spurb.] APPLICATION OF GEOLOGICAL PRINCIPLES. 27 
regarded with great favor, but the fact that the veins are in this posi- 
tion has no connection with the mountain; they are there rather in 
spite of the mountain than by virtue of it, and it would certainly have 
been much better for mining operations if the rhyolite had not been 
erupted and the mountain formed. 
Most of the dacite breccias and some of the brecciated rli3 T olite are 
in the nature of flows, and ma}^ be sunk through by shafts to lower 
formations, such as the later andesite or the early andesite. Much of 
the fine-grained, brecciated rhyolite, however, is intrusive, so that, as 
a rule, this rock also is not a good place to sink, except with caution. 
The tuff formation, of course, is always sedimentaiy and can be sunk 
through to reach the underlying formation, which may be any one of 
the older rocks. 
To sum up, then, the later andesite is the most favorable of the 
capping rocks; the pumiceous dacite breccia with interstratified, 
coarse, tuffaceous rocks is next, while the brecciated rhyolite should 
be prospected very cautiously to see whether it is intrusive. The tuff 
may be pierced to discover the underlying rock where the geological 
relations do not make this plain, while intrusive dacite and rhyolite 
necks should be approached very cautiously indeed. 
It must be understood that these characterizations appty only to the 
immediate vicinity of Tonopah, or the Tonopah district proper. On 
account of the sudden rise and fame of the camp the name Tonopah 
has spread far beyond its limits, and has been applied to mines lying 
in quite other districts, sometimes 50 or 60 miles away. Here, and 
also in neighboring districts which have not borrowed the name Tono- 
pah, the geological conditions are different. In the Gold Mountain 
district, for example, about 4 miles 'south of Tonopah, the veins lie 
in rhyolites and tuffs and evidently belong to the rhyolitic period; yet 
they carry in some cases considerable values in gold and silver. 
The position of the early andesite away from the outcrops can be 
foretold to only a limited and uncertain extent, for in dealing with a 
complex of volcanic flows and dikes like that in Tonopah there is no 
rule which governs their distribution and by which their position in 
an unknown locality can always be defined with certainty. There- 
fore, the prospecting should be done cautiously and the district should 
be developed from the known region as a center; thus little by little 
a. greater knowledge of the geological detail will be gained. On 
account of the lack of certainty of results in this work, in many 
cases it would probably be more economical, both of time and of 
mone}^, to explore with a diamond drill rather than with shafts. 
Certainly the sinking of a two- or three-compartment shaft in an 
unproved portion of the district is often unwise. 
The early andesite once located, the question of finding the veins 
remains, and after that the question of finding pay ore within the 
veins. The geological evidence goes to show that the mineralizing 
