428 



Remarks on ike 



stancy, some veins of granite being as coarse-grained as the 

 mass to which they belong. 



In a former part of this paper I had occasion to notice an 

 alteration which appears to take place in the texture of killas 

 when in the vicinity of granite. This circumstance was so re- 

 markable in Galloway, at the Louran and other places, that I 

 took the strata so situated for mica-slate, although I had observed 

 no line of separation between it and the killas ; I was forcibly 

 struck with this at the moment, but having then no time to 

 follow it up I was obliged to leave the country without any par- 

 ticular examination. It will be observed, by the specimens from 

 St. Michael's Mount, that the killas there assumes the appear- 

 ance of fine-grained gneiss. At Wasseldale Crag, between 

 Kendal and Shap, 1 noticed a rock^, in the immediate vicinity of 

 granite, quite similar; and I am told that the texture of the 

 strata near the granite of the mountains of Morne is altogether 

 the same. 



This alteration is always of a gradual nature ; and is so im- 

 perceptible that it afibrds a good example of what might be 

 understood by the German term passage^ or transition from one 

 species to another; this passage, even admitting the substance 

 altered, is of too limited a nature to constitute a distinct and 

 totally different rock. This alteration, if traced with attention, 

 may lead to some very important results; but, without entering 

 upon it at present, 1 ^hall content myself with recommending it 

 to the notice of geologist?, some of whom may consider it of 

 too minute a nature to deserve attention. They may, however, 

 rest assured that it is only by an accurate examination, and a 

 faithful detail of such objects, thcit we can hope to arrive ulti- 

 mately at truth, the only solid basis of philosophic inquiry. 



1 may he accused of generalising too much in the foregoing 

 statement, on grounds so limited ; it must be remembered, 

 however, that 1 have purposely confined myself to the examples 

 of the relations which exist within my own knowledge, between 

 the transition rocks and granite. The same phenomena are 

 familiar where gp.eiss and mica-slate come in contact with that 

 rock ; but as these strata are considered to be of a very different 

 age, the facts wliich 1 might have cited, had my object been to 

 prove the age of granite with respect to all other rocks, were 

 unnecessary when my purpose was to point out the relative ages 

 of killas and granite. 



From what I have said I consider myself warranted in finish- 

 ing this paper with the following conclusions : 



The killas of Cornwall belongs to the transition series of 

 Werner. 



The granite of Cornwall is possessed of every character by 

 which the oldest varieties are distinguished. 



