Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/194

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the identity of their composition throughout. This is the rock on which the elements seem to make no impression, and on which no soil accumulates, causes which equally determine the permanent and rugged nature of their spiry outline. I have no hesitation in saying that the whole of the group as far as it is visible from Coruisk, as well as the opposite side of Garsven far on towards Loch Brittle seaward, and the smaller mountains which separate this valley from Blaven, consist of the same rock, but to what extent it may reach northwards cannot be determined until the whole shall have been traversed, unless the rugged outline and external general characters are admitted as a proof of identity of composition. In this case the whole of the Cuchullin is a mass of hypersthene rock, with the exception only of the veins which it contains, which consist of basalt, compact felspar, augit rock, syenite, felspar-porphyry, and lead-blue claystone. I have chosen the term hypersthene rock to designate this new and important member of the trap family, since like that of augit rock it is explicit, and introduces no confusion into the existing nomenclature.

There appear but two prevalent varieties of composition. In the first the mixture consists of hypersthene with greenish compact felspar, and in the second with crystallized white felspar possessing generally a slight glassy lustre. This latter variety seems to predominate, and is much more easily recognized than the former, which from its greenish hue and the minuteness and intimacy of the admixture, is often difficult to be distinguished from common greenstone. The principal variations of appearance are produced by the greater or less proportion of the hypersthene, by the varieties of its colour, and by the unequal magnitude of the crystals. While on the subject of Coruisk I ought to add, that the sonorous rock formerly described is either a compact felspar or a