Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/88

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Dr. MacCulloch on the Granite Tors of Cornwall.

forms of the Cheshire rock salt, and the igneous explanation of the forms of basaltic columns, add to this supposition.

On a smaller scale, a phenomenon of rare occurrence in Nature may also be suggested in aid of it. I mean the spherically disposed granite of Corsica, which exhibits the various constituents of granite formed round numerous centres, and producing those beautiful specimens still so rare in the cabinets of collectors. Similar radiating tendencies in the smaller parts have been noticed by Saussure, and although I had not the good fortune to see them in Arran, my friend Professor Jameson has described them as existing there. I have also witnessed a similar disposition in the mica which is included in the granite veins near Portsoy, and the same structure is well known to exist in that variety of granite which is called Tyger granite, where the hornblende or shorl forms radiating spheres.

It is sufficiently apparent from the history of this granite, and from its progress in decomposition now described, that the migration of stony masses may to a certain extent be explained, at least as far as this variety of granite is concerned, even without having recourse to any very violent mechanical action. But the decision and complete explanation of this very common and puzzling phenomenon, must in most cases rest upon a question of a different nature, and of greater difficulty, namely, the alterations which the surface of the earth has undergone at different eras, as well as the comparative antiquity of those changes. This phenomenon is only one of many, which prove the former existence of a different distribution of those parts of the globe which are at this present time land and sea, hill and valley.