PAHUTE MESA, STRUCTURE. 
described (see p. 133) shows that there are two thin (lows of this rhyo- 
lite. The lower flow is a dense grayish-purple rock containing () 
vast number of small phenocrysts. Under the microscope the ground- 
mass is seen to be a glass with highly developed flow lines. The phe- 
nocrysts all appear to be orthoclase. The upper rhyolite is similar to 
the lower, although it contains slightly larger orthoclase phenocrysts, 
and in addition a few tiny biotite and quartz phenocrysts arc visible. 
The younger rhyolite of Pahute Mesa and that of Goldfield hills 
are similar lithologic units occupying like stratigraphic positions, 
and without doubt are contemporaneous and probably of late Pliocene 
age. 
Basalt. — Flows of basalt cover Pahute Mesa northwest of Tolicha 
Peak, similar flows extend eastward from Quartz Mountain to 
Thirsty Canyon, and a third area of considerable size lies west of 
Timber Mountain. The last-named mass appears from a distance 
to have flowed from two cones, one 5 miles northeast and the other 
8 miles southeast of Timber Mountain. Black Mountain, from a dis- 
tance, also resembles a volcanic cone, and the north peak of Mount 
Helen may have been an ancient vent. 
The basalt of Pahute Mesa includes vesicular and nonvesicular 
facies, the former red, the latter black. In some of the basalt white 
striated feldspars, reddish altered olivines, and black pyroxenes arc 
prominent, while in other portions these phenocrysts are inconspicu- 
ous. Stratigraphically the basalts of Pahute Mesa and of the Gold- 
field hills have identical positions and they are considered of late 
Pliocene age. 
STRUCTURE. 
Pahute Mesa northwest of Tolicha Peak is capped by lava flows, 
which dip gently in all directions from a center 5 miles southwest of 
Mount Helen. The only structural feature of interest in this portion 
of the mesa is the slight rise of the lava sheets as they approach the 
older hill groups, well seen at Gold Crater and on the southeastern 
face of Stonewall Mountain. A similar gentle dome is present in the 
mesa between Quartz Mountain and Thirsty Canyon, but eastward 
from the canyon the mesa gradually rises, owing probably to compar- 
atively recent uplifts of the Kawich and Belted ranges. In this por- 
tion of the mesa normal faults Avith north-south strike, few of which 
have a displacement of over 50 feet, are common. Although of late 
Tertiary age, these faults do not exhibit fault scarps, since erosion has 
planed down the upthrown side. In some instances, however, the 
downthrown side, being more resistant, is now a scarp. 
ECONOMIC GEOLOGY. 
Three mining camps are situated in Pahute Mesa — Trappmans 
Camp, Wilsons Camp, and Gold Crater. 
