rtant mines no igneous rocks have been observed, and the origin of
the ores has not been worked out.

In the Central Plateau of France the numerous antimony deposits are
stibnite veins cutting granites and the surrounding schists and
sediments. An origin related in some way to hot ascending solutions
seems probable.

The deposits of the National district of western Nevada, the most
important war-developed antimony deposits of the United States, consist
of stibnite veins with a gangue of fine-grained drusy quartz, cutting
through flows of rhyolite and basalt. They are intimately related to
certain gold- and silver-bearing veins, and all are closely associated
with dikes of rhyolite, which were the feeders to the latest extrusion
in the district. The wall rocks have undergone alteration of the
propylitic type. These relations, and the presence of the mercury
sulphide, cinnabar, in some of the ores (see pp. 258-259), suggest an
origin through the work of ascending hot waters or hot springs. These
waters probably derived their dissolved matter from a magmatic source,
and worked up along vents near the rhyolite dikes soon after the
eruption of this rock.

In the weathering of antimony deposits, the stibnite usually alters to
form insoluble white or yellowish oxides, which are sometimes called
"antimony ocher." These tend to accumulate in the oxide zone through the
removal of the more soluble accompanying minerals. Secondary sulphide
enrichment of antimony deposits, if it occurs at all, is negligible.

ARSENIC ORES

ECONOMIC FEATURES

About two-thirds of the arsenic consumed in recent years has been used
in agriculture, where various arsenic compounds--arsenic trioxide or
"white arsenic," Paris green, lead arsenate, etc.--are used as
insecticides and weed killers. Arsenic compounds are also used in
"cattle-dips" for killing vermin. The only other large use of arsenic is
in the glass industry, arsenic trioxide being added to the molten glass
to purify and decolorize the product. Small quantities of arsenic
compounds are used in the preparation of drugs and dyeing materials, and
metallic arsenic is used for hardening lead in shot-making.

The principal arsenic-producing countries are the United States,
Germany, France, Great Britain, Canada, and Mexico. Spain, Portugal,
Japan, and China are also producers, and recent trouble with the
"prickly-pear" pest in Queensland, Australia, has led to local
development of arse