14 ORE DEPOSITS OF TONOPAH, NEVADA. [bull. 219. 
altered and bleached it has a dull clayey luster, which is rarely seen 
in the early andesite. Rocks which are highly siliceous from altera- 
tion are apt to belong to the early rather than the late andesite. 
Distribution and characteristics of the rhyolites. — There have been 
several different rhyolite flows and intrusions, some before, some 
mainly subsequent to the f aultings ; and the rocks have a variety of 
appearances which defies any description in common. The lava of 
Oddie, Rushton, and Ararat hills is a characteristic siliceous white 
rhyolite. The Rescue shaft is mainly in this rock, and two long 
tunnnels, the North Star and the G. & H., have been driven in it into 
Mount Oddie. 
Besides this, however, there is a formation of fine-grained gray or 
red, largely glassy rhyolite, which covers a still larger area. A large 
body of it occupies the northern portions of the district, where it is 
intrusive, with a sinuous contact, into the later andesite. It continues 
from here, along the western edge of the district, to the vicinity of 
the Ohio Tonopah, and occupies a considerable part of the basin 
inclosed between Butler, Brougher, and Siebert mountains. The for- 
mation here occurs as complicated dikes cutting the dacite-breccia 
formation, and as sheets underlying the tuff, or, to a less extent, inter- 
calated in the tuff. In the rest of the district it is practically absent. 
It is in general bet ween the dacite breccia and the stratified tuff in 
point of age, is older than the faults, and is therefore displaced by 
the fault movements. In the northern portion of the district a num- 
ber of shafts are located on or near the contact of this rhyolite and 
the later andesite. Among these may be mentioned the King To- 
nopah, Belle of Tonopah, Miriam, Silvertop, Little Tonopah, etc. In 
the western portion the Ohio Tonopah and the New York Tonopah 
each encountered a considerable quantity of this rhyolite mixed with 
dacite breccia, while the Fraction extension is entirely in this rock, 
after having passed through the overlying tuff. 
This finer grained gray or red rhyolite has many different aspects. 
Fine brecciation, with bright red, gray, and white colors, is frequent; 
in other cases it is dense and characterless. The best test of it is the 
detection of the small quartz crystals that dot the glassy groundmass, 
which distinguishes the rock satisfactorily from the andesites. Some- 
times, however, these quartz crystals may be rare and small. Near 
the contacts of the rhyolite (intrusive into the earlier rocks) there 
has also frequently been alteration — silicification, the formation of 
pyrite, etc. — and then the rock may become almost identical in 
appearance with the early andesite which has been similarly altered. 
The best method for distinguishing the rhyolite from andesite in this 
case is the tracing of the connection with the less-altered rhyolite 
farther away from the contact. 
Distribution and characteristics of the dacites. — The bold knob 
north of Butler Mountain, known as Heller Butte, is made up of what 
