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GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



the contact above referred to it is fairly typical. The contact is probably not 

 due to faulting, but is normal and indicates that the veins in the earlier andesite 

 outcropped at the surface at the time of the later andesite extrusion. 



VEINS IN THE EARLIER ANDESITE. 



At a depth of 230 feet, in the earlier andesite, a heavy vein was cut near the 

 shaft. This has been developed by levels at depths of 244 and 385 feet, and by an 

 incline between the levels. The general strike of the vein is west-northwest and 

 the dip north from 30 to 45. The vein is from 3 to 8 feet thick and shows shoots 

 of high-grade sulphide ore like that of the Montana Tonopah. So far as had been 

 developed at the time of the writer's visit, in November. 1904, the vein has not 

 been faulted. 



tofeet 



Fio. 69. Diagrammatic vertical cross section of Tonopah Extension vein. a. Altered earlier andesite, wall rock; 6, 

 typical white vein of earlier andesite period, containing black silver sulphides, with values of several hundred dollars 

 per ton; c, black, jaspery quartz of later introduction than original vein, of which it contains fragments. Values of 

 black quartz and fragments, 820 to $30 per ton. 



An interesting phenomenon is displayed by the Tonopah Extension vein. Where- 

 ever it has been followed, a portion of the vein, generally that lying next to the 

 hanging wall, is of different character from the rest. The main body is composed 

 of white quartz containing black silver sulphides, and has exactly the same char- 

 acter as the other earlier andesite veins in the camp. The upper portion, however, 

 is of black or gray jaspery quartz, like so many of the veins in the Tonopah 

 rhyolite-dacite. Moreover, this portion contains angular fragments of the ordinary 

 quartz vein in such a way as to show conclusively that the jaspery quartz was of 

 later introduction than the main vein. Evidently renewed pressure reopened the 

 vein subsequent to the first ore deposition, and caused a new fracture or fissure, 



