100 GEOLOGY OB' TONOPAH MINING DISTRICT, NEVADA. 



extend to the contact of the andesite and do not enter it, raises at first a doubt 

 as to whether the andesite is not really the younger rock instead of the older. 

 In some of the shafts mentioned the andesite is soft and very little silicified, 

 while the amount of silicification in the rhyolite-dacite is very great. However, 

 there is no doubt of the relative age of the rhyolite-dacite as given on p. 43, 

 and the reason for the described phenomenon appears upon reflection. The 

 rhyolite-dacite consists mainly of volcanic glass and was injected into the earlier 

 andesite after this was thoroughly decomposed and softened as the result of the 

 action of hot spring waters that accompanied and caused the principal minerali- 

 zation. Any slight subsequent strains in the earth resulting from volcanic action 

 shattered this fresh and glassy rock, but formed no fractures or fissures in the 

 soft adjacent andesite. The hot waters that rose immediately after the rhyolite- 

 dacite eruptions found almost their only channels in the fractured and fissured 

 glassy rock to which they owed their origin. Therefore the veins that they 

 formed are confined chiefly to this rock. Evidence of the correctness of this 

 explanation is furnished by the thick veins of this period that are found on the 

 contact of the rhyolite-dacite sheet with the overlying decomposed andesite. Such 

 veins are often found at this place and the accompanying silicification is very 

 great, but is almost invariably confined to the rhyolite-dacite near the contact. 

 Such, for example, is the situation in the Mizpah Extension, the MacNamara, 

 Tonopah Extension, and West End, and to a less degree in the Ohio Tonopah. 

 These things show that the ascending hot waters, circulating through the fractured 

 rhyolite-dacite, rose until at the contact with the overlying soft andesite they 

 found a practically impervious barrier, along whose lower contact they circulated 

 and deposited the materials which they held in solution. 



Subsequent to this formation of quartz veins and attendant silicification, 

 similar differences between the rhyolite-dacite and the andesite with reference to 

 brittleness continued, so at the present day the silicified rhyolite-dacite is found 

 to be extremely faulted and fractured and to contain open fissures, features which 

 are not present to the same extent in the adjacent andesite. 



EFFECT OF WATERS PRODUCING THE TONOPAH RHYOLITE-DACITE VEINS ON 



EARLIER FORMED VEINS. 



Although as a rule decomposed andesite seems to have presented a formidable 

 barrier to the circulating waters accompanying the Tonopah rhyolite-dacite, in 

 some places the waters must have traversed the andesite and found their way 

 along the andesitic veins. Indeed, it is along these brittle veins and the brittle 

 silicified adjacent andesite that fractures and fissures must have been most easily 

 formed at this period. In the case of the Tonopah Extension, as described 



