6o STUDIES IN GEOLOGY, No. i 



from i m. to 5 m. in width, averaging about 2-3 m. They 

 carry native copper and its oxidation products, the oxides 

 and carbonates of copper, near the outcrop. The first veta, 

 the Copocabana, was the most important. It averaged 4 m. 

 wide, but varied between 2 and 6 meters. The upper part 

 usually consisted of dark sulphide ore, which in depth 

 changed to native copper ore. It contained much charque 

 associated with gypsum and in druses crystals of calcite in 

 all stages of transformation to native copper. (This remark 

 probably refers to. the pseudomorphs of copper after arago- 

 nite.) The fourth veta, the Buen Pastor, is described more 

 fully because of its silver content. The discovery of the 

 silver ores of this veta caused considerable excitement, but 

 numerous shafts and tunnels quickly proved the area of 

 workable silver ore to be very restricted and included in the 

 Buen Pastor mine. This was worked with reasonable 

 success until the mine was abandoned and became flooded 

 during the revolution of 1859. Below the narrow oxidized 

 part of the veta, were copper sulphides, and still deeper 

 native copper. The silver was first encountered at a depth 

 of 20 m. and alternated with the copper in such a way that 

 at some places the one metal predominated, at other points 

 the other metal. For a depth of 60 m. there was eight times 

 as much silver as copper; at still greater depths silver pre- 

 dominated but not in so great ratio. Rich ores contained 

 2400 ounces of silver per ton, and medium grade ores 

 yielded on amalgamation 700 to 1200 ounces. 



One of the interesting mmeralogical phenomena at Coro- 

 coro are the hexagonal prisms of native copper pseudo- 

 morphic after aragonite which are encountered disseminated 

 in the ramos. They are found in all stages of replacement 

 from the pure aragonite to almost pure copper. Domeyko 

 describes these crystals and the manner in which their 

 transformation is affected quite fully. He says they are 

 never completely changed to copper but that the densest do 

 not exceed 5.8 sp. gr. and include cavities and small patches 

 of unreplaced aragonite. In conclusion he adds a brief 



