180 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



feet, was supposed to represent the same rock, for no sharp division had been 

 noted, but was judged, after microscopic study, to have rather the characteristics 

 of the earlier andesite. This specimen was altered to quartz, sericite. and pyrite. 



CHARACTER OF ANDESITE ON 220-FOOT LEVEL. 



At a depth of 220 feet from the surface, drifts were run 338 feet to the north 

 of the shaft and 285 feet to "the south. In both these drifts only andesite was 

 encountered and no general distinction was noted between the andesite in the 

 different parts of the drifts. In both drifts the rock strongly resembles certain 

 phases of the earlier andesite; in the south drift perhaps more than in the 

 north. This resemblance also holds good on microscopic study. Some sections 

 of the rock in the north drift showed occasional original phenocrysts of quartz, 

 such as are occasionally found in the earlier andesite. This original quartz was 

 found also in the specimen obtained in the shaft at a depth of 196 feet. On both 

 these drifts there was evidence of considerable movement, the general strike of 

 the slip or fracture planes being north and south and the dip west rather steeply. 

 The andesite when examined microscopically was found to be highly altered, the 

 chief alteration products being quartz, calcite, chlorite, serpentine, pyrite. siderite. 

 kaolin, and adularia. 



COKKELATION OF ANDESITES IN WEST END AND FRACTION WORKINGS. 



After studying the delicate question as to whether this rock is the earlier or 

 the later andesite the writer has satisfied himself that the andesite of the south 

 drift in the West End is identical with that shown in the long north drift from 

 the -100-foot level of the Fraction No. 2 shaft. The faces of the two drifts are 

 only about 250 feet apart in a straight line, but there may be, and very likely is. 

 intervening faulting. The writer was not able to distinguish between the general 

 type of the andesite in this north drift of the Fraction and the typical Fraction 

 andesite, which is often relatively dark and chloritic. In the Fraction No. 1 

 workings the andesite contains a large vein, carrj'ing in places at least good values. 



EXTENSION OF CORRELATION TO THE WANDERING BOY AND GOLD HILL. 



It seems to the writer, moreover, that the andesite in the Fraction No. 1 i> 

 identical with that in the Wandering Boy, which is more nearly the Mizpah Hill 

 type of earlier andesite. On following the chain still farther, the andesite in the 

 Fraction and that in the Wandering Boy seem to be identical and are probably 

 in the same fault block as the Gold Hill andesite. The rock of Gold Hill has 

 certain peculiarities which at one time caused the writer to study for some time 

 the question carefully as to whether or not it belonged to the earlier or later 



