16 OEE DEPOSITS OF TONOPAH, NEVADA. [bull. 219. 
Indeed, there is every evidence that the veins were formed by ascend T 
ing hot waters succeeding and connected with the early andesite intru- 
sion, and that these waters had apparently become inactive by the 
time of the later andesites. 
The mineralization at this period was extraordinarily active, as the 
profound alteration of the early andesite testifies. Among the known 
veins formed at this period those of the Valley View, Mizpah, and 
Montana Tonopah groups are the most important, though certainly 
there are others which have not yet been discovered. These veins 
carry gold and silver, in the proportion of about 1 part of gold to 
100 of silver, by weight. They are unusually free from base metals — 
no lead, arsenic, etc., has been detected. In some places there is a 
very little copper, in others none. The gangue is quartz, with fre- 
quently a mineral which is a variety of orthoclase feldspar (valen- 
cianite). The sulphide ores, so far as developed, show primary step- 
hanite, with probably some polybasite, scant pyrite, and comparatively 
rare chalcopyrite. Secondary sulphides coating the cracks in these 
ores are rubj 7 silver, argentite, and probably pyrite and chalcopyrite. 
The oxidized ores show abundant silver chloride, with occasional 
bromides, etc. , and sometimes free gold. 
Mineralisation subsequent to the early rhyolite intrusion. — Along 
the borders of the gray or red glassy rhyolite intrusions, especially in 
the northern portion of the district, there has been, as before noted, 
considerable alteration and mineralization, which must be attributed 
to a cause similar to that which produced the veins in the early ande- 
site — namely, the action of hot ascending waters immediately suc- 
ceeding and genetically connected with the rhyolite intrusion. This 
alteration is in the form of silicification and the formation of pyrite, 
and has acted. on the rhyolite as much as the intruded rock. Quartz 
veins have been formed, in the majority of cases relatively small. 
These veins contain precious metals, but usually very irregularly 
distributed. High assays, especially on the surface, may even be 
obtained, but it is likely that some of these are the result of the well- 
known process of increase of values at the surface during oxidation. 
Specimens of the veins have a general resemblance to those of veins 
of the early andesite, and show pyrite, often finely disseminated in a 
quartz gangue. Some of these veins have been found to be of consid- 
erable size; therefore it may often be difficult to distinguish them 
from the andesitic veins without study of the rock in which they lie. 
Chemically they appear to be characterized, so far as yet developed, 
by very low average values and by the frequent but not regular 
preponderance of the gold values over those of silver. 
Mineralization subsequent to the later rhyolite intrusion. — It has 
been described how the white rhyolite of Oddie, Rushton, and Ararat 
hills is probably later than the dense, usually gray rhyolites referred 
to above. For example, the top of Ararat seems to be a plug or neck 
