The deep dark hollow of the Coire Uaigneich has been cut out of the very core of Blath Bheinn, and lays bare the structure of the east part of the mountain in the most impressive as well as instructive way ([Fig. 334]). By ascending into this recess from Loch Slapin, we pass over the whole series of rocks, and can examine them in an almost continuous section in the bed of the stream and on the bare rocky slopes on either side. Sandstones and shales of the Jurassic series extend up the Allt na Dunaiche for nearly a mile, much veined with basalt and quartz-porphyry, by which the sandstones are locally indurated into quartzite. At last these strata are overlapped by the basalts of the Strathaird plateau, which with a marked inclination to N.N.W., here dip towards the mountains. But by the time these rocks have reached the valley, they have already lost their usual brown colour and crumbling surfaces, and have assumed the indurated splintery character, though still showing their amygdaloidal structure. They are much traversed by felsitic veins and strings which proceed from a broad band of fine-grained hornblende-granite that runs up the bottom of the Coire Uaigneich and, ascending the col, crosses it south-westwards into the Glen nan Leac. On the left or south-eastern side of this intrusive mass, a portion of Lias shales and limestone (here and there altered into white marble) is traceable for several hundred yards up the stream.[346]
[346] This limestone was formerly identified by me with the Cambrian strata of the district. It was noticed by Von Oyenhausen and Von Dechen, who, as Mr. Harker has recently ascertained, correctly believed it to be a portion of the Lias torn off and carried upward by the eruptive rocks (Karsten's Archiv, i. p. 79).
The bedded basalts of Strathaird, after dipping down towards the N.N.W., bend up where they are interbanded with dolerites and gabbros, and form the prominence called An Stac, which rises as the eastern boundary of the Coire Uaigneich. Their steep dip away from the mountain is well seen from the east side, and their outward inclination is continued into the ridge to the southward. Similar rocks appear on the other flank of the band of granite, and form the base of Blath Bheinn. They are likewise continued in the mountains further north called Sgurr nan Each and Belig, where they dip in a northerly direction away from Blath Bheinn, which seems to be the centre of uprise, with the gabbro-sheets dipping away from it. The bedded basalts have been traced by Mr. Harker up to a height of well over 2000 feet on the Blath Bheinn range. They are of the usual altered, indurated, and splintery character. The intrusive sheets interposed between them become thicker and more abundant higher up, until they constitute the main mass of the mountain. But that they are in separate sheets, and not in one amorphous mass, can be recognized by the parallel lines that mark their boundaries. The junction of the gabbro sills and the lavas is a very irregular one, portions of the latter rocks being enveloped in the intrusive sills.
The granite which sends out veins into the surrounding rocks is obviously the youngest protrusion of the locality, except of course the basalt-dykes which cross it, and which are nowhere seen in a more imposing display than round the flanks of Blath Bheinn. A section across the corry shows the structure represented in [Fig. 334].
It is thus demonstrable that when its line of junction with the surrounding plateau-basalts is traced in some detail, the gabbro is found to overlie them as a whole, but also to be intercalated with them in innumerable beds, bands, or veins which rapidly die out as they recede outwards from the main central mass; that these interposed beds are intrusive sheets or sills from that mass which have cut off and enveloped portions of the basalts, and that the contiguous bedded basalts show more or less marked metamorphism.
We have now to consider the structure of the interior of the gabbro area of the Cuillin Hills. The first impression of the geologist who visits that wild district is that the main mass of rock is as thoroughly amorphous as a core of granite. Yet a little further examination will reveal to him many varieties of texture, sometimes graduating into, sometimes sharply marked off from, each other, and suggesting that the rock is not the product of one single protrusion. He will notice further indications of successive discharges or extravasations of crystalline material during probably a protracted period of time, and in the intricate network of veins crossing each other and the general body of the rock in every direction, as well as in the system of basalt-dykes that traverse all the other rocks, he will recognize the completion of the evidence of repeated renewals of subterranean energy.
Fig. 334.—Section across the Coire Uaigneich, Skye.
a, b, Jurassic sandstones and shales; c c, bedded basalts and dolerites; d d, gabbros and dolerites with indurated basalts; e, fine-grained hornblende-granite sending veins into surrounding rocks; f f, basalt-dykes running through all the other rocks.
But the observer will be struck with the absence of the more usual proofs of volcanic activity in such forms as vesicular lavas and abundant masses of slag, bombs and tuffs, which are commonly associated with the idea of the centre of a volcanic orifice, though he will meet with isolated masses of coarse volcanic agglomerate within the gabbro area and along some parts of its junction with the granophyre. The general characters of the rocks around him suggest that he stands, as it were, far beneath that upper part of the earth's crust which is familiar to us in the phenomena of modern volcanoes; that he has been admitted into the heart of one of the deeper layers, where he can study the operations that go on at the very roots of an active vent.
When the geologist begins a more leisurely and systematic examination of the interior of the gabbro area of Skye he soon sees reason to modify the impression he may at first have received that this rugged region presents the characters of one single eruptive mass. The more he climbs among the hills the more will he meet with evidence of long-continued and oft-repeated extravasation, one portion having solidified before another broke through it, and both having been subsequently disrupted by still later protrusions.