in the copper, lead, and zinc ores has already been referred to in
the discussions of these ores.

Reference will be made here only to the vein deposits in which gold,
with silver, constitutes the principal values. Because of their common
gangue of quartz these are often called "dry" or "siliceous" ores. Their
principal occurrence is in distinct fissure veins in igneous rocks, with
more or less replacement of the wall rock. The igneous rocks are
commonly acid intrusives of a granite or porphyry type, less commonly
intrusives of gabbro and diabase and surface lavas of rhyolite and
basalt. In a few cases the ores are contact-metamorphic deposits of the
type described under copper ores. In still rarer cases they are in
pegmatites. Gold is commonly associated with minerals and wall-rock
alterations indicating deposition by hot solutions, which are inferred
to have come from the igneous rocks.

Because of the resistant nature both of ore minerals and gangue,
weathering and secondary concentration have had little effect in
enriching gold deposits. So far as there has been any noticeable effect
on the gold content of the ores, it has been due to the leaching out of
other constituents, principally pyrite and other sulphides, leaving the
gold present in slightly larger proportions. Locally there is evidence
of solution of gold in weathered zones and its deposition in the
sulphide zones below. Solution is believed to be accomplished by
chloride solutions, and is favored by the presence of manganese which
delays precipitation. The precipitating agent below may be ferrous
sulphate, various sulphides, native metals, or organic matter.

Of the vein or lode gold ores in the United States some of the most
productive and best known have the following geologic features:

The California gold belt extends north and south along the west slope of
the Sierra Nevada Mountains. The ore is in a series of parallel and
overlapping veins striking with the trend of the range, associated with
granodiorite intrusives in schist and slate. There is no pronounced
secondary concentration. These deposits are the source of most of the
great placer deposits of California, hence the name "Mother Lode"
applied to a part of them. The principal ore deposits are somewhat
removed from the main mass of intrusive which forms the crest of the
Sierra Nevada range, and are more closely related to the smaller similar
intrusive masses farther down the slope. The gangu