402 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 per- 

 fectly analogous to that of the sandstone which lies under the Cu- 

 chullin 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 for- 

 bear to dwell on it here. The basalt veins, so common through- 

 out 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 to- 

 wards 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 moun- 

 tains, and which are perfectly distinguished in their general cha- 

 racters from those which, on account of their bedded and parallel 

 appearance, are well designated by the term foetz trap. 



But the composition of this rock is peculiar, and, as far as I 

 know, resembles no other in Scotland. It consists of two ingre- 

 dients 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 di- 

 mensions on approaching nearer to the summit of the mountain. 

 The crystals of augite are not unfrequently an inch or more in 

 length. Although I traversed a considerable space over these 



