SPUHK] PERIODS AND NATURE OF MINERALIZATION. 17 
of white rhyolite forced up into the earlier gray rhyolite. This later 
white rhyolite shows near its contacts, and to a less extent away from 
these, in itself or in the intruded rock, veins which are ascribed to 
causes similar to those which probably brought about the preced- 
ing mineralizations — namely, ascending heated waters following the 
lava intrusion, genetically connected with it, and, like the previous 
similar processes, dying out after the lapse of some time. These 
veins, which are in places very large, are of calcite, siderite or fer- 
riferous calcite, chalcedony, etc. The}^ contain some p}^rite and some- 
times give low values in the precious metals, gold preponderating. 
Mineralization phenomena may also be observed in some places near 
the contact of the Oddie Mountain rhyolite, where there is silicifica- 
tion and the formation of pyrite, with quartz veins of various sizes, 
sometimes containing calcite. These veins are generally small, but 
may be large. Galena and chalcopyrite have been exceptionally found 
in them. They carry low values in gold and silver, the gold being 
apt to predominate. 
Mineralization subsequent to the dacite intrusions. — Finally, the 
contacts of the large dacite necks (Butler, Golden, and other moun- 
tains) are accompanied in many places by the deposition of chalce- 
dony, manganese, etc., in cracks and crevices. These are not usually 
in veins, or when so are small and irregular. Considerable values in 
the precious metals have been claimed in some cases, but it is the 
writer's experience that the content of these metals is insignificant. 
Resume. — To sum up, no fewer than four periods of hot-spring 
action, accompanied by more or less vein formation and mineraliza- 
tion, have been noted at Tonopah. Each of these periods was conse- 
quent upon a lava intrusion, and the mineralization was the result of 
a process which is known to accompany and follow volcanic eruption 
at the present day. In general, the values found in the veins of these 
different periods increase with increasing age. Also, the general phe- 
nomena indicate that the younger veins were formed nearer the sur- 
face than the older ones. Although ascending hot waters are not 
known to deposit more than traces of metals at the surface, it is sup- 
posed that an important precipitation takes place at some depth; so in 
the case of Tonopah it may be that the later periods of mineralization 
have produced some rich ores, but that erosion has not yet had time 
to expose the level at which they lie, whereas in the case of the oldest 
period this has been accomplished. At Gold Mountain, about 4 miles 
south of Tonopah, veins belonging to the rhyolitic period and having 
the same characteristics as some of these veins at Tonopah contain 
considerable values in gold and silver, while at Tonopah no such 
values have been discovered in this class of veins. 
At Tonopah, therefore, the earliest and most important class of 
veins can be found only in the earlier andesite. The rhyolite veins, 
on the other hand, may occur in the rlryolites, as in any of the older 
Bull. 219—03 2 
