130 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



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. 37. 



It is thus demonstrable that when its line of junction with the surrounding plateau- 

 basalta 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 proceeding from that mass, 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 recognise 

 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 



Fig. 37. --Section across the Coire Uaigneich, Skye. a, Silurian limestone and quartzite; b, Jurassic sandstones and shales; 

 cc, bedded basalts and dolerites ; del, gabbros and dolerites with indurated basalts ; e, quartz-porphyry sending veins into 

 surrounding rocks ; //, basalt-dykes running north-west through all the other rocks. 



the system of steady N.W. basalt-dykes that traverse all the other rocks, he will recognise 

 the completion of the evidence of repeated renewals of subterranean energy. 



But he 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. Everything around 

 him suggests 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. He will notice that, on the whole, the rock is largest 

 in grain towards the centre, some features of it around Loch Coruisk reminding him of 

 the most coarsely-crystalline granites. Here and there too, he will observe details of 



