portation,
and which have a density sufficiently high so that they are partially
sorted out and concentrated from the accompanying quartz and other
minerals. To warrant their recovery they must also be of such high
intrinsic value that it pays to mine small quantities. The most
important of such minerals are gold, tin, platinum, and the precious
stones. Iron, copper, lead, and zinc minerals are often somewhat
concentrated as placers, but their intrinsic value is not high enough to
warrant the attempt to recover them in the large amounts necessary to
make them commercially available.

Placers are forming now and have formed at all stages of the earth's
history. Early placers may be reworked and further concentrated by
renewal of the proper erosional and transportational conditions. Old
placers may be buried beneath younger rocks, cemented, and more or less
recrystallized. "Fossil" placers of this kind are best represented by
deposits in the Black Hills of South Dakota and probably by the South
African gold deposits.

In the Witwatersrand deposits of South Africa, the gold is concentrated
in the lower parts of large conglomerate and quartz sand layers of great
areal extent. Pebbles of the conglomerate are mainly quartz and
quartzite. The gold, in particles hardly visible to the eye, is in a
sandy matrix and is associated with chloritoid, sericite, calcite,
graphite, and other minerals. The origin of the gold deposits of this
district is not entirely agreed on, but the evidence seems on the whole
to favor their placer origin. Some investigators of these ores believe
them to have been introduced into the conglomerate and sand by later
solutions, possibly by hot solutions related to certain diabase
intrusions that cut the beds.

In the vein or lode or hard-rock deposits, the gold is mainly metallic
gold, and to a minor extent is in the form of gold tellurides. It is
usually closely associated with iron pyrite in a matrix or gangue of
quartz. Seldom is a gold deposit free from important values in other
minerals. About 84 per cent of the gold mined in vein or lode deposits
of the United States is associated with silver minerals, the combined
value averaging about $6 per ton; about 13 per cent comes from copper
ores which have an average yield of gold and silver of 50c. per ton; and
3 per cent comes from zinc and lead ores, with an average gold and
silver yield ranging from $1 to $6 per ton. The geologic occurrence of
gold