chap. xv. of the Cordillera. 587 



siliceous sandstones, of coarse conglomerates, lime- 

 stones, tuffs, dark mud-stones, and those singular fine- 

 grained rocks which I have called pseudo-honestones, 

 vast beds of gypsum, and many other jaspery and 

 scarcely describable varieties, vary and replace each 

 other in short horizontal distances, to an extent, I 

 believe, unequalled even in any tertiary basin. Most 

 of these substances are easily fusible, and have appa- 

 rently been derived either from volcanos still in quiet 

 action, or from the attrition of volcanic products. If 

 we picture to ourselves the bottom of the sea, rendered 

 uneven in an extreme degree, with numerous craters, 

 some few occasionally in eruption, but the greater 

 number in the state of solfataras, discharging calcareous, 

 siliceous, ferruginous matters, and gypsum or sulphuric 

 acid to an amount surpassing, perhaps, even the exist- 

 ing sulphureous volcanos of Java, 1 we shall probably 

 understand the circumstances under which this singular 

 pile of varying strata was accumulated. The shells 

 appear to have lived at the quiescent periods when only 

 limestone or calcareo-argillaceous matter was deposit- 

 ing. From Dr. G-illies's account, this gypseous or 

 cretaceo-oolitic formation extends as far south as the 

 Pass of PJanchon, and I followed it northward at in- 

 tervals for 500 miles : judging from the character of the 

 beds with the Terebratula cenigma, at Iquique, it 

 extends from 400 to 500 miles farther; and perhaps 

 even for ten degrees of latitude north of Iquique to the 

 Cerro Pasco, not far from Lima : again, we know that 

 a cretaceous formation, abounding with fossils, is largely 

 developed north of the equator, in Columbia : in Tierra 

 del Fuego, at about this same period, a wide district of 

 clay-slate was deposited, which in its mineralogical 



' Von Buch's * Descript. Physique des lies Canaries,' p. 428, 

 26 



