64 STUDIES IN GEOLOGY, No. i 



and as the sandstone beds are more prominent and thicker 

 in the former series, the vetas are believed to average 

 thicker than the ramos. In both series, mineralization has 

 occurred at a number of closely spaced horizons, but at 

 only a few to form important oreboclies. The thickness 

 of the mineralized beds varies from a few inches up to as 

 much as 10 or 12 feet. Active mining operations are 

 centered about the Cerro Corocoro and the south end of 

 the ridge on the west side of the Corocoro valley. Here 

 the vetas outcrop, but the ramos are first encountered in 

 depth. On account of the geologic structure along the 

 fault plane, the zone of the ramos . pitches upward toward 

 the north and emerges to the surface in the northern part 

 of the district in the vicinity of the Libertad mine. 

 Wherever mineralized, the beds have lost their red color 

 and are white or light-green. Residual patches of red rock 

 are unmineralized and the impervious shales which enclose 

 the cupriferous sandstones retain their red color. The 

 bleached rock is, as a rule, soft and friable as though much 

 of the cement that held together the sand grains had been 

 removed. In the ramos the ore is entirely native copper, 

 and occurs chiefly in the form of small particles dissemi- 

 nated through the sandstone. The ordinary rich ore is 

 called "tacana ;" if the particles of native metal are coarse, 

 it receives the name "chafra." In very rich ore, the copper 

 grains are knitted together and isolate the sand grains to 

 such an extent as to make the ore soft, yet difficult to break 

 on account of its toughness. Plates and arborescent forms 

 of copper are abundant as fillings of joints, cracks, and 

 openings along bedding planes. These are called charque. 

 F. A. Sundt is cited as authority for the statement that 

 the rock in the vicinity of charque is poorer than the 

 average, suggesting that the copper had been deposited in 

 that form in those places instead of having been dissemi- 

 nated in the bed. Gypsum, and less abundantly, barite and 

 celestite occur as gangue minerals, but chiefly in association 

 with charque. In the lower part of the vetas the mineral- 



