VEINS OF TONOPAH RHYOLITE-DACITE. 97 



Just beyond the western corner of the area mapped (PI. XI), opposite Siebert 

 Mountain, a group of three low hills rises above the plain. One of these 

 hills is capped by a patch of dacite, whose resistance to erosion has probably 

 caused the hill. The rest of this hill and all of the other two are composed of 

 white tuff mixed with beds of conglomerate, plainly referable to the white tuffs 

 of the area mapped. The origin of the two hills, which are composed entirely of 

 tuffs, is due to two elliptical areas where these tuffs and conglomerates have been 

 thoroughly silicitied and changed to a quartzite-like condition. Some mineral- 

 ization has accompanied the silicification. A random specimen of the silicified 

 material from the smaller of the two hills thus formed was reported to the 

 writer to have yielded on assay $8 in gold and no silver. This silicitication and 

 mineralization is evidently the work of powerful hot springs, and the elliptical 

 shape of the silicitied areas shows that the springs rose along pipe-like channels 

 and not along definite fractures. These deposits are probably of practically the 

 same age and origin as the veins in the Tonopah rhyolite-dacite. 



CHARACTERISTICS OF RHYOLITE-DACITE VEINS. 



The veins of this period are characterized by irregularity and by lack of 

 definition and persistence, though their size may locally be great. As a rule they 

 are elongated and have the appearance of veins, but can not be followed as far either 

 on the strike or dip as true veins may. They may disappear by scattering and 

 passing into a silicified wall rock, or may be cut off along a cross-wall fracture in 

 the same manner as some of the veins in the earlier andesite described on p. 85. 

 The quartz is as a rule dense and jaspery, and is white, gray, or black; it is therefore 

 usually of different appearance from the white quartz of the earlier andesite veins. 

 The veins are usually barren or contain only very small quantities of gold and silver, 

 except locally, as in the Desert Queen, where rich bunches of ore may occur, though 

 usually of limited and irregular extent (fig. 13). Like the veins of the earlier 

 andesite the rhyolite-dacite veins very frequently contain adularia, and in one case 

 probable albite was noted (see p. 197), a mineral which has not been detected in the 

 andesite veins. In the Ohio Tonopah barite has been found as a gangue mineral with 

 the rhyolite-dacite veins. This mineral has not yet been found in connection with 

 the earlier andesite mineralization. In the Desert Queen and the North Star, where 

 quartz of the rhyolite-dacite period has been cut by drifting, a green stain forms on 

 the walls, which is a basic copper sulphate. This phenomenon has not yet been 

 noted in connection with the earlier andesite mineralization. A characteristic of the 

 rhyolite-dacite veins, to which there are, however, numerous exceptions, is the 

 1684S No. 4205 7 



