Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/412

From Wikisource
Jump to navigation Jump to search
This page has been validated.
Dr. Mac Culloch on the Geology of

although at times putting on the appearance of clay. The quartz is of a brownish red colour. My examination of it was by no means sufficiently extensive to enable me to say what varieties may exist in different places, but it has an aspect and disposition so perfectly analogous to that of the sandstone which lies under the Cuchullin mountain on the opposite shore of Sky, and which forms the greater part of the island of Soa Vretil, that I consider them as belonging to one bed. As I shall have occasion to describe this rock again in the remarks which I propose to offer on Sky, I forbear to dwell on it here. The basalt veins, so common throughout all the islands of this coast, traverse this sandstone, but are not so abundant immediately about Loch Skresort, as is common on these shores. Where the sandstone terminates on ascending towards the mountain, it is succeeded by a mass of rock, which bears the general aspect and features of the mountain trap rocks, of the rocks for example which constitute the Cuchullin mountains, and which are perfectly distinguished in their general characters from those which, on account of their bedded and parallel appearance, are well designated by the term floetz trap. But the composition of this rock is peculiar, and, as far as I know, resembles no other in Scotland. It consists of two ingredients only, and has at first sight the aspect of those syenitic traps so common in the Cuchullin, which are formed of hornblende and felspar, in large crystals: but instead of hornblende it contains augite. These substances, the felspar and augite, are confusedly crystallized together; and although the concretions vary much in size, they are most generally large, and appear to increase in dimensions on approaching nearer to the summit of the mountain. The crystals of augite are not infrequently an inch or more in length. Although I traversed a considerable space over these