spurr.] ORE DEPOSITS OF TONOPAH AND VICINITY, NEVADA. 83 
diate vicinity of Tonopah, however, only volcanic rocks are found. 
These consist of flows, breccias, and derived tuff and ash accumula- 
tions. These volcanics are probably of Tertiary age, and represent a 
number of successive flows, with intervening showers of ash and 
breccia, and erosion intervals. On account of the confusion of these 
volcanics the relative order of eruption has not yet been certainly 
made out, but at the present time the sequence is considered to be 
somewhat as follows : 
1. Earlier andesite (latite?). 
2. Earlier rhyolite and breccias. 
3. Later andesite. 
4. Erosion interval. 
5. Volcanic breccias and flows. 
6. Great water-laid tuff formation containing infusorial silica. 
7. Later rhyolite. 
8. Latest lava flow (tlacite?). 
The oldest volcanic rock is the earlier andesite, which is commonly 
called the lode porphyry. Although originally an andesite, it is now, 
so far as examined, everywhere almost entirely decomposed and trans- 
formed into secondary products, consisting of fibrous muscovite, 
secondary quartz, pyrite, chlorite, iron carbonate, etc. In its present 
decomposed form, therefore, it is not an andesite, although originally 
one. Some forms of the altered rock are what has been described by 
early writers in this region as propylite; but Dr. G. F. Becker showed 
that the propylite of the Comstock region was an altered andesite, 
just as it is at Tonopah. The Tonopah rock and the Comstock rock 
are, as a matter of fact, apparently similar in composition, and prob- 
ably are also in point of age. 
The important veins of the district occur only in this earlier andes- 
ite or lode porphyry, and not, so far as yet found, in the later rocks. 
So it seems that the mineralization must have followed the first 
andesitic eruption. Therefore it is that the overlying volcanics do 
mot show these veins, and constitute cap rocks which overlie them, 
and which must be pierced in order to reach the lode porphyry and 
its associated ores. 
The ores are in the form of quartz veins of the kind which have 
been described by some writers as the noble quartz formation — that 
lis to say, the quartz constitutes almost the whole of the ore, and the 
valuable metals are very finely distributed, so as to appear barely or 
jnot at all to the naked eye. There is also a very small quantity of 
the less valuable metallic minerals. Silver is found in the form of 
chloride, sulphide (argentite), and ruby silver. Gold has been seen 
fin a free state in the ore. 
The deposits are well-defined veins, maintaining a nearly regular 
strike and dip. The principal veins average a few feet in width. 
,rhe chief trend of the veins is east and west, but in the developed 
Region the different veins diverge regularly from one another, so that 
