116 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



is cut off by nearly flat fault, which dips west at a moderate angle. This may he 

 called the Siebert fault (fig. 17). On the upper side of the fault the rock is chiefly 

 light-colored, partly oxidized, silicified earlier andesite, mixed with much barren 

 quartz; on the lower side it is unoxidized and has a different appearance, and 

 though study shows it to be probably the earlier andesite there is much chlorite 

 and sometimes calcite among its decomposition products, thereby separating it 

 sharply from the andesite inclosing the veins, which has characteristically been 



Surface 



Surface 



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Scale 

 o 10 20 30 40 so feet 



H;. lf>. Vt-rticiil cross section of a portion of 

 Mi/pith vein as exposed in the Oddie shaft, 

 showing reversed dip near the surface. 



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Scale 

 o so mo 200 300 feet 



FIG. 17. Vertical cross section of Mizpah vein along 

 Brougher shaft and inclines. 



altered to quartz and sericite. Below the 700-foot level in the Siebert shaft 

 (PI. XVIII) this rock in places is altered chiefly to quartz and sericite, and ev<-ii 

 contains silicified zones or quartz veins giving assays of a few dollars to the ton. 

 At a depth of about 935 feet there was encountered a body of dacitic or rhyolitic 

 rock resembling the rock in the lower part of the Mizpah Extension, and probably 

 referable to the Tonopah rhyolite-dacite; below this a vertical drill hole shows that 

 the same rock is continuous to nearly 1,400 feet from the surface, where the boring 

 was stopped. 



