8 STUDIES IN GEOLOGY, No. i 



cement. Intercalated in these are the ore beds which are of 

 light-brown to greenish-blue color and somewhat harder. 



L. Sundt saw more clearly the lithologic characteristics 

 and differences between the two formations. He says in 

 Corocoro are two distinct geologic formations, resting dis- 

 cordantly on each other. The older beds (the Vetas) 

 extend to the north and west, strike N. 30 W. and dip W., 

 are principally sandstones of yellow or red color, never 

 contain beds of gypsum, and extend with great uniformity 

 in strike and dip several kilometers to the west and north, 

 having a thickness of more than looo meters. The sand- 

 stones are composed especially of rounded fragments of 

 quartzites, which often have the size of a nut, forming 

 conglomerates. Besides the quartzites, there are often 

 small fragments of felsite, half kaolinized and not rounded, 

 and also fragments of an eruptive rock composed of feld- 

 spar and hornblende. The other formation (the Ramos) 

 rests on top of the former and surrounds it to the south 

 and east and is composed of sandstones and conglomerates, 

 often similar to those of the former, but usually of a darker 

 color, chocolate and coffee. It is distinguished from the 

 Vetas by its beds of red shale with intercalated beds of 

 gypsum or aragonite. Besides the sandstones, conglome- 

 rates, shales, and gypsum, Sundt says there are encountered 

 in the two formations of Corocoro, beds of tuffs and very 

 characteristic trachytic conglomerates. In a white tuff are 

 rounded fragments, up to the size of a fist, of trachyte com- 

 posed of sanidine, black mica, and quartz. Small fragments 

 of black mica, feldspar, and at times crystals of bipyramidal 

 quartz also frequently form an integral part of the quartz- 

 itic sandstones. 



Sundt's interpretation of the stratigraphic relation of the 

 two formations will be disregarded at this point. It is 

 evident that he clearly recognized the prominence of red 

 shales and the presence of gypsum as distinguishing char- 

 acteristics of the Ramos formation in contrast to the Vetas 

 formation, and that the sandstones and conglomerates of 



