ttractive for exploration and the discovery of new
deposits are in Siberia and South America, which in the opinion of many
engineers may eventually rival South Africa. Mexico, with the
establishment of a stable government, should also have a greatly
increased production.

GEOLOGIC FEATURES

The principal gold mineral is native or metallic gold. This occurs in
nature in small scales, crystals, and irregular masses, and also in
microscopic particles mechanically mixed with pyrite and other
sulphides. Chemically, gold is very inactive and combines with but few
other elements. A small part of the world's supply is obtained from the
gold-silver tellurides--calaverite, sylvanite, krennerite, and petzite.

Gold deposits are of two general classes--placers, and veins or lodes.

Placers, which are in general the more easily discovered and more easily
worked deposits, have in the past been the chief source of the world's
gold supply. It is estimated that in the first twenty-seven years of the
modern era of gold-mining, beginning with the discovery of gold in
California in 1848, 87 per cent of the world's production was obtained
from placers. At present the placers of recent geologic age supply a
tenth to a fifth of the gold, and ancient or fossil placers in the
Transvaal supply another two-fifths. In the United States about a fourth
of the gold production comes from placers, mainly from California and
Alaska.

Placers are detrital or fragmental sediments containing the ore in
mechanical fragments, which are derived from the erosion and
transportation of solid-rock veins or lodes, sometimes called the
"mother lode." During