Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/316

 the rock which I have hitherto spoken of by the general name of schist. To have described the several varieties on all occasions where the geological history only was required, would have created much confusion, as they rarely maintain the same character for any considerable space. The most abundant is a clay slate, rarely fissile, and generally of a very compact texture and dark blackish blue colour. It sometimes possesses a glossy and unctuous surface, and passes into a sort of talcaceous schist. It is often also much penetrated with quartz, and on some occasions finely interlaminated with the same substance, very beautifully banded specimens of this variety occur near Gow's bridge. On some occasions it becomes intermixed with hornblende, which by degrees predominating, the compound passes into a hornblende schist similar to that which is so common in gneiss. This occurrence is frequent at the junctions, and here also the schist, even where it is merely argillaceous, generally displays unusual hardness. Hornblende schist is also found in beds unconnected with the clay slate.

Micaceous schist of aspects infinitely varied is also found in this ridge of hills. In general however it is characterized by the predominance of the quartzy ingredient, and in this way it passes into quartz rock by a series of gradations which cannot be defined. I have already had occasion to speak of those places where it assumes the character of gneiss, and need not therefore repeat the description here. I have only to add that green fibrous actinolite is occasionally to be found in this micaceous schist, a substance which, however radically identical with hornblende, is sufficiently distinguished in its appearance by characters which need not be detailed here. In some of the rolled porphyries which are found in the hills which bound this Glen I have observed pinite. This mineral