58 STUDIES IN GEOLOGY, No. i 



concordant with the stratification and as fine particles of 

 metallic copper penetrating white sandstones. Neither the 

 red sandstones nor the shales contain ore and as sandstones 

 become red or argillaceous, they lose their richness. Twelve 

 principal beds from y$ to 3 m. wide, had been worked. 

 Locally the copper ores grade over into silver ores. Where 

 this takes place the white sandstones become red and shaly, 

 the copper content diminishes, and the silver increases and 

 exceeds the copper in amount. The metallic silver occurs 

 in the same manner as the metallic copper except that salt 

 is associated with it. There is no regularity in the position 

 of the silver which is encountered at some places at 70 m. 

 and at others at more shallow depths. 



A fuller description of the ore deposits and of their 

 structural relations is given by Forbes (pp. 40-47). He 

 says the main object of exploitation is native copper which 

 occurs as metallic grains or larger masses, disseminated 

 irregularly in certain beds of sandstone, but west of the 

 fault combinations of copper with oxygen, arsenic, etc., are 

 also found. The metalliferous district is divided into two 

 parts by the Corocoro fault. On the west side of the fault 

 are three principal veins, called vetas. The main vein, 

 nearest the fault, the Umacoya, was producing native copper 

 disseminated irregularly through a coarse grit, in grains, 

 irregular lumps, or plates, sometimes of very considerable 

 size. Beyond it lay the Rejo vein, also called the Copaca- 

 bana and Negra, which produced a very dark ore because 

 of its richness in copper sulphides and arsenides as chalcocite 

 and domeykite. Farthest from the fault was the Buen 

 Pastor, an impregnation of a fine-grained sandstone not 

 only by copper but also with native silver. This bed was 

 being worked exclusively as an argentiferous exploration. 

 On the east side of the fault the metalliferous beds are 

 described as differing considerably from the vetas and as 

 they are of much less thickness are called, in contrast, ramos. 

 There were 5 workable ramos and 9 lesser ones. The ramos 

 ore is in a finer state of aggegation than that from the vetas 



