Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/289

 the drawing as it is to me, and it is probably not very obvious to any one.

It is not far from this place, and on the left bank of the river, that a greenstone vein may be seen traversing the strata at nearly right angles. It is the only specimen of genuine greenstone which I observed in Glen Tilt, and it appeared to me to resemble those veins which are found in Cruachan and in the neighbouring mountains, where they also frequently assume a porphyritic aspect. Still however they differ considerably in their general appearance from those greenstone veins which are most usual in Scotland, and which are found to predominate in the vicinity of the larger formations of trap in the Western islands, and in many well known parts of the continent of Scotland.

The great mass of limestone which we shall hereafter find forming the whole of the left boundary of Glen Tilt, is of a dark blue colour, with one or two exceptions which I have already described in the progress down the river. But the beds at this place are of various colours, and offer some of the most beautiful ornamental marbles which Scotland has yet produced.

The basis of nearly the whole is a white, rather larger grained, and crystalline marble. Beds of this variety occur in a pure state, and of considerable dimensions. But as all these marbles contain more or less of mica, with which substance they are interstratified, the white colour is seldom pure, being mottled with the slight grey tint which mica in similar cases always produces. It cannot therefore be considered as a statuary marble, since modern artists, acquainted