spurr.] TOPOGRAPHY AND GEOLOGICAL HISTORY. 9 
The values in the ores are entirely gold and silver. The propor- 
tion of gold to silver is usually remarkably uniform, and is the same 
in both the oxidized and the sulphide ores, namely: The gold is to 
the silver, by weight, as 1 : 90 or 1 : 100, making the values about two- 
sevenths gold and five-sevenths silver. In some of the excessively 
rich ores the proportion of gold becomes greater, occasionally making 
up half of the total value. 
TOPOGRAPHY. 
That the mountain range in which Tonopah is situated is of volcanic 
origin is shown by its topography. It consists of a series of eroded 
mesas and detached or connected hills. These mesas and hills are 
irregularly distributed, with no definite valley systems between. They 
are separated by patches of sloping or rolling country, largely covered 
with "wash," or debris worn from the hills. The mesas are the rem- 
nants of volcanic flows ; the isolated or connected irregular hills are 
denuded volcanic necks. 
An excellent topographic map of the country immediately around 
Tonopah, on the scale of 800 feet to an inch, has been made by Mr. 
W.J. Peters, of the United States Geological Survey. On this it is 
seen that the general elevation of the slight valley-like depression in 
which the town of Tonopah lies is about 6,000 feet above sea level, 
while the top of Butler Mountain, which is the highest mountain near 
the town, is 7,160 feet. 
GENERAL GEOLOGICAL HISTORY. 
Pre-Tertiary formations. — In the immediate vicinity of Tonopah the 
rocks are all Tertiary volcanics or sedimentary tuffs belonging to the 
volcanic period, and composed partly of ash and partly of silt derived 
from the erosion of the lavas. Eight or nine miles south of the camp, 
however, there is limestone, very likely of Cambrian or Silurian age, and 
in this granitic rock is intrusive. Limestones and granites occur also 
several miles north of Tonopah, and at intervals between there and 
Belmont. At Belmont the limestone, into which granite is intrusive, 
is known to be Silurian. 
At Tonopah itself occasional limestone fragments and more abun- 
dant fragments of a fine, even-grained pegmatite occur in the volcanic 
breccias. They are blocks which were hurled out from the volcanoes 
at the time of their eruption. Thus it is shown that at an uncertain 
depth below the present surface the ascending lavas broke through 
rocks of this character. 
■Early andesite eruption. — The oldest rock outcropping in the imme- 
diate vicinity of the camp is a hornblende-andesite, or, rather, was 
such, for no fresh portions have been found and the rock has been 
altered so as to acquire an aspect in general very different from its 
original one. 
