78 Dr. Mac Culloch 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 speci- 

 mens 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 thwe. 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. 



