90 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



cles which showed under the microscope the corroding action of the reagents used. 

 When boiled with concentrated nitric acid, these black particles became golden in color, 

 and the solution contained little or no selenium, but of this last I am not positive. 

 So far as can be judged, the whole of the gold exists in the form of this malleable 

 black alloy, which is so high in silver that the latter can all be extracted by strong 

 nitric acid. The cause of the black color is not apparent, and it puzzles me not a 

 little." 



SUMMARY OF VEIN MINERALS. 



The principal minerals of the primary veins are, then, quartz, adularia, and 

 some sericite, carbonates of lime, magnesia, iron, and manganese, sulphides of 

 silver, antimony, copper, iron, lead, and zinc (sulphides occurring in the form of 

 argentite, stephanite, polybasite, chalcopyrite, pyrite, galena, and blende), silver 

 selenide, and gold in a yet undetermined form. The remarkable thing about the 

 metallic contents is the scarcity of the common elements and the abundance of the 



rare ones. 



OXIDATION. 



The chief alteration of the rocks, as will hereafter be explained, is due to 

 the action of ascenaing underground waters. The effects of descending surface 

 waters are seen chiefly in oxidation and similar processes acting upon the altered 

 rocks. The oxidation or other alteration of metallic sulphides is the chief change, 

 and, on account of the universal presence of pyrite formed by hot-spring action, 

 this change can be observed both in the veins and in the country rocks. 



DEPTH OF OXIDATION. 



The depth to which this oxidation of pyrite has penetrated is exceedingly 

 irregular, being quite different in neighboring shafts, and is very variable in 

 different parts of the same workings. The difference plainly depends on the 

 porosity and fracturing of the rock. Where these are greatest the oxidizing 

 waters have penetrated farthest downward. Along veins the oxidation generally 

 penetrates much deeper than in the rock, so that the ores may be oxidized while 

 the country rock is pyritiferous. This is plainly due to the greater rigidity and 

 brittleness of the vein as compared with the rock, so that it has been more 

 fractured by .strains, and therefore offers a readier channel. Even in veins the 

 depth of oxidation is very irregular, dependent upon the amount of fracturing. 



CAP ROCKS AS PROTECTION FROM OXIDATION. 



The veins which outcrop are most deeply oxidized, as the Mizpah and Valley 

 View veins. The former is for the most part oxidized down to a depth of nearly 

 700 feet; the latter is oxidized at the lowest level developed (about 500 feet). At 

 a depth of 400 feet in both mines the vein is almost completely oxidized or 



