1813.] On Transition Rocks. 10? 



mineralogists will conclude from this statement, that it really is 

 fine-grained gneiss ; and that Mr. Allan's conclusion, that the 

 transition rocks are here connected with granite, is entirely 

 unfounded and fallacious. The granite is probably the second 

 conformable formation of that rock as discovered by Professor 

 Jameson ; and I have little doubt that, on farther examination, 

 it will be found that this famous discovery of the metamorphosis 

 of grey-wacke into gneiss has as little foundation in nature as 

 the metamorphosis of Battus into a touch-stone, or any other of 

 the fine and fanciful changes of bodies into new forms, or if we 

 translate literally, forms into new bodies, to be met with in the 

 writings of Ovid. 



Mr. A. infers this change, or passage of grey-wacke into 

 gneiss, from appearances at St. Michael's Mount, and at the 

 Lauran in Galloway. He tells us that at the latter place he took 

 the killas in the vicinity of the granite for mica-slate, " although 

 he had observed no line of separation between it and the killas." 

 Docs he mean that the " alteration " produced on grey-wacke, 

 or killas, as he prefers calling it, was actually observed by him 

 to be of " a gradual nature," and " imperceptible " in the 

 neighbourhood of the Lauran, so as to make it small-grained 

 gneiss? or does he only intend to say, that he had no opportunity 

 of seeing the junction of these two rocks? If the former be 

 Mr. Allan's meaning, I wish very much he had mentioned any 

 particular spot where such gradual alteration is to be seen ; for 

 though 1 have been at some pains to discover the junction of 

 what I conceive to be fine-grained gneiss with the grey-wacke of 

 that district, I have not been so fortunate as to meet with any 

 place where the rock is sufficiently exposed for this purpose. 

 But if Mr. A. means merely to affirm that this was the case with 

 hi in too, and that he had no opportunity of observing the junc- 

 tion of the two rocks of which we are speaking, then certainly 

 his conclusion is such as no mineralogist can admit, and of a 

 very different nature from what might have been expected. That 

 he did not see the distinction, can never be a good reason why 

 lie should conclude that there was none. 



But admitting, Sir, that Mr. A. did observe a seemingly 

 gradual transition of the grey-wacke into gneiss, is this a proof 

 that these two rocks are one and the same rock? As well might 

 he argue that the colours green and blue in the prismatic spec- 

 trum arc the same ; because the alteration of the green to the 

 blue is of a gradual nature, and imperceptible ; or as well might 

 I argue that gneiss and mica-slate are the same, because they 

 are often seen at their junction apparently passing into one 

 mother. 



Much BtfCM is laid on the miners of Cornwall making no 

 distinction between grey-wacke and the fine-grained gneiss; but 

 including both under the general name of killas. But, to use 



