MINERALOGY OF THE PENTLAND HILLS. iQQ 



frequently covered with claystone. The finer gra- 

 nular varieties, gradually pass into claystone ; and 

 probably it is contained in great cotemporaneous 

 portions in the claystone, or alternates in beds 

 with it. 



It occurs in Turnhouse Hill, Caernethy, East 

 Side Black Hill, Kirk Hill, Castlelaw, and the 

 Kirkyetten Hills, and in general it is situated on ^ 

 higher level than the clinkstone. 



VIIL Porphyry. 



The basis of this rock varies in colour from grey, 

 through blue to red. The fracture is earthy and 

 dull. It is opaque, soft, sectile, rather easily 

 frangible, and rather heavy \ so that it appears 

 to be nearly of the same nature as claystone. It 

 contains imbedded crystals of felspar, which are 

 generally small, almost always in a soft or disinte- 

 grated state, and in the bluish varieties nearly in 

 the state of steatite. It passes on the one hand 

 into claystone and compact felspar, and on the 

 other it inclines to basalt and clinkstone. 



It rests on claystone, and is frequently inter- 

 mixed with it. It forms the higher parts of 

 Turphouse Hill, Caernethy, East Side Black Hill, 

 Castlelaw, and occurs also in Kirk Hill, Black 

 flfcil}, and amongst the Kirkyetten Hills. 



