PART 1. CHAPTER XL 



145 



Alterations of Strata in contact with Granite. 



strong resemblance to certain shales of the coal found at Rus- 

 sell's Hall, near Dudley, where coal mines have been on fire for 

 ages. Beds of shale of considerable thickness, lying over the 

 burning, have been baked and hardened so as to acquire a flinty 

 fracture, the layers being alternately green and brick-coloured. 



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 appearances 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 Mr. De la 

 Beche, has intruded itself into the slate and slaty sandstone 

 called greywacke, twisting and contorting the strata, and send- 

 ing veins into them. Hence some of the slate rocks have be- 

 come *' micaceous, others more indurated, and with the charac- 

 ters of mica-slate and gneiss, while others again appear convert- 

 ed into a hard-zoned rocjk strongly impregnated with felspar."* 



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

 eastern Pyrenees there are mountain masses of granite poste- 

 rior in date to the formation called lias and chalk of that dis- 

 trict, and that these fossiliferous rocks are greatly altered in 

 texture, and often charged with iron-ore, in the neighbourhood 

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

 Paul de Fenouillet, the chalky limestone becomes more crystal- 

 line and saccharoid as it approaches the granite, and loses all 

 traces 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 Ran- 

 cie 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 Py- 

 renees where it occurs, " couzeranite." 



Now the alterations above described as superinduced m rocks 

 by volcanic 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, nay, often absolutely identical with 

 that of gneiss, mica-schist, and other stratified 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 wo 



N 



* Geol. Manual, p. 479. 



