the lath-shaped felspars being drawn out parallel to the planes of movement, and giving thereby the peculiarly schistose structure which is so deceptive.

The massive and coarsely crystalline gabbros below and above this troctolite are all more or less affected by the same laminar structure. Some of those in higher parts of the mountain are quite massive in part, but also include bands of lamination. Banding like that of the Skye gabbros is generally developed among them, the individual bands varying from less than an inch to a foot or more in thickness. This structure, like the lamination, is parallel to the general bedding of the sheets. As in the Cuillin Hills, the bands differ from each other in the relative proportions of the constituent minerals, especially the predominant pyroxene and olivine. The crystals or crystalline aggregates are often from a quarter of an inch to an inch in diameter, and in these large forms are crowded together in certain bands. Magnetite, on the whole, is rather less conspicuous than in the Cuillin gabbro: at least, it is not so prominently aggregated in special layers. In one or two instances I have observed curvature of the banding, but no example so striking as that cited from the Cuillin area (Fig. 337).

On weathered surfaces, where the felspars decay into a creamy white and the ferro-magnesian minerals assume tints of green, brown and red, the resemblance of the rocks to schists is striking. This external likeness is combined with a tendency to split into thin plates parallel to the lamination, which still further increases their schistose appearance. Though less developed than in Skye, the banding appears to be of the same kind and origin; but in Rum it is combined with the remarkable lamination above mentioned, produced by the arrangement of the component minerals with their longer axes parallel to the planes of bedding, as in flow-structure--a combination which I have not yet observed in Skye.

The bedded arrangement of the gabbros of Rum, so conspicuous in the great eastern cones (Figs. 339 and 340), is emphasized by the fact that some sheets, of a more durable kind, stand out boldly as prominent ribs, while the softer crumble into a kind of sand, which forms talus-slopes between the others. Alternations of this nature are continued up to the very top of the mountains. The beds are nearly flat, but dip slightly into the interior or towards the south-west. On the west side of the island also, beyond Loch Sgathaig, a distinct bedding may be traced, the inclination being here once more inwards or to the east. But from Glen Harris and the base of Askival this structure becomes less marked, and gradually disappears. There is thus a central or southern more amorphous region, while round the margin towards the north and east the rock appears in frequent alternating beds.

It is clear that in the broad features of their architecture the hills of Rum follow closely the plan shown in the Cuillin Hills of Skye. But, unfortunately, in the former island denudation has gone so far that no connection can be traced on the ground between the gabbros and the plateau-basalts. As already stated, the latter rocks have been almost entirely stripped off from the platform of sandstones and schists which they undoubtedly at one time covered, and the few outliers of them that remain lie at some little distance from the margin of the gabbro area (_ante_, p. 216). Nevertheless, we are not without some indications of them underneath the gabbros. I have alluded to the basalts that lie at the base of the eastern cones. As we follow the bottom of the gabbro southward round the flanks of the hills, dull compact black shattery basalts, with a white crust, appear from under the more crystalline sheets. These at once remind one of the altered basalts of Skye and Mull. On the west side also, beds of basalt emerge from under the gabbro, but they have been so veined and indurated by the granophyre of that district, that their relations to the gabbro are somewhat obscured. If we could restore the lost portions of the plateau, I believe we should find the gabbros of Rum resting on part of the volcanic plateau, and some of the gabbro-beds prolonged as sills between the sheets of basalt.

3. _The Gabbro of Ardnamurchan_

The promontory of Ardnamurchan reveals as clearly as the flanks of the Cuillin Hills, though in a less imposing way, the relations of the gabbros to the plateau-basalts (Map VI.). From the southern shore at Kilchoan to the northern shore at Kilmory, bedded basalts, of the usual type, amygdaloidal and compact, weathering into brown soil, may be followed along the eastern slopes of the hills, resting upon the schists and Jurassic series of western Argyleshire. These rocks are a continuation of those that cap the ridges further to the south-east and cross Loch Sunart into Morven. They dip westwards, and followed upwards in that direction, they soon present the usual marks of alteration. They weather with a white crust and become indurated and splintery. Sheets of dolerite with many veins and dykes of basalt run between and across them. Bands of gabbro make their appearance, and these, as we advance westwards, increase in number and in coarseness of grain until this rock, in its rudely bedded form, constitutes practically the whole of the promontory from Meall nan Con to the light-house. Many admirable sections may be seen on the coast-cliffs and in the rugged interior, showing the irregular bedding of the gabbro, and how prone this rock is to develop its component minerals in bands or ribbons, sometimes made up of large crystals, as in Skye, Rum and Mull.

