chap. xv. Gypseous Formation. 555 



merate, with crystallised particles and veins of carbonate 

 of lime, from 300 to 400 feet in thickness. The 

 pebbles are of claystone porphyries of many varieties ; 

 they are tolerably well rounded, and vary in size from 

 a large apple to a man's head. This bed includes 

 three layers of coarse, black, calcareous, somewhat 

 slaty rock : the upper part passes into a compact red 

 sandstone. 



In a formation so highly variable in mineralogical 

 nature, any division not founded on fossil remains, 

 must be extremely arbitrary : nevertheless, the beds 

 below the last conglomerate may, in accordance with 

 all the sections hitherto described, be considered as 

 belonging to the porphyritic conglomerate, and those 

 above it to the gypseous formation, marked [E] in the 

 section. The part of the valley in which the following 

 beds are seen is near Potrero Seco. Bed (5), compact, 

 fine-grained, pale greenish-gray, non-calcareous, indu- 

 rated mudstone, easily fusible into a pale green and 

 white glass. Bed (6), purplish, coarse-grained, hard 

 sandstone, with broken crystals of feldspar and crystal- 

 lised particles of carbonate of lime ; it possesses a 

 slightly nodular structure. Bed (7), blackish-gray, 

 much indurated, calcareous mudstone, with extraneous 

 particles of unequal size ; the whole being in parts 

 finely brecciated. In this mass there is a stratum, twenty 

 feet in thickness, of impure gypsum. Bed (8), a 

 greenish mudstone, with several layers of gypsum. Bed 

 (9), a highly indurated, easily fusible, white tuff, thickly 

 mottled with ferruginous matter, and including some 

 white semi-porcellanic layers, which are interlaced with 

 ferruginous veins. This stone closely resembles some 

 of the commonest varieties in the Uspallata chain. 

 Bed (10),- a thick bed of rather bright green, indurated 

 mudstone or tuff, with a concretionary nodular structure 



