270 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



mines, in which part of the veins strike northeast and dip southeast and others have 

 their course toward the northwest and dip northeast. The gangue is quartz, carrying 

 galena, pyrite, chalcopyrite, and tetrahedrite. In the San Rafael mine, Jalisco, the 

 veins have a course N. 25 W. In the mines of Hostotipaquillo the veins contain 

 calcite and quartz with some rhodochrosite, a small amount of pyrite and black blende, 

 argentite, galena, chalcocite, and chalcopyrite. In the oxidized zone they contain 

 native silver, carbonates of copper, and a very small amount of copper oxide. It 

 would be tiresome to enumerate all the silver veins of Mexico which occur in 

 andesites, but as has been said, the majority of the silver veins of the country are in 

 various species of this rock, which Humboldt designated as metalliferous porphyries." 

 Rarely similar veins are found in rhyolite. 6 



Perusal of the instances mentioned above by Aguilera shows that the veins are 

 all closely alike, not only in regard to their country rock, but to their tilling. 



THE COMSTOCK LODE. 



Pachuca is about 2,000 miles southwest of Tonopah, but a similar analogous 

 deposit (the Comstock) lies 150 miles to the northwest. 



The Comstock lode is a vein 4 miles long which has formed in Tertiary eruptive 

 rocks, chiefly andesites, along a fault line having a maximum displacement of 3,000 

 feet. At both ends it branches and so dies out. It strikes east of south and dips 

 east. It was discovered in 1859, and worked up till the present day, but most 

 actively from 1861 to 1880. Up to June, 1902, it had yielded $369,566,112.61 worth 

 of ore, of which about 42^ per cent was gold and 57 per cent silver/ The rocks of 

 the district in the order of their succession are, according to Hague and Iddings/ 

 andesite, dacite, rhyolite, andesite, and basalt. The andesites are coarse grained in 

 depth (diorites and diabases). Near the lode, and for some distance away, in a space 

 about 5 by 2 miles, the country rock (chiefly andesitic) is extremely decomposed, 

 the period of alteration having succeeded an andesitic eruption. The hornblende, 

 augite, and biotite have altered to chlorite, pyrite, epidote, etc., the feldspar to quartz 

 and an undetermined white aggregate. This altered andesite is the famous "propy- 

 lite." The basalt, which is the latest rock of the district, has not been altered in the 

 same way as the andesites. The alteration of the rocks and the lode was due to 

 solfataric action which accompanied the faulting. 



The lode material is quartz, certain limited portions of which contained large 

 quantities of silver and gold (bonanzas), while the rest is low grade. Calcite is 

 much less than quartz in amount and is generally insignificant. Most of the bullion 

 has been derived from a bluish quartz, like that at Tonopah, the color being mainly 



"Trans. Am. Int. Min. Eng., vol. 32, pp. 515-516. 

 6 Ibid., p. 517. 



e Becker, G. F.. Mon. f. S. Oeol. Survey, vol. 3, pp. 9, 11. Also Kept, of the Director of the Mint for 1901, p. 169. 

 >l Hague, A., and Iddlngs, J. R., Bull. U. S. Oeol. Survey No. 17. Doctor Becker's determinations anil succession are 

 somewhat different, a follows: Granite, diorite, quartz-porphyry, diabase, andesitc, basalt. 



