137 



From a study of the fissure silver veins of central Arizona it would 

 appear — firstly, that they have in common, quartz, galena and te- 

 trahedrite; secondly, that there is a close connection between bary- 

 tes and copper-glance, more or less argentiferous, in their occur- 

 rence in a vein ; and thirdly, that the projjortion of silver in the 

 galena is largely increased when this mineral is associated with te- 

 trahedrite. A large number of assays made on the gray copper 

 ores of different mines showed a range of from one to eight and a 

 half per cent, of silver.. In many, if not all the richer varieties 

 examined, a large per centage was undoubtedly contained as me- 

 chanically mixed sulphuret of silver. 



In the Santa Cruz mountains, south of Fort Buchanan, is a series 

 of lead mines, several of which were excavated by Mexicans several 

 years since. They appear to follow the line of contact between an 

 argillaceous limestone, in whi«h corals have been found, and a 

 probably metamorphic porphyry. In places the deposits are of con- 

 siderable extent, often many yards in thickness, but, apparently, 

 very irregularly developed. Near the surface the galena is often 

 entirely changed into carbonate of lead associated with porous 

 quartz. 



At the Patagonia mine the ore consists of galena sufficiently 

 altered, at the present depth of working, to render its reduction 

 extremely simple. The average yield of silver from this ore has 

 been, thus far, about eighty dollars per ton. 



There is another class of contact veins bearing both lead and 

 copper ores. 



To this'class belongs the deposit near San Xavier on the Santa 

 Cruz. The ore is galena with copper pyrites and tile ore, associated 

 with oxyd of iron and quartz, the whole interstratified with meta- 

 morphic limestone. 



The galena examined contained 0.20 per cent., the copper 

 pyrites 0.25 per cent., and the tile ore 0.10 per cent, silver. 



Near Caborca, in northwestern Sonora, are deposits of a some- 

 what similar character. The strata of metamorphic limestone are 

 almost vertical, and near their contact with granite become highly 

 impregnated with lime garnets. Along the hne of contact between 

 the two formations, the presence of copper ores is indicated by fre- 

 quent occurrence of green and blue carbonates and impure red 

 oxyd. These indications often lead to the discovery of limited de- 

 posits containing a few hundred tons of copper. 



One of these, worked in 1861, yielded from two hundred and 

 fifty to three hundred tons of twenty-five per cent. ore. 



There was no vein ; the ore, which was accompanied by calcareous 

 spar, being gradually replaced at the bottom of the deposit by the 

 limestone of the formation. 



