1813.] Mineralogical Observations on Cornwall. 253 



bed. The whole northern base of the mount consists of clay 

 slate. I could not determine either the direction or amount of 

 its dip ; but it seemed to lie much more horizontally than the 

 clay slate at Merazion. It does not extend to any height up the 

 mount. The upper part of the mount consists of granite. On 

 the south side this granite continues down to the water's edge, 

 and it continues to constitute the whole of the hill both on the 

 east and west side for about fths of its whole extent. Then we 

 come to the clay slate. On the east side, where the granite 

 terminates, numerous veins of it run into the clay slate in 

 different directions. I traced some of them for a length of 125 

 feet, and then lost them only because they ran under heaps of 

 loose stones, which I was unable to remove. These veins vary 

 in width from an inch or two to about a foot and a half; but 

 their common width is above a foot. Sometimes they contain 

 fragments of clay slate; sometimes they consist of felspar; but 

 most commonly of perfect granite. I consider the descriptions 

 which Mr. Playfair andMr. Allan have givenof them as correct.* 

 No person who will be at the trouble to examine them can 

 doubt that they are real veins, that they run from the granite 

 into the clay slate, and therefore that the granite must have been 

 deposited after the clay slate : but if there be any person not 

 satisfied with the appearance of these veins, he has only to go 

 to the west side of the mount, where he will find two beds of 

 granite in the clay slate, and the position of these beds such as 

 to indicate in the clearest manner that the great body of the 

 granite had been deposited after the clay slate. 



These facts put it beyond dispute that the granite of St. 

 Michael's Mount is not primitive but transition granite. This 

 is the conclusion that Mr. Allan ought to have drawn from his 

 premises, and the conclusion that he would have drawn if he had 



f riven his reasoning powers fair play. He proved that the granite 

 ay over the clay slate, and that it must have been deposited 

 alter the clay slate. He proved, likewise, that the clay slate is 

 a transition rock. From all this it follows irresistibly that the 

 granite likewise is a transition rock. 



This is not the first time that granite has been observed as a 

 transition rock. Von Buch observed it in a similar position in 

 the neighbourhood of Christiana in Norway. In all probability, 

 when transition rocks, which are so common in this country, 

 come to be examined more carefully than they have hitherto 

 been, granite will be frequently found among them. Why 

 should it not, as well as clay slate, greenstone, and porphyry? 



(7'o b* continued.) 



• Tfie clay ilale in ihj mit;libotirliood of the veins contains so much tniia as 



iu i.jvc 'tie asurct of mica liatc. 



