192 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



across the contact into the rhyolite. It is 160 feet long and runs N. 60 C E. 

 At the breast of the tunnel the rock is very much shattered Oddie rhyolite 

 containing 1 openings filled with brown iron-lime carbonate and white calcite. 

 From this point to the contact with the later andesite the rock is mostly a dense 

 rhyolite breccia of volcanic origin, the fragments being of very large size. 

 Strong open fractures striking N. 25 W. and dipping east at an angle of 60 

 are lined with white and brown carbonates, oxidized in places to iron oxide 

 and manganese oxide. Throughout the breccia, tilling all the interspaces, are 

 veinlets, filled chiefly with ferriferous carbonate and to a less degree with calcite 

 and chalcedony. Veins of smooth brown or bluish jasper, indicating silicification 

 of the rhyolite, have the same course and the whole breccia is largely silicitied. 

 Some of this material is claimed to run $8 or $9 to the ton, the values being all 

 in gold. 



The contact of the andesite with the rhyolite is 70 feet from the mouth of 

 the tunnel, and strikes N. 35 W. and dips east at an angle of 50. The rhyolite 

 is plainly intrusive. The brecciation, fracturing, and silicitication of the rhyolite 

 increase in measure as the contact is approached. Near the mouth of the tunnel 

 two dikes of rhyolite breccia, one 6 inches thick and one 3 feet thick, lie in the 

 andesite. These are in general parallel to the main contact, but dip 50 in an 

 opposite direction. The fracturing and brecciation are confined to the rhyolite, 

 and are not notable in the later andesite, which, however, is highly decomposed 

 and crumbling, while the rhyolite is hard. 



The evident interpretation of these phenomena is that this rhyolite column 

 was intruded into the andesite and that the upward movement continued after 

 the beginning of cooling. The result of this upward impulse was that the cooler 

 rhyolite for a zone of nearly 100 feet thick near the contact was intensely 

 brecciated while in n semisolid state. The upward pressure continued even after 

 further cooling, causing open fractures, mostly parallel to the contact, but 

 sometimes cutting across the rhyolite, as has been described elsewhere (p. 101). 

 Along these open fractures ascending hot waters, whose advent followed the 

 eruption, deposited iron and lime carbonates, silica, some manganese, and probably 

 some gold. 



BOSTON TONOPAH SHAFT. 



The Boston Tonopah shaft, lies 200 or 300 feet south of the Wingtield tunnel, 

 farther down the slope. At the time of the writer's visit it was 300 feet deep, 230 

 feet in the later andesite and the last 70 feet in white rhyolite like that constituting 

 the central plug. The contact between the andesite and the rhyolite in the shaft, 

 according to Mr. McCambridge, the superintendent, pitches northwest. 



