

Ch. XVII.] 



PETROLEUM SPRINGS. 



413 







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Disintegrating effects of carboni 

 of granite is a striking feature of large districts in Auvergne, 

 especially in the neighbourhood of Clermont. This decay 

 was called by Dolomieu, ' la maladie du granite ; ' and the rock 

 may with propriety be said to have the rot, for it crumbles to 



pieces in the hand. The phenomenon may, without doubt, be 

 ascribed to the continual disengagement of carbonic acid gas 



-G 



from numerous assures. 



In the plains of the Po, between Verona and Parma, 

 especially at Villa Franca, south of Mantua, I observed great 

 beds of alluvium, consisting chiefly of pebbles of crystalline 

 rock, percolated by spring-water, charged with carbonate of 

 lime and carbonic acid in great abundance. They are for 

 the most part incrusted with calc-sinter ; and the rounded 



blocks of gneiss, which have all the outward appearance 





of solidity, have been so disintegrated by the carbonic acid as 



readily to fall to pieces. 



The subtraction of many of the elements of rocks by the 

 solvent power of carbonic acid, ascending both in a gaseous 

 state and mixed with spring-water in the crevices of rocks, 

 must be one of the most powerful sources of those internal 

 changes and re-arrangements of particles so often observed 

 in strata of every age. The calcareous matter, for example, 

 of shells, is often entirely removed and replaced by carbonate 

 of iron, pyrites, silex, or some other ingredient, such as 

 mineral waters usually contain in solution. It rarely happens, 

 except in limestone rocks, that the carbonic acid can dissolve 

 all the constituent parts of the mass ; and for this reason, 

 probably, calcareous rocks are almost the only ones in w 7 hich 

 great caverns and long winding passages are found. 



Petroleum springs. — Springs of which the w aters contain a 

 mixture of petroleum, or rock oil, which is a compound of 

 hydrogen and carbon, and the various minerals allied to it, 

 such as naphtha and asphalt or mineral pitch, are very nu- 

 merous. All these substances, says Mr. T. Sterry Hunt, 

 forms of bitumen, some, like petroleum, being fluid, others, 

 like asphalt, being solid at ordinary temperatures. They are 

 supposed to be all of organic origin, derived partly from 

 terrestrial, partly from marine plants, and sometimes from 



are 



