204 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



CHARACTERISTICS OF THE RHYOLITE-DACITE. 



The specimens of the rhj-olite-dacite examined microscopically are of a highly 

 altered, very glass}' lava. The groundmass is glassy, often kaolinized. It is very 

 abundant, constituting nearly all the rock, and often shows marked flow structure. 

 The phenocrysts are rare and small, and consist chiefly of short, blunt feldspars, 

 biotite, and occasional very small quartz grains. Some of the feldspars are 

 striated. The feldspars are usually almost or entirely altered to kaolin, sericite, 

 and secondary adularia. The biotite is usually altered, sometimes to chlorite. 

 Secondary quartz and pyrite are usually common in the rock, and sometimes 

 there is calcite. 



MINERALIZATION. 



As a rule the rock is very much silicilied. Cracks in this rock are filled 

 with coatings of calcite, quartz, and pyrite, and excellent free crystals of barite. 

 Some streaks are considerably silicitied, and contain silver and gold, as is shown 

 by assay. Up to the time of this writing, however, no veins of importance have 

 been struck in this formation. 



The chief veins are irregular, barren, and nonpersistent. They have a 

 general northeast or east strike, and die out along the strike by scattering into 

 the silicitied rock, or are cut off by faulting. At the upper contact of the 

 rhyolite-dacite with the patch of andesite above mentioned, there are 2 feet of 

 jaspery barren quartz, illustrating again the tendency of the rhyolitic quartz to 

 form at the upper contact of the rhyolite-dacite body, under the impervious 

 decomposed andesite, as elsewhere described in the discussion of the MacNamara, 

 the Tonopah Extension, the Mizpah Extension, and other mines. 



Some faulting is shown in this level, the chief being in a north-northwest 

 direction, and indicating in places considerable displacement. 



PITTSBURG SHAFT. 



The Pittsburg shaft lies near the eastern edge of the area mapped, on the 

 south side of the main road which runs east out of Tonopah. It is not shown 

 on the topographic map, having been started since this was made. At the time 

 of the writer's visit, in November, 1904, it was 570 feet deep, all in volcanic 

 breccia, probably belonging chiefly to the Tonopah rhyolite-dacite period. This 

 formation contains some harder layers, which may be flows, but as a whole is to 

 be considered a surface formation, the product of volcanic explosions. 



RED ROCK SHAFT. 



This shaft lies about halfway between the Pittsburg and the Ohio Tonopah. 

 It was at the time of the writer's visit, in November, 1904, 230 feet deep in 

 volcanic breccia like the Pittsburg and the upper part of the Ohio Tonopah. 



