198 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



CORRELATION OF VEINS WITH OTHER OCCURRENCES. 



The formation of pyritiferous quartz veins in this rock is therefore a contact 

 phenomenon near the edge of the intrusion, for the glassy rhyolite-dacite outcropping 

 away from the contact is usually quite fresh and unsilicified. This idea gains cor- 

 roboration from the fact that near by, at other points on the contact, namely, in the 

 vicinity of the Belle of Tonopah shafts and elsewhere, silicification and the formation 

 of veins has occurred. This vein is, then, of the same class as the veins at the contact 

 of the glassy Tonopah rhyolite-dacite in the Mizpah Extension and other shafts. 



BELLE OF TONOPAH SHAFT. 

 GEOLOGICAL CONDITIONS. 



The Belle of Tonopah is situated in the northern corner of the area mapped, about 

 1,600 feet west of the King Tonopah, on the irregular contact of the glassy Tonopah 

 rhyolite-dacite and the later andesite. At a number of places along this contact the 

 phenomena show that the rhyolite-dacite is intrusive into the andesite. Just south 

 of the Belle of Tonopah shaft a number of rhyolite-dacite dikes occur in the later 

 andesite near the contact. These are considerably decomposed and are accompanied 

 by small and nonpersistent quartz veins, which give assays showing generally small 

 and irregular quantities of gold, with little silver. 



The Belle of Tonopah shaft starts in such a rhyolite-dacite dike, very close to the 

 contact, and passes downward through 20 feet of this material, when it enters the 

 later andesite. The contact of the two rocks strikes west-northwest and dips south- 

 west at angles ranging from 65 or 70. The contact is marked by a decomposed 

 zone, and the later andesite below is soft and is very full of pyrite which, however, 

 is quite barren. 



At the time of the writer's visit the shaft was 230 feet deep, all except the 

 upper 20 feet being in the later andesite. Since then, in January, 1904, Mr. 

 A. C. Stock, the manager, has sent the writer a specimen of the rock from the 

 bottom of the shaft at a depth of 460 feet. This is later andesite. 



The rhyolite-dacite in the upper part of the shaft resembles the rock from 

 the King Tonopah. It is highly decomposed, but has the structure of a nearly 

 glassy volcanic rock and contains many very small crystals, nearly all of which 

 seem to have been feldspar, with no original quartz. Now the whole rock is 

 altered to kaolin, chert, hematite, siderite, etc. This rock unquestionably belongs 

 to the glassy Tonopah rhyolite-dacite and is intrusive into the later andesite at 

 this place. 



