152 MINERALOGY OF ST ANDREW's. 



cannot be easily investigated on account of the 

 thick continuous bed of soil with which it is co- 

 vered. 



V. Trap^Tuff. 



The principal rock of this formation is trap^ 

 tuff, similar in composition to the rock of the 

 same kind which occurs in Arthur's Seat. It 

 contains globular pieces of quartz and granite, 

 which have been water- worn ; — likewise frag- 

 ments of grey-wacke, floetz- sandstone, and lime- 

 stone containing petrifactions of entrochi. Small 

 pieces of augite and felspar are likewise contained 

 in it. The inclosed fragments of the older rocks 

 are sometimes of considerable size, measuring 

 two or three entire feet. 



It generally happens, that the fragments of the 

 oldest rocks, when observed imbedded in the new- 

 er formations, are much rounded and water-worn : 

 Whereas the fragments of the newer rocks, are 

 rather angular, and seem to have suffered less from 

 attrition : — a circumstance which points out with 

 considerable precision the relative antiquity of 

 rocks, and is entitled to a more careful examina- 

 tion than has hitherto been bestowed upon it. 



The most curious circumstance which attracted 

 my notice, in examining this bed of tuff, was the 

 rock, which is well known in the neighbourhood 



