Dr. Mac Culloch on the Granite Tors of CornwalL 77 



of these spheroids approached each other, the successive crusts would 

 interfere, and the remaining intervals would be filled by portions of 

 spheroidal crusts, until the cuboidal figures of all the contiguous 

 masses were completed, thus forming that aggregated mass of cuboids, 

 which we witness in the granites of this aspect which remain unin- 

 jured in their places. We need not be surprised that this regularity 

 is not more constant, nor the forms more perfect, as we are unac- 

 quainted with the numerous circumstances which may determine the 

 several centres of crystallization, or which may interfere with the 

 ultimate regularity of the resulting masses. It is certain from che- 

 mical experiments, that the fact which is the basis of the foregoing 

 supposition, occurs in various instances of the cooling of slags and of 

 rocks artificially fused, as Mr. Watt's experiments have so well 

 shown. But in these experiments, certain as they are, we are 

 unacquainted with the causes which determine the places of the 

 several centres of crystallization, and though equally unacquainted 

 with those which may have influenced the centres on which the 

 granite masses were formed, we may yet from analogy understand 

 how the irregularity of these masses may have been caused by a 

 corresponding irregularity in the position of their centres. 



We can also easily conceive that in certain cases, the peculiar 

 circumstances of which lie equally hid from us, the approximation of 

 the spheres of crystallization may have caused the crystalline polarity 

 of the several masses to interfere with each other, so as to have pro- 

 duced in many cases an irregularity still greater than this, and in 

 some instances even entirely to have obliterated the appearance of 

 central tendencies. To the chemical facts above adduced in support 

 of this explanation, I might subjoin, what every one's mind will 

 immediately suggest, the illustration which the commenced spheroidal 



