Van Horn and Cook — New Occurrence of Pearceite. 519 



which constitute the Sierra. The ore occurs at or near the 

 contact of the limestone with a rock which is locally called 

 conglomerate, although others have named it a porphyritic 

 breccia.* However, the rock seems to be either a decomposed 

 rhyolite or a rhyolite tuff, since it consists chiefly of fine-grained 

 quartz and orthoclase, much decomposed. Along the contact 

 of the two rocks there are many indications of faulting, such 

 as breccias, slickensides, and clay selvages. It seems not 

 improbable that the valley was made by faulting, which has 

 left the cliffs as a fault scarp. There are about 19 mines in the 

 district, which, even in 1900, produced about 200,000 metric 

 tons of ore. Up to 1893, it was a silver-lead camp in which 

 the predominant ore was argentiferous cerussite with small 

 amounts of galena. In 1893, however, in the western part of 

 the region, large bodies of silver-copper ore were found in the 

 San Jose mine in addition to the silver-lead ore bodies. Simi- 

 lar silver-copper ores were afterward found on adjoining prop- 

 erties, of which the Yeta Rica mine is one. The latter has 

 proven to be the richest if not quite the largest mine in the 

 camp. 



Ores and Minerals Found at Veta Rica Mine. 



Although Sierra Mojada is still predominantly a silver-lead 

 camp, nevertheless the chief output of the Yeta Rica, which is 

 one of the largest mines in the district, is known as a siliceous 

 silver lime containing workable amounts of copper. The chief 

 ore is either a red or dark gray magnesian limestone, impreg- 

 nated with quartz, cerargyrite, native silver, and sometimes 

 barite. In the specimens of this type which were subjected to 

 investigation, no well-defined copper minerals were observed, 

 although it is said to contain from 0*6 up to 2 per cent of 

 copper. 



In another part of the mine, along a fissure in the limestone, 

 copper minerals containing silver are found. A rather inter- 

 esting fact is that on the same level along this fissure are two 

 ore bodies of this type, but one consists of sulphides while the 

 other is entirely oxidized. Specimens from the former presented 

 by Mr. Cochran consist of massive chalcocite, chalcopyrite, 

 and a little covellite. Also small amounts of galena and sphaler- 

 ite are said to occur at this point. About 120 meters west of 

 this ore body, but at the same horizon and on the same fissure, 

 is the oxidized body, which consists of native copper, cuprite, 

 azurite, and malachite associated with gypsum. The water 

 channel which caused the oxidation of this body has evidently 

 been prevented in some manner from reaching the sulphides. 



* The Sierra Mojada, Coahuila, Mexico, and Its Ore Deposits, by James 

 W. Maleolmson, Trans. Amer. Inst, of Min. Eng., xxxii, 105, 1902. 



