ItiNERALOGY OF THE PENTLAND HILLS, 183 



Section IIL 

 transition rocks. 



The transition rocks which I have had an op^ 

 portunity of observing in this group, of hills, are 

 Clay- Slate, Grey-Wacke, Greenstorte, and I be-- 

 iieve also Porphyry. 



1. Clay-Slate, 



The colour of this rock is greenish-gtey, or 

 greyish-black ; scales of mica are in general in- 

 termixed with it, and these give it a glimmering 

 lustre. Its fracture is sometimes fine, sometimes 

 coarse slaty, and in some instances the slaty is as- 

 sociated with a splintery fracture. It is opaque. 

 The streak is grey-coloured. It is soft, inclining 

 to semi-hard ; it is easily frangible. 



It is distinctly stratified, and the strata run from 

 south-west to north-east, and are nearly verti- 

 cal. 



I did not observe in it either imbedded portions 

 of quartx, or cotemporaneous veins of that mi- 

 neral ; and no felspar, or other imbedded mi- 

 nerals, appear in it. Several of the varieties 

 are nearly allied to Slate -clay, and even appear to 

 pass into it ; others again, particularly those ha-^ 



