RHYOLITES AND DACITE8. 43 



Age and origin. There is a great deal of evidence concerning the age of 

 the Tonopah rhyolite-dacite. In the northern part of the area mapped this 

 formation is intrusive into the earlier andesite, and in many places into the later 

 andesite. In the southern half of the area it contains numerous inclusions of 

 later andesites, as well as probable earlier andesites, vein quartz, and granitic 

 fragments. It is often intrusive into or overlies the Fraction dacite breccia, which 

 therefore in general seems to be older (fig. 3). 



Above the Fraction dacite breccia proper is a series of coarse, pumiceous tuffs 

 which are rudely layered and rarety well stratified, and in which Tonopah rhyolite- 

 dacite sheets are often interbedded, with no sign of intrusion. This shows that 

 the flows were poured out intermittently and alternated with explosive eruptions, 

 which caused the great intervening accumulation of pumice and the 3 r ellow ash 

 derived from its disintegration. Occasionally also, but not commonly, thin sheets 

 of the same rhyolite-dacite are found in the lower part of the waterlaid Siebert 

 tuffs, which ovejlie the pumiceous unassorted tuffs and breccias and their inter- 

 calated Tonopah rhyolite-dacite flows. 



/-;-;-;,-,;-;_< ;'ih%'^v''-""^^^^ 



,-'"' Scale 



o 5 10 20 30 feet 



FIG. 3. Vertical section of part of tunnel north oi Brougher Mountain and southeast of Ohio Tonopah shaft, showing 

 (1) Tonopah glassy rhyolite-dacite overlying Fraction dacite breccia; (2) faulting subsequent to both; (31 contact 

 dipping down and in general faulted down toward the volcanic neck of Brougher Mountain (mouth of the tunnel 

 is about 800 feet distant from the border of this neck). 



The geologic position of the Tonopah rhyolite-dacite is, then, pretty clearly 

 fixed. The eruptions of Fraction dacite breccia were soon followed by those of the 

 Tonopah rhyolite-dacite, which mingled with the Fraction dacite, as indicated by 

 numerous observations where these rocks are intimately associated. The eruptions 

 of Fraction dacite became subordinate to those of the Tonopah rhyolite-dacite 

 and were probably chiefly explosive, contributing material to the brown pumice 

 beds, which are often several hundred feet thick and which alternate with the 

 Tonopah rhyolite-dacite flows. At this period the Tonopah rhyolite-dacite 

 eruption was at its height, though for some time subsequently, after the formation 

 of the Tertiary lake and the accumulation of the Siebert tuffs therein, scanty 

 flows were occasionally and locally emitted. Near Rushton Hill, however, pebbles 

 of glassy Tonopah rhyolite-dacite in a conglomerate at the base of the Siebert 

 tuffs indicate that the older Tonopah rhyolite-dacite flows contributed by erosion 

 their material to the upbuilding of the tuffs, at the same time that the last 

 tardy flows of the same lava were being brought forth. 



