254 GEOLOGICAL DESCRIPTION OF THE EARTHS CRUST. 



some difficulties connected with the interposition of beds 

 of altered rock between unaltered strata, which we cannot 

 wholly explain. 



Having described the transformation of compact car- 

 bonate of lime into granular limestone and dolomite, we 

 have to notice a third mode of alteration in the same rock, 

 occasioned by the emission at some ancient epoch of the 

 vapours of sulphuric acid. The gypsum thus produced 

 offers analogies with beds of rock salt and sulphur (the 

 latter deposited from aqueous vapour charged with that 

 mineral) . In the lofty Cordilleras of Quindiu, far from any 

 volcano, I have observed deposits of sulphur in fissures in 

 gneiss, while in Sicily (at Cattolica near Girgenti), sulphur, 

 gypsum, and rock salt, are found in the most recent secon- 

 dary formations. ( 283 ) At the edge of the crater of Vesuvius 

 also I have seen fissures filled with rock salt, sometimes 

 in masses sufficiently considerable to occasion a contra- 

 band trade ; while on the northern and southern declivi- 

 ties of the Pyrenees, one cannot doubt the connection of 

 dioritic (pyroxenic?) rocks with the occurrence of dolo- 

 mite, gypsum, and rock salt. ( 284 ) In these phenomena 

 every thing indicates the action of subterranean forces on the 

 sedimentary strata deposited by the ancient sea. 



There is much difficulty in assigning the origin of those 

 vast masses of pure quartz which are characteristic of the 

 Andes of South America. ( 285 ) In descending towards the 

 Pacific, from Caxamarca to Guangamarca, I have found beds 

 of quartz from seven to eight thousand feet in thickness, 

 resting sometimes on porphyry devoid of quartz, and some- 

 times on diorite. Can these be a metamorphosed sandstone, 

 such as Elie de Beaumont conjectures to be the origin of the 



