Fig. 332.—Section across Glen Brittle, to show the general relations of the Bedded Basalts (a) and the Gabbros (b).
The first point to be ascertained in regard to the gabbro and associated basic rocks of the mountainous tract is their connection in geological structure and age with the bedded basalts of the plateau. This initial and fundamental relation, as Forbes long ago said, can be examined along the whole western and southern flank of the Cuillin Hills, from the foot of Glen Sligachan round to the mouth of Loch Scavaig. Even from a distance, the observer, who is favoured with clear weather, can readily trace the almost level sheets of basalt till they dip gently under the darker, more massive rock of the hills. Tourists, who approach Skye by way of Loch Coruisk, have an opportunity, as the steamer nears the island of Soay, of following with the eye the basalt-terraces of the promontory of Rudh' an Dunain until they disappear under the gabbro of the last spur of the Cuillins that guards the western entrance to Loch Scavaig.
What is so evident at a distance becomes still more striking when viewed from nearer ground. Nowhere can it be more impressively seen than at the head of Glen Brittle. Looking westwards, the traveller sees in front of him only the familiar level terraces and green slopes of the basalt-plateau, rising platform above platform to a height of nearly 1500 feet above the sea. But turning to the east, he beholds the dark, gloomy, cauldron-like Corry na Creiche, from which rise some of the ruggedest and loftiest crests of the Cuillins. On the hills that project from either side of this recess and half enclose it, the bedded basalts mount from the bottom of the valley, with their lines of parallel terrace dipping gently inward below the black rugged gabbro that crowns them and sweeps round to form the back or head of the corry. Down the whole length of Glen Brittle the same structure conspicuously governs the topographical features. On the right hand, the ordinary terraced basalts form the slopes; and they rise for some 500 or 600 feet up the eastern side, until they pass under the darker, more rugged, and less distinctly bedded rocks of the mountains ([Fig. 332]). The dip of the whole series is here at a gentle angle towards south-east, that is, into or under the main mass of the Cuillin group.
When, however, we proceed to examine the junction between the two rocks we find it to be less simple than it appears. It is not an instance of mere superposition. The gabbro unquestionably overlies the basalts, and is therefore of younger date. But it overlies them, not as they rest on each other, in regular conformable sequence of eruption, but intrusively, as a sill does upon the rocks on which it appears to follow in the unbroken order of accumulation. This important structure may be ascertained in almost any of the many sections cut by the torrents which have so deeply trenched with gullies the flanks of the hills. Starting from the ordinary bedded basalts, we observe, in mounting the slopes and approaching the gabbro, that the rocks insensibly assume that indurated shattery character, which has been referred to as characteristic of them round the margins of vents, and which will be shown to be not less so in contact with large eruptive masses of basic or acid rock.[345] Beds of dolerite make their appearance among the basalts, so distinctly crystalline, and so similar in character to the rocks of the sills, that there can be little hesitation in regarding them as intrusive. These sills increase in size and number as we ascend, though hardened amygdaloidal basalts may still be observed. True gabbros then supervene in massive beds, and at last we find ourselves entirely within the gabbro area, where, however, thin bands of highly altered basalt may still for some distance appear. One further fact will generally be noticed, viz. that before reaching the main mass of gabbro, veins and sills of basalt, as well as of various felsitic and porphyritic members of the acid group, come in abundantly, crossing and recrossing each other in the most intricate network. The base of the thick gabbro-sheets is thus another horizon on which, as on that below the plateau-basalts, intrusive masses have been especially developed. Through all these rocks numerous parallel basalt-dykes, running in a general persistent N.N.W. direction, with a later N.E. series, rise from below the sea-level up even to the very crests of the Cuillins ([Fig. 333]).
[345] This indurated, altered character of the bedded basalts near the intrusive bosses and sills will be more particularly described in a later chapter in connection with the granophyre intrusions (see [p. 386]). The metamorphism induced by the basic rocks has generally been less pronounced than that effected by the acid masses.
The sections on the western side of the gabbro area of Skye thus prove that this rock inosculates with the bedded basalts by sending into them, between their bedding planes, sheets which vary in texture from fine dolerites at the outside into coarse gabbros further towards the central mass, and that this intrusion has been accompanied by a certain amount of induration of the older rocks.
Fig. 333.—View of the crest of the Cuillin Hills, showing the weathering of the gabbro along its joints, and of a compound basic dyke which rises through it. (From a photograph by Mr. Abraham, Keswick.)
On the eastern side, the same structure can be even more distinctly seen, for it is not only exposed in gullies and steep declivities, but can be traced outward into the basalt-plateau. In the promontory of Strathaird, Jurassic sandstones and shales, which form the coast-line and lower grounds, are surmounted by the bedded basalts. Denudation has cut the plateau into two parts. The smaller of these makes the outlier that rises into Ben Meabost (1128 feet). The larger stretches continuously from Glen Scaladal and Strathaird House northward into Blath Bheinn. Hence from the ordinary terraced basalts, with their amygdaloids, thin tuffs, red partings, and seams of lignite, every step can be followed into the huge gabbro mountain. Starting from the black Jurassic shales on which the lowest basalt lies, we walk over the successive terraces up into the projecting ridge of An da Bheinn. But as we ascend, sheets of dolerite and gabbro make their appearance between the basalts, which gradually assume the altered aspect already noticed. The dip of the whole series is at a low angle northwards, and the beds can be followed round the head of the Glen nan Leac into the southern slopes of Blath Bheinn. Seen from the eastern side of this valley, the bedded character of that mountain is remarkably distinct, but it becomes less marked towards the upper part of the ridge where the gabbros preponderate. One of the most striking features of the locality is the number and persistence of the dykes, which strike across from the ordinary unaltered basalts of the plateau up into the highest gabbros of the range. Where less durable than the intractable gabbro, they have weathered out on the face of the precipices, thereby causing the vertical rifts and gashes and the deep notches on the crest that form so marked a feature in the scenery. On the other hand, they are often less destructible than the plateau-basalts, and hence in the Glen nan Leac they may be seen projecting as low dams across the stream which throws itself over them in picturesque waterfalls. The youngest dykes in the Blath Bheinn group of hills, have been found by Mr. Harker to have a north-easterly trend, and a north-westerly hade of about 40°, and to give a stratified appearance to the gabbro when viewed from a distance.