126 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



that the amorphous bosses, both basic and acid, are younger than the surrounding bedded 

 basalts, and that the acid protrusions are on the whole younger than the basic. I shall 

 now proceed to show how these conclusions are established by the evidence of each of 

 the areas where the several kinds of rock occur. 



(a) Skye. — By far the largest, most picturesque, and to the geologist most im- 

 portant area of gabbro in Britain, is that of Skye. Though like every other portion of 

 the volcanic region, it has suffered enormous denudation, and has thereby been trenched 

 to the very core, it reveals, far more conspicuously and clearly than can be seen anywhere 

 else, the relation of the gabbro to the bedded basalts on the one hand, and to the acid 

 protrusions on the other. Its chief portion is that which rises into the group of the 

 Cuillin Hills, which for blackness of hue, ruggedness of surface, jaggedness of crest, and 

 general grimness of aspect, have certainly no rivals within the limits of the British Isles. 

 It has long been known to extend eastwards into Blath Bheinn (Blaven) and its immediate 

 northern neighbours. There is, indeed, no break whatever between the rock of the 

 Cuillins and that of the hills on the east side of Strath na Creitheach. In Strath More 

 the gabbro is interrupted by the granitoid mass of the Red Hills. Patches of it, however, 

 occur further to the east, even as far as the Sound of Scalpa. If we throw out of 

 account the invading granitoid rocks, and look upon the whole tract within which the 

 gabbro occurs as originally one connected area, we find that it covered an elliptical space 

 measuring about nine miles from S.W. to N.E. and six miles from N.W. to S.E., and 

 embracing at least forty square miles.* But that its original size was greater is strikingly 

 shown more particularly on the western margin, which like that of the basalt-escarpments, 

 has obviously been determined by denudation, for its separate beds present their trun- 

 cated ends to the horizon all along the flanks of the Cuillins, from the head of Glen 

 Brittle round to Loch Scavaig (fig. 36). 



The first point to be ascertained in regard to the gabbro and its 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 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 



* Though this and the other bosses are here spoken of as consisting of gabbro, it will be understood that this rock 

 only constitutes the larger portion of their mass, which includes also dolerites, basalts, and other basic compounds. 



