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.
Fig. 341.—Section of foliated gabbros in the Tertiary volcanic series of Allival, Rum.
a, massive gabbro with rude lamination parallel to bedding, only seen in some weathered surfaces; b, laminated troctolite; c, massive coarsely crystalline gabbro rudely laminated.
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.