4. _The Gabbro of Mull_

In the island of Mull, the conclusions to which the geology of the other volcanic districts leads us as to the position of the gabbros in the series of volcanic phenomena, are further confirmed. The first geologist who appears to have observed the relation of these rocks in that island was Jameson, who classed them under the old name of "greenstone," including in the same designation rocks now termed dolerites and gabbros. He ascended one of the hills above Loch Don, probably Mainnir nam Fiadh (2483 feet), which he found to consist of "strata of basalt and greenstone," with some basalt-breccia or tuff and a capping of basalt. He speaks of the "singular scorified-like aspect" of the weathered greenstone--a description which applies to some of the coarser gabbro bands of that locality. But he appears to have recognized the general bedded arrangement of the rocks up even to the summit of the hill.[353]

[Footnote 353: _Mineralogy of the Scottish Isles_ i. p. 205.]

It was not, however, until the visit of Professor Zirkel in 1868, that the true petrographical characters of the gabbro of Mull were recognized. This observer remarked that the rock is regularly interstratified with the basalt.[354] Professor Judd, as already stated, has supposed the gabbros to be the deep-seated portion of the masses which when poured out at the surface became the plateau-basalts, and he represents them in his map and sections of Mull as ramifying through the granitic rocks.[355]

[Footnote 354: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. (1871) p. 58.]

[Footnote 355: _Quart. Jour. Geol. Soc._ xxx. (1874).]

In Mull the disposition of the gabbro in beds, sheets or sills is well displayed, for there is here no great central complicated mass of interlacing banded and amorphous sheets. We have seen that a higher group of plateau-basalts has survived in this island better than in the other plateaux, and it would seem that denudation has not yet succeeded here in cutting down so deeply into the gabbro core as in Skye, Rum and Ardnamurchan. Only the upper or outer fringe of intrusive sheets among the bedded basalts has been laid bare. The district within which this fringe may be observed is tolerably well-defined by the difference of contour between the long terraced uplands of the ordinary basalts and the more conical forms of the southern group of gabbro hills between Loch na Keal and Loch Spelve. The number and thickness of the gabbro-sheets increase as we proceed inwards from the basalt-plateau. These sheets are specially prominent along the higher parts of the ridge that runs northwards from the northern end of Loch Spelve, and along the west side of Glen Forsa. But they swell out into the thickest mass in the south-western part of the hilly ground, where, from above Craig, in Glenmore, they cross that valley, and form the rugged ridge that rises into Ben Buy (2354 feet), and stretches eastward to near Ardara (Map VI.). It is in this southern mass that the Mull gabbro approaches nearest in general characters to that of Skye. But even here its true intercalation above a great mass of bedded basalt may readily be ascertained in any of the numerous ravines and rocky declivities.

One of the best lines of section for exhibiting the relations of the rocks is the declivity to the west of Ben Buy and Loch Fhuaran. Ascending from the west side, we walk over successive low escarpments and terraces of the plateau-basalts with a gentle inclination towards north-east or east. These rocks weather in the usual way, some into a brown loam, others into spheroidal exfoliating masses. But as we advance uphill they gradually assume the peculiar indurated shattery character already referred to. The soft earthy amygdaloids become dull splintery rocks, in which the amygdales are no longer sharply defined from the matrix, but rather seem to shade off into it, sometimes with a border of interlacing fibres of epidote. The compact basalts have undergone less change, but they too have become indurated, and generally assume a white or grey crust, and none of them weather out into columnar forms. Strings and threads full of epidote run through much of these altered rocks. Abundant granophyric and felsitic veins traverse them. Sheets of dolerite likewise make their appearance between the basalts, followed further up the slope by sheets of gabbro until the latter form the main body of the hill.

On the north side of the same ridge similar evidence is obtainable, though somewhat complicated by the injections of granophyric and felsitic veins and bosses, to which more detailed reference will afterwards be made. But the altered basalts with their amygdaloidal bands and their intercalated basalt-tuffs and breccias, can be followed from the bottom of the glen up to a height of some 1700 feet, above which the main gabbro mass of Ben Buy sets in. Many minor sheets of dolerite and gabbro make their appearance along the side of the hill before the chief overlying body of the rock is reached. Some of these can be distinctly seen breaking across or ending off between the bedded basalts which here dip gently into the hill (Fig. 342). A conspicuous band of coarse basalt-agglomerate, containing blocks of compact and amygdaloidal basalt a yard or more in diameter, shows by the excessive induration of its dull-green matrix the general alteration which the rocks of the basalt-plateau have here undergone. An almost incredible number of veins of fine basalt, porphyry and felsite has been injected into these rocks--a structure which is precisely a counterpart of what occurs under the main body of gabbro in Skye, Ardnamurchan and Rum.

