Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/87

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77
Dr. MacCulloch on the Granite Tors of Cornwall.

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 uninjured in their places. We need not be surprised that this regularity is not more constant, nor the forms more perfect, as we are unacquainted 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 chemical 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 produced 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