10 ORE DEPOSITS OF TONOPAH, NEVADA. [bull. 219. 
Subsequent to the eruption of this rock fractures were formed in 
it, and these became the channels for circulating solutions, probably 
ascending hot waters, closely connected with the volcanism. The 
waters were remarkable for intensity of action. The rocks which 
they traversed have been chemically transformed, the chief process 
being silicification, so that they are now found more or less completely 
altered to quartz and sericite (fine muscovite). Along fracture zones 
the rock has been replaced almost entirely by quartz carrying silver 
and gold, and these constitute the veins by which Tonopah is known. 
Some parts of the hornblende-andesite, on the other hand, show 
chiefly calcite and chlorite, with pyrite, as alteration products; but 
such phases have not yet proved to lie immediately adjacent to 
important veins, and, as a rule, do not seem to be indicative of the 
proximity of the best class of ore. 
Later andesite eruption. — There is evidence that these events were 
followed by a considerable period of erosion, long enough to bring the 
veins to the surface. Then another eruption of andesite occurred. 
This later andesite, which was abundant in amount, differed from 
the earlier andesite in containing, as dark minerals, pyroxene and 
biotite rather than hornblende, and it also differed in some structural 
and textural points. It is probable that this andesite came up through 
the hornblende-andesite in places, and also flowed over it, completely 
covering it and its contained veins. 
Rhyolite mid dacite < ruptions. — Following probably another period 
of erosion and volcanic rest, eruption was vigorously renewed and 
there were outbreaks at many closely adjacent points. The lavas 
were rhyolites and siliceous dacites, closely related to one another. 
Some vents emitted one, some the other rock. This period of volcan- 
ism was long and a number of different lava flows of dacite and rhyo- 
lite alternated or mingled. The eruptions were often explosive, as is 
shown by the considerable quantity of pumiceous and fraginental 
material in the volcanic breccias of this period. 
Lake sediments. — During this volcanic epoch a large fresh- water 
lake formed, of unknown, but probably great extent. In it were 
deposited white, finely stratified ash and tuff, and some beds are 
entirely made up of late Tertiary infusoria. In places the sediments 
of this lake, exposed near Tonopah, are several hundred feet thick. 
It is probable that this lake was partly filled by sediments and then 
drained by an uplift. Near the top of Siebert Mountain (a white tuff 
mountain southwest of Mount Brougher) there is what appears to be 
a river channel belonging to this post-elevation epoch, containing 
rounded waterworn pebbles of rocks such as now outcrop to the north 
and east of Tonopah, and fragments of silicified wood, showing the 
presence of vegetation at that time. This uplift may be ascribed to 
volcanism, for it was soon followed by renewed outbursts of lava. 
Eruption of basalt and intrusion of later dacites and rhyolites. — 
Above the tuffs on Siebert Mountain is a thin sheet of slaggy basaltic 
