The mountain range in which the mines of this district occur is isolated
from all others, although evidently a part of a chain extending in a
northwest and southeast direction for many miles. This particular group
of hills is about 4 miles in length by 1½ in width, and consists of
rocks, which are all of plutonic origin. They are mostly quartz porphyry
of the normal type, consisting of a fine-grained felsitic ground mass,
with macroscopical crystals of quartz and feldspar.
Of several thin sections made for the purpose of microscopic study of
these rocks, their behavior under the microscope is so similar that
general description will suffice for all. The section is characterized
by a micro-crystalline to micro-granular ground mass, sometimes to
globulitic. The feldspars are so completely clouded as to leave little
clue to their identity. Some still show faintly the parallel lines which
are so characteristic of plagioclase, but some of these feldspars are
probably orthoclase. These feldspars are plainly distinguishable in the
rock mass with the unaided eye. Quartz blebs as large as small peas,
with many smaller ones, occur plentifully in all of these rocks. In the
section they seem to have been corroded, the outlines being nearly
always rounded, though frequently showing hexagonal forms. Most of these
quartz grains polarize in brilliant colors. All of the quartzes contain
a great abundance of fluid inclusions, some of which show included air
bubbles. Numerous very small, colorless, needle-like crystals, which
occur in all the quartz, are no doubt apatite prisms. Green, dust-like
hornblende as inclusions are not at all uncommon, and in one section a
mass of green, fibrous material, having all the optical properties of
hornblende, is seen. This inclusion is large enough to be easily
distinguished with the unaided eye. There are numerous globulitic,
granular, and sac-like inclusions of the ground mass in many of the
quartzes, which is characteristic of the quartz porphyries generally.
Besides these macroscopic crystals of quartz and feldspar are many
hornblendes, some of which are of good size, a few having the typical
crystal outlines. It is usually of a bright green color, strongly
dichroic, and polarizes in the usual colors. Some of the hornblende is
altered to chlorite. Borders of iron ore, probably magnetite, are
common, frequently preserving the original outline where the hornblende
has suffered great decomposition.
Dark-green basic dikes, which seem to be greatly altered diabase, occur
in the district, but have no connection with any of the ore deposits as
far as observed.
Numerous dikes, large and small, of a red felsitic rock, occur
throughout the district, and seem to bear an important relation to some
of the ore deposits. All of the rock is much decomposed, and its
identification is not an easy matter. It appears under the microscope to
be a finely porphyritic rock, having a somewhat fluidal structure, as
shown in the arrangement of the numerous small, lath-like crystals of
feldspar. Small blebs of quartz occur quite abundantly. In a general way
the rock resembles some rhyolites.
On the northern flank of the range immense masses of red and brown tufa
occur, besides great flows of black basaltic lava. These rocks form a
terrace-like ridge that extends for several miles along the base of the
mountains. Two large cinder cones, one on the northeast end of the
range, the other about 4 miles distant in the desert valley to the
northward, form prominent landmarks.
The basalt is coarse to fine grained, usually black or dark gray, and is
often scoriaceous. It contains plagioclase, augite, and olivine, and
abundant magnetite in a micro-crystalline or granular base. Near the
foot of the range the basalt has overflowed the beds of tufa, which
latter, it is said, contains from a trace to as high as four ounces of
silver per ton. As far as I have any knowledge of it, no prospecting has
been done in these tufa beds, but the fact that silver exists there at
all would lead one to believe that under proper conditions ore bodies of
great value may occur.
At the base of the mountains, at an altitude of 1,800 feet, is a dry
lake, which drains a large area of country. In this basin water has been
obtained by sinking a well 80 feet in depth. Though the well is located
near the edge of the basin, there is little doubt that sufficient water
can be obtained in this basin for milling purposes.
The principal vein in the district lies along the north side of the main
range, and is known as