Ch. xxxy.] alteratioxs of strata. 739 



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

 ern Pyrenees there are mountain masses of granite posterior in date 

 to the formations called lias and chalk of that district, and that these 

 fossiliferons rocks are greatly altered in texture, and often charged 

 with iron-ore, in the neighborhood of the granite. Thus in the envi- 

 rons 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." 



Xow the alterations above described, as superinduced in 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 charac- 

 ter, similar to, hay, 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 we must suppose the influence of heat to 

 be in some way connected with the transmutation, if, for reasons 

 before explained, we concede the igneous origin of granite. 



The experiments of Gregory Watt, in fusing rocks in the laboratory, 

 and allowing them to consolidate by slow cooling, prove distinctly that 

 a rock need not be perfectly melted in order that a rearrangement of 

 its component particles should take place, and a partial crystallization 

 ensue.* We may easily suppose, therefore, that all traces of shells and 

 other organic remains may be destroyed ; and that new chemical com- 

 binations may arise, without the mass being so fused as that the lines 

 of stratification should be wholly obliterated. 



We must not, however, imagine that heat alone, such as may be 

 applied to a stone in the open air, can constitute all that is comprised 

 in plutonic action. We know that volcanoes in eruption not only emit 

 fluid lava, but give off steam and other heated gases, which rush out 

 in enormous volume, for days, weeks, or years continuously, and are 

 even disengaged from lava during its consolidation. 



We also know that long after volcanoes have spent their force, hot 

 springs continue for ages to flow out at various points in the same area. 

 In regions also subject to violent earthquakes such springs are fre- 

 quently observed issuing from rents, usually along lines of fault or 

 displacement of the rocks. These thermal waters are most commonly 

 charged with a variety of mineral ingredients, and they retain a 



* Phil. Trans., 1804. 



