422 



Remarks on the 



[June, 



of nature should bave been so multiplied and complicated, as to 

 afford the endless distinctions which are thus required. Indeed, 

 I cannot help thinking that if the killas of Cornwall had been 

 sufficiently known it would have excluded entirely the introduc- 

 tion of that harsh" sounding German term graywacke. Killas 

 appears to me to be as proper a translation of that word as spe- 

 cular iron ore is of eisen-glanz, and I think may be used with 

 great propriety; distinguishing graywacke and graywacke-slate, 

 by amorphous and sciiistose killas. 



The only other rock of any importance in Cornwall is granite, 

 termed grauen by the common people, — ^a name also given to 

 clay-porphyry, a substance found pretty frequently in large veins. 

 The shades of distinction chronicled by the mineralogist cannot 

 be expected to attract the attention of the miner, who knows 

 but two rocks, grauen and killas, throughout the Stannaries. It 

 has been thought that a distinct rock was understood by the 

 term elvan; but this is a mistake; elvan may sometimes be 

 greenstone, but in general is either killas or granite, and is so 

 termed by the miner when he finds the rock harder to work in 

 one place than in another. 



Before 1 entered Cornwall I was led to believe that it abounded 

 in two kinds of granite, primary and secondary. Never having 

 had an opportunity of comparing them in situ, I was anxious to 

 do so here, and different locahties were pointed out to me; these 

 I examined with care, but could discover no grounds to justify 

 any distinction. Dr. Berger makes no mention of secondary 

 granite ; and another gentleman, whose opinion on this, as on 

 most subjects, will be received with the utmost deference, and 

 who had the same object in view, during a visit made since I 

 was there, informs me that he could discover no distinction at 

 all. It is therefore of importance to ascertain whether the gra- 

 nite of Cornwall be new or old; which will easily be done by 

 comparing the appearances it presents with the descriptions of 

 these rocks as given in the Wernerian school ; it is there taught 

 that three formations of granite have been ascertained. 



The oldest is the basis or nucleus, round w^hich all other rocks 

 have been deposited. The second occurs only in veins, tra- 

 versing only the granite of the older formation. The third rests 

 on some of the older primitive rocks, in an unconformable and 

 overlying position. From this description of its external rela- 

 tions, it is evident that the granite of Cornwall can nehher be 

 the second nor third. With respect to its internal structure, we 

 have the following definition ; granite is a granular aggregated 

 rock, composed of felspar, quartz, and mica. These alternate 

 from large to small, and even to very fine granular. The large 

 and coarse granular usually belong to the oldest; the small and 

 line granular to the newest granite formationso Besides felspar,, 



