^08 Dr. Mac CuLLOCH o« the Geology of Glen Tilt. 



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 cre- 

 ated 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 interla- 

 minated with the same substance, very beautifully banded speci- 

 mens of this variety occur near Gow's bridge. On some occa- 

 sions 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 occur- 

 rence 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 pre- 

 dominance 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 v/here it as- 

 sumes 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 distin- 

 guished 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 



