um, rhodium, and palladium. Most of the platinum as used
in jewelry and for electrical purposes contains iridium, which serves to
harden it. Paladium-gold alloys are a substitute for platinum, chiefly
in dental uses.

The original home of platinum is in basic igneous rocks, such as
peridotites, pyroxenites, and dunites, where it has been found in small,
scattered crystals intergrown with olivine, pyroxene, and chromite.
Platinum is very dense and highly resistant to oxidation and solution.
In the breaking up and washing away of the rocks, therefore, it is
concentrated in small grains and scales in stream and beach placers. Of
the world production of platinum over 99 per cent has been derived from
placers.

The Ural Mountain deposits of Russia are gold- and platinum-bearing
placers, in streams which drain areas of dunite rock containing minute
quantities of native platinum. The deposits of Colombia and Australasia
are placers of a similar character. In the United States small
quantities of platinum are recovered from the gold-bearing gravels of
California and Oregon, where the streams have come from areas of
serpentine and peridotite.

A platinum arsenide, called sperrylite, is sometimes found associated
with sulphide minerals in basic igneous rocks. At Sudbury, Ontario, this
mineral, together with palladium arsenide, is found in the nickel ores,
especially in the weathered zone where it is concentrated by removal of
more soluble materials. It has also been found in the copper mines of
Rambler, Wyoming. In the Yellow Pine district of southern Nevada,
metallic gold-platinum-palladium ore shoots are found in association
with copper and lead ores, in a fine-grained quartz mass which replaces
beds of limestone near a granitic dike. No basic intrusives are known
in the district. The deposit is unusual in that it has a comparatively
high content of platinum (nearly an ounce to the ton), and is probably
genetically related to acid intrusives. From all these deposits, only
small quantities of platinum are mined.

FOOTNOTES:

[34] Report of a joint committee appointed from the Bureau of Mines and
the United States Geological Survey by the Secretary of the Interior to
study the gold situation: _Bull. 144, U. S. Bureau of Mines_, 1919. See
also Report of Special Gold Committee to Secretary of the Treasury,
February 11, 1919.

[35] Ransome, F. L., The geology and ore deposits of Goldfield, Nevada:
_Prof. Paper 66, U.S. Geol.