202 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



FAULT. 



This same rhyolite-dacite outcrops about 35 feet east of the shaft, on the 

 farther slope of a little gull}-. A northeast fault running along this gully is thus 

 evidenced, and indeed is shown farther northeast up the hill slope. By this fault 

 the block in which the Fraction Extension is located is downthrown in respect to 

 that on the east side. 



TONOPAH CITY SHAFT. 

 GEOLOGICAL SECTION. 



The Tonopah City shaft lies on the outskirts of the town, about 1,100 feet 

 south of the Fraction No. 2. It was driven to a distance of 500 feet before work 

 was stopped. On the surface at this point is a very thin covering of black glassy 

 rhyolite or dacite (latest rhyolite-dacite flow), generally only a few feet thick, and 

 often broken up into bowlders rather than in place. 



Practically none of this black lava is exposed in the shaft itself, the first solid 

 formation cut being the Fraction dacite breccia. The upper 100 feet of this was a 

 coarse breccia, evidently detrital, which contained large and small inclusions, mostly 

 of later andesite. From 100 to 300 feet the breccia was finer grained and denser, 

 and apparently had an explosive origin, being full of small, angular, white pulveru- 

 lent fragments, which are probably decomposed pumice. At a depth of 300 feet 

 solid Heller dacite (see p. 37) came in and continued for 200 feet to the bottom of 

 the shaft. 



At a depth of 400 feet in the shaft this dacite was observed to be cut by 

 a dike of exactly similar material, the only difference being the presence in the 

 dike of a greater abundance of light-colored intrusions. This dike is 10 inches 

 thick and has a N. 70 W. strike and a dip of 75 to the northeast. 



INDICATED DISPLACEMENT OF FAULT BLOCKS. 



Since neither the earlier nor the later andesite was encountered in this shaft, 

 and the dacite breccia is so much thicker than in the Fraction shaft to the 

 north, it is plain that the fault block in which the Tonopah City is situated is 

 depressed relatively to that in which the Fraction shafts lie. 







OHIO TONOPAH SHAFT. 

 DACITE TUFFS IN SHAFT. 



The Ohio Tonopah is situated about l,f>00 feet west of the MacNamara shaft. 

 At this point the surface formation is a volcanic tuff due to dacitic outbursts. 

 Home of the harder portions are more clearly referable to the glassy Tonopah 

 rhyolite-dacite, while other portions, especially where the rock is softer, approach 



