1813.] 



Trafisition Rocks of Werner. 



419 



of the country to the west composed of transition rocks; while, 

 so far as I know, none o'ccurs to the east of it ; akhough it is 

 probable that at Mount Sorrel, in Leicestershire, some of the 

 same series may be found. 



We are still less acquainted with the precise limits of its ex- 

 tent in Ireland: v/e know, however, that it occupies the coast 

 from Belfast Lough to the mountains of Morne, which are of 

 granite ; it also extends westward as far as Monaghan, and pro- 

 bably much beyond that point. From what Mr. Weld states, in 

 his account of Killarney, it appears to be the principal rock of 

 the Kerry mountains, and I know it occurs in great abundance 

 in the county of Cork. Hence, even with the little information 

 we possess respecting its exact limits, we have enough to know 

 that the transition rocks form a very large proportion of the 

 superficial extent of Great Britain and Ireland, and also com- 

 prehend the principal mining districts. 



Having thus imperfectly chalked out the boundaries, or rather 

 localities of the transition districts in the islands, i shall endea- 

 vour to show that some of the rocks of Cornwall are graywacke, 

 in all respects similar to some of the south of Scotland; and if 

 strata maybe compared to the leaves of a book, a few decided 

 and indisputable specimens are sufficient to characterise a dis- 

 trict. 



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 nature 

 of ilie stratified rock of which that country is principally com- 

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

 were other circumstances, however, vaiich severally contributed 

 to prevent me from supposing that graywacke could occur in this 

 position. 



First, the unlimited use to which Dr. Hutton applied the 

 term alpine schisius, left us quite uncertain with respect to the 

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

 graywacke, 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 

 gneisfe and mica-slate. 



Before I visited Cornwall, I knew that granite abounded io 

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



2 D 2 