The gabbro mass of the Ben Buy ridge is thus undoubtedly a huge overlying sheet, which probably reaches a thickness of at least 800 feet. It seems to descend rather across the bedding into the hollow of Glen More, and possibly its main pipe of supply lay in that direction. Being enormously thicker than any other sheet in the island, it exhibits the crystalline peculiarities which are so well developed in the central portions of the larger bosses of gabbro. It presents more coarsely crystalline varieties than appear in the thinner sheets, some portions showing crystals of diallage and felspar upwards of an inch in length. It likewise contains admirable examples of banded structure, which, as in Skye and elsewhere, is best developed where the texture becomes especially coarse. Veins or bands, in which the constituent minerals have crystallized out in more definite and conspicuous forms, here and there succeed each other so quickly as to impart a bedded or foliated look to the body of rock, recalling, as in Skye, the aspect of some coarsely crystalline granitoid gneiss. In these respects the Mull gabbro closely resembles that of the Cuillin Hills. Occasionally, on the exposed faces of crags, portions of such bands or veins are seen to be detached and enveloped in a finer surrounding matrix. The thick belts or bands of coarser and finer texture alternate, and give an appearance of bedding to the mass. Nevertheless they are really intrusive sills, which run generally parallel with beds of finer gabbro or with sheets of highly indurated basalt, that may be detached portions of the ordinary rocks of the plateau. The thick sheet of Ben Buy, like the mass of the Cuillin Hills, is thus the result not of one but of many uprises of gabbro.

Of the thinner sheets of dolerite and gabbro in Mull little need here be said. I have referred to their great abundance in the range of eastern hills that rise from the Sound of Mull between Loch Spelve and Fishnish Bay. Though obviously intrusive, they lie on the whole parallel to the bedding of the basalts. The latter rocks exhibit the usual dull indurated shattery character which they assume where large bosses of gabbro have invaded them, and which gradually disappears as we follow them down hill away from the intrusive sheets to the shores of the Sound. They dip towards the centre of the hill group, that is, to south-west in the ridge of Mainnir nam Fiadh, Dùn da Ghaoithe, and Beinn Meadhon, the angle increasing southwards to 15°-20°, and at the south end reaching as much as 35°-40°. Some fine crags of gabbro and dolerite form a prominent spur on the east side of the ridge of Ben Talaidh, in the upper part of Glen Forsa. These consist of successive sheets bedded with the basalts, and dipping south-west. A large sheet stands out conspicuously on the north front of Ben More, lying at the base of the "pale lavas," and immediately above the ordinary basalts. It circles round the fine corry between Ben More and A'Chioch, some of its domes being there beautifully ice-worn. This is the highest platform to which I have satisfactorily traced any of the intrusive sheets of Mull. Another dyke-like mass emerges from beneath the talus slopes of A'Chioch, on the southern side, and runs eastward across the col between the Clachaig Glen and Loch Scridain.

5. _The Gabbros of St. Kilda and North-east Ireland_

Sixty miles to the westward of the Outer Hebrides lies the lonely group of islets of which St. Kilda is the chief. As the main feature of geological interest in this group is the relation of the acid protrusions to the other rocks, the account of the geology will be more appropriately given as a whole in Chapter xlvii. I need only remark here that the predominant rocks of these islands are dark basic masses, chiefly varieties of gabbro, but including also dolerites and basalts. Reasons will be afterwards brought forward for regarding these rocks as parts of the Tertiary volcanic series. They present a close parallel to the gabbros and associated rocks of Skye. But in one important respect they stand alone. No certain trace remains of any basalt-plateau at St. Kilda such as those through which the gabbros of Skye, Mull and Ardnamurchan have been injected. In regard to their mode of production they have doubtless been intruded at some considerable depth beneath the surface. But no relic appears to have survived of the overlying cover of rock under which they consolidated, and into which they were injected.

In the remarkable volcanic district of the north-east of Ireland a series of basic rocks appears, which in its mode of occurrence and its relation to the other members of the series presents many points of resemblance to the gabbros of the Inner Hebrides. The Irish gabbros are well developed in the Carlingford district, where they form intrusive bosses and sheets which have been erupted through the Palæozoic rocks (Map VII.). They are themselves pierced by later masses of granophyre and other acid rocks. Further reference will be made to these gabbros in later pages, where an account will be given of the granite masses of Mourne, Barnavave and Slieve Gullion.

* * * * *

It is interesting to observe that, while in St. Kilda no relic of any basaltic plateau has been preserved, in the Faroe Islands, on the other hand, no sign has been revealed by denudation that the volcanic plateau of that region is pierced by any eruptive core of gabbro or of granophyre. During my cruises round these islands and through their channels, I was ever on the outlook for any difference in topography that might indicate the presence of some eruptive boss like the gabbro and granophyre masses of the Inner Hebrides. But nothing of that nature could be discerned. Everywhere the long level lines of the bedded basalts were seen mounting up to the crests of the ridges and the tops of the highest peaks. Though I cannot assert that no intrusions of gabbro or of granophyre exist among the Faroe Islands, I feel confident that any such masses which may appear at the surface must be of quite insignificant dimensions, and do not make the important feature in geology and topography which they do among the Inner Hebrides. It is, of course, possible that, vast as the denudation of these islands has undoubtedly been, it has not yet trenched the plateau deeply enough to expose any great intrusive bosses and sills which may underlie and invade the basalts.