ut 400 miles long. The ore bodies are
in fissured zones in serpentine and Jurassic sediments, and are related
in general to recent volcanic flows. A considerable amount of bituminous
matter is found in the ores, and is believed to have been an agent in
their precipitation.

The Terlingua ores of Texas are found in similar fractured zones in
Cretaceous shales and limestones associated with surface igneous flows.
The occurrence of a few ore bodies in vertical shoots in limestone,
apparently terminating upward at the base of an impervious shale,
furnishes an additional argument for their formation by ascending
waters.

In the few deposits (_e. g._, those of Almaden, Spain, and of the deep
mines of New Almaden and New Idria, California,) where there is no such
clear relation to volcanic rocks as generally observed, but where the
ores contain the same characteristic set of minerals, it is concluded
that practically the same processes outlined above have been active in
their formation; and that the volcanic source of the hot solutions
either failed to reach the surface or has been removed by erosion. The
same line of reasoning is carried a step further, and in many
gold-quartz veins in volcanic rocks, where cinnabar and its associated
minerals are present, it is believed that waters of a hot-spring nature
have again been effective. Thus cinnabar, when taken with its customary
associations, is regarded as a sort of geologic thermometer.

In the weathering of mercury deposits, cinnabar behaves somewhat like
the corresponding silver sulphide, argentite. In the oxide zone, native
mercury and the chloride, calomel, are formed. In the Texas deposits a
red oxide and a number of oxychlorides are also present. The carrying
down of the mercury and its precipitation as secondary sulphide may
have taken place in some deposits, but this process is unimportant in
forming values.

TIN ORES

ECONOMIC FEATURES

The largest use of tin is in the manufacture of tin-plate, which is
employed in containers for food, oil, and other materials. Next in
importance is its use in the making of solder and of babbitt or bearing
metal. Tin is also a constituent of certain kinds of brass, bronze, and
other alloys, such as white metal and type metal. Minor uses include the
making of tinfoil, collapsible tubes, wire, rubber, and various
chemicals. Tin oxide is used to some extent in white enameling of metal
surfaces. Roughly a third of the tin cons