Van Horn — Silver, Copper, and Cobalt Minerals. 25 



Sulphur. — This mineral is found massive in considerable 

 quantities in the upper levels, and has been exploited for 

 market in other mines of the region. It is often very pure, 

 but in some places is mixed with gypsum, cerussite, and limon- 

 ite. Chism ascribes the formation of the sulphur to heat, but 

 it was more probably formed by the normal oxidation of sul- 

 phide ores, due to the action of hydrogen sulphide on limestone 

 or sulphur dioxide, or by the reduction of gypsum which is so 

 generally present. 



Silver. — Silver is found massive, in wires, plates, and irreg- 



FlG. 1. 



Fig. 1. Cubes of Copper in matrix of Selenite from the Veta Rica mine. 



ular globular forms. It has been found in three different 

 associations : as grains and wires with argentite in barite ; with 

 embolite in the siliceous limestone ores ; and in hair-like 

 masses, and globular incrustrations on erythrite in barite. 



Copper. — This is present as perfect cubes which have a vary- 

 ing diameter up to five-eighths of an inch. The edges are in 

 some places slightly modified by a tetrahexahedron. It is rare 

 to find such un distorted crystals of native copper. The crys- 

 tals are partially coated with cuprite, and are embedded in a 

 matrix of white cleavable gypsum. This is an association 

 which the writer has never seen, although copper is said by 

 Dr. E. O. Hovey, of the American Museum of Natural History, 

 New York City, in a personal communication, to be found 

 with gypsum at Painesdale, Michigan. A photograph of the 

 Yeta Rica occurrence is shown in fig. 1. 



Argentite. — This mineral occurs massive, and also in irregu- 

 lar rounded forms. On two specimens it was present in dis- 



