erground (indicating
unusual sources for the hot springs), the contrast in composition
between the ores and the country rock, and the general relation of these
ores to a large number of similar occurrences in Tertiary lavas in the
same general area.

Under weathering conditions, the silver sulphide minerals in general are
oxidized to form native silver and cerargyrite, which are relatively
insoluble and remain for the most part in the oxide zone. Silver is less
soluble than copper and zinc, but more soluble than gold; and to some
extent it is removed in solution, particularly where the oxidation of
pyrite forms ferric sulphate. Farther down it may be reprecipitated as
native silver, argentite, and the sulpho-salts, by organic matter or by
various sulphides. The secondarily enriched ores are in a few districts,
as at Philipsburg, Montana, the most valuable portions of the deposits.
In other cases, sulphide enrichment does not appear to have contributed
greatly to the values. The zones of oxide ores, secondary sulphide ores,
and primary or protores are in most silver deposits much less regular
and much less definitely marked than in the case of copper ores.

PLATINUM ORES

ECONOMIC FEATURES

The principal uses of platinum are: as a catalytic agent in the contact
process for the manufacture of sulphuric acid, and in the making of
nitric acid from ammonia; for chemical laboratory utensils that must be
resistant to heat and acids; for electrical contacts for certain
telephone, telegraph, and electrical control instruments, and for
internal combustion engines; in dental work; and for jewelry. In normal
times before the war, it is estimated that in the United States the
jewelry and dental industries used 75 per cent of the platinum metals
consumed, the electrical industry 20 per cent, and the chemical industry
5 per cent. During the war, with the extraordinary expansion of
sulphuric and nitric acid plants, these proportions were reversed and
the chemical and electrical industries consumed about two-thirds of the
platinum. Substitutes have been developed, particularly for the
electrical uses, and the demand from this quarter may be expected to
decrease.

About 90 per cent of the world's crude platinum produced annually comes
from the Ural Mountains in Russia. The deposits next in importance are
those of Colombia. Small amounts are produced in New South Wales,
Tasmania, New Zealand, Borneo, British Columbia, United Sta