MINING IN CALIFORNIA AND NEVADA. 53
Part II.
NEVADA.
Section I.—Geological Features.
An
examination of the celebrated Comstock lode was made during the latter
period of my stay on the Pacific slopes, and I regret that
circumstances beyond my control had intervened previously which limited
the time at my disposal at Virginia City. The outcrop of this
magnificent deposit is situated 6,200 feet above the sea-level, at the
base of Mount Davidson, 7,240 feet above the bay of San Francisco. The
lode is essentially of a quartziferous nature, and the "ore" occurs
therein irregularly distributed. At some places along this vein, or
lode, accumulations of very rich ore are found, however, and are well
known under the term "bonanza" or ore bodies; these assume,
with a northerly dip in the line of strike, a lenticular form, and are
therefore not to be depended on for a regular supply of ore.
Considering the great length of this lode, which has been proved
ore-bearing for over 20,000 feet, there would be rather more ground
from payable to very rich, than that which has been proved unprofitable
to work. Portions of the lode, nearer to and at the surface, consist
of a friable and reddish kind of quartz, in which silver is not
traceable until at a greater depth; gold of the ordinary purity
(Californian) predominates in this crystalline quartz. This lode
differs in every respect from any found, to my knowledge, in Australia,
and it constitutes simply what is known as a "contact vein," i.e., a
deposit of ore bearing all the characteristics of a "true" lode,
occurring, however, between walls of two different kinds of rock, which
are also of different age in geological time. The west or hanging wall
of the older "syenite" averages 45 degrees underlay, and is, in its
very regular descent, accompanied by a strong fluccan, or "dig." The
opposite eastern hanging wall of the more recent "propylite," 600 feet
east at the outcrop, that being the width of the lode at the
surface, has no such defined division with the lode, which gradually
loses its solidity by breaking up into numerous bands, bunches, and
cross-veins in that direction; practical observers hold that, owing to
the absence of any defined wall, other ore bodies may be found against
a proper limit, or defined wall, should such exist to the east. The
Comstock lode is subject to the intrusion of "horses," or detached
masses of wall rock, in its regular occurrence, besides which, the ore
therein is traversed by strong clay veins, which dam the surface or
other water to the detriment of mining operations, inasmuch as by any
level or shaft inadvertently piercing these clays, disastrous swamping
of the workings has taken place, though latterly the mining diamond
drills have been used with excellent effect, both for prospecting the
ground hundreds of feet ahead of the workings, and by tapping these
subterranean and pent-up waters.
The
eastern wall rock (propylite) causes the diminution in width of the
lode from 600 feet at the surface to 260 feet at the 1,400-foot level,
owing to such wall rock assuming firstly a western underlay of 75
degrees, which lower down, however, curves round to nearly the same
eastern underlay as that of the western foot wall at 45 degrees. This
propylite is joined or passes at its eastern extension into a coarse
feldspathic porphyry. At the lower levels, however, there are strong
indications of the lode increasing its width considerably.
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