ROCKS. 265 



phyre and the transformation of compact lirJestone into a crys- 

 talline mass difiering in its chemical character, we are, to a 

 certain degree, justified in believing, where the second phe- 

 nomenon is manifested unattended by the appearance of the 

 first, that this apparent contradiction is owing to the absence, 

 in certain cases, of some of the conditions attendant upon the 

 exciting causes. Who would call in question the volcanic na- 

 ture and igneous fluidity of basalt merely because there are 

 some rare instances in which basaltic veins, traversing beds 

 of coal or strata of sandstone and chalk, have not materially 

 deprived the coal of its carbon, nor broken and slacked the 

 sandstone, nor converted the chalk into granular marble ] 

 Wherever we have obtained even a faint light to guide us in 

 the obscure domain of mineral formation, we ought not un- 

 gratefully to disregard it, because there may be much that is 

 still unexplained in the history of the relations of the transi- 

 tions, or in the isolated interposition of beds of unaltered strata. 



After having spoken of the alteration of compact carbonate 

 of lime into granular limestone and dolomite, it still remains 

 for us to mention a third mode of transformation of the same 

 mineral, which is ascribed to the emission, in the ancient pe- 

 riods of the world, of the vapors of sulphuric acid. This trans- 

 formation of limestone into gypsum is analogous to the pene- 

 tration of rock salt and sulphur, the latter being deposited 

 from sulphureted aqueous vapor. In the lofty Cordilleras of 

 Quindiu, far from all volcanoes, I have observed deposits of 

 sulphur in fissures in gneiss, while in Sicily (at Cattolica, near 

 Girgenti), sulphur, gypsum, and rock salt belong to the most 

 recent secondary strata, the chalk formations.^ I have also 

 seen, on the edge of the crater of Vesuvius, fissures filled with 

 rock salt, which occurred in such considerable masses as occa- 

 sionally to lead to its being disposed of by contraband trade. 

 On both declivities of the Pyrenees, the connection of dior:t> 

 and pyroxene, and dolomite, gypsum, and rock salt, can not be 

 questioned ;t and here, as in the other phenomena which we 

 have been considering, every thing bears evidence of the ac- 

 tion of subterranean forces on the sedimentary strata of the 

 ancient sea. 



There is much difficulty in explaining the origin of the beds 

 of pure quartz, which occur in such large quantities in South 

 America, and impart so peculiar a character to the chain of 



* Hoffman, Geogn. Reise, edited by Von Dechen, s. 113-119, and 

 380-386; Poggeud., Annalen der P^sik, bd. xxvi., s. 41. 



r Duf'renoy, in the Memoires GiM^iqnes, t. ii., p. ] 45 and 17.9. 

 Vol. I.— M 



