Ch. XXXV ALTERATIONS OF STRATA. 593 



right angles to their strike, as will be seen by the accompanying ground 

 plan.* 



The indurated and ribboned schists above mentioned bear a strong re- 

 semblance to certain shales of the coal found at Eussel's Hall, near Dud- 

 ley, where coal-mines have been on fire for ages. Beds of shale of con- 

 siderable thickness, lying over the burning coal, have been baked and 

 hardened so as to acquire a flinty fracture, the layers being alternately 

 green and brick-colored. 



The granite of Cornwall, in like manner, sends forth veins into a coarse 

 argillaceous-schist, provincially termed killas. This killas is converted 

 into hornblende-schist near the contact with the veins. These appear- 

 ances are well seen at the junction of the granite and killas, in St. 

 Michael's Mount, a small island nearly 300 feet high, situated in the bay, 

 at a distance of about three miles from Penzance. 



The granite of Dartmoor, in Devonshire, says Sir H. de la Beche, 

 has intruded itself into the slate and slaty sandstone called greywacke, 

 twisting and contorting the strata, and sending veins into them. Hence 

 some of the slate rocks have become " micaceous ; others more indu- 

 rated, and with the characters of mica-slate and gneiss ; while others 

 again appear converted into a hard-zoned rock strongly impregnated with 

 felspar."f 



"We learn from the investigations of M. Dufrenoy, that in the eastern 

 Pyrenees there are mountain masses of granite posterior in date to the 

 formations called has and chalk of that district, and that these fossiliferous 

 rocks are greatly altered in texture, and often charged with iron-ore, in 

 the neighborhood of the granite. Thus in the environs of St. Martin, near 

 St. Paul de Fenouillet, the chalky limestone becomes more crystalline 

 and saccharoid as it approaches the granite, and loses all trace of the 

 fossils which it previously contained in abundance. At some points, also, 

 it becomes dolomitic, and filled with small veins of carbonate of iron, and 

 spots of red iron-ore. At Rancie the lias nearest the granite is not only 

 filled with iron-ore, but charged with pyrites, tremolite, garnet, and a 

 new mineral somewhat allied to felspar, called, from the place in the 

 Pyrenees where it occurs, " couzeranite." 



Now the alterations above described as superinduced in rocks by vol- 

 canic dikes and granite veins prove incontestably that powers exist in 

 nature capable of transforming fossiliferous into crystalline strata — powers 

 capable of generating in them a new mineral character, similar to, nay, 

 often absolutely identical with that of gneiss, mica-schist, and other strati- 

 fied members of the hypogene series. The precise nature of these altering 

 causes, which may provisionally be termed plutonic, is in a great degree 

 obscure and doubtful ; but their reality is no less clear, and we must 

 suppose the influence of heat to be in some way connected with the trans- 

 mutation, if, for reasons before explained, we concede the igneous origin 

 of granite. 



* Keilhau, Gsea Xorvegica, pp. 61-63. f Geol. Manual, p. 479. 



38 



