another fabulously rich ore
body. The lode occupies a fault fissure parallel to the trend of the
range and dipping about 40 degrees to the east, which can be traced
about two and a half miles along the strike, with igneous rocks forming
both hanging and foot walls. There are no sedimentary rocks in the
district. The high-grade part of the vein is several hundred feet in
thickness, with many irregular branches; the great thickness has been
thought to be at least in part due to the tremendous pressure exerted by
growing quartz crystals. The wall rocks have undergone a "propylitic"
alteration, with development of chlorite, epidote, and probably
sericite, much as at Butte. The ore contains rich silver sulphide
minerals and native gold, in a gangue composed almost entirely of
quartz. The ore was doubtless formed by hot solutions, but the exact
nature of these solutions, whether magmatic or meteoric, has not been
proven. The hypothesis was early developed that the ores were deposited
by surface waters,--which are supposed to have fallen on the summits of
the Sierra Nevadas, to have sunk to great depths where they were heated,
enabling them to pick up metallic constituents from the diabase forming
one wall of the ore body, and to have risen under artesian pressure
along the fault plane, where loss of heat and pressure resulted in
deposition. Later studies have emphasized the similarity of the
ore-depositing conditions with those in other districts where the ores
are believed to have come directly from magmatic sources, and this
origin is now generally favored for the Comstock Lode. However, the
earlier theory has not been disproved.

The Tonopah, Nevada, district is very similar to the Goldfield district
(p. 230). Silver and gold are found in veins and replacements in a
series of Tertiary volcanic flows and tuffs, all of which have been
complexly faulted. Silver is the dominant constituent of value. The
formation of fissures and faults accompanying and caused by the
intrusion and cooling of lavas was first clearly shown in this district.
Evidences of origin through the work of hot solutions, probably
magmatic, are the close association of the ores in place and in time
with the igneous rocks--ore deposition in most of the flows having taken
place before the next overlying flows were put down,--the presence of
fluorine, the nature of the wall-rock alterations, the fact that both
hot and cold springs are found close together und