TRANSITION ROCKS. 117 



Having thus imperfectly chalked out the boundaries, or ra^. 

 ther localities of the transition districts in these islands, I shall 

 endeavour to shew that some of the rocks of Cornwall are 

 grauwacke, in all respects similar to some of the south of Scot- 

 land ; and if strata may be compared to the leaves of a book, a 

 few decided and indisputable specimens are sufficient to cha- 

 racterise a district. 



It was in consequence of some observations during a tour 

 through Cornwall and Devon last summer, that I was led to 

 suspect this class stood in a very different relation in point of 

 period, with respect to granite, from that which I had hitherto 

 conceived ; greater experience, or perhaps sufficient attention 

 to the writings of Dr Hutton, might have pointed out this before. 

 Had I looked more attentively into his description of the granite 

 district of Galloway, and at the same time attended to the na- 

 ture of the stratified rock of which that country is principally 

 composed, this fact would not have been new to me now. 

 There were other circumstances, however, which severally 

 contributed to prevent me from supposing that grauwacke,- 

 could occur in this position. 



First, The unlimited use to which Dr Hutton applied the 

 term Aljnne Schistus, left us quite uncertain with respect to 

 the species of rock he meant : secondly, the alteration induced 

 on grauwacke,near its junction with granite, — a circumstance so 

 strikingly exemplified in Galloway, that I own it deceived my- 

 self ; and, lastly, the assertion I have so often heard repeated 

 by the Wernerian geognosts, that granite veins never occurred 

 excepting in rocks formed of the same constituents, alluding to 

 gneiss and mica-slate. 



Before I visited Cornwall, I knew that granite abounded in 

 the Stannaries, and that tin and wolfram, metals which are con- 

 sidered nearly of the highest antiquity, were there common. 



productions^ 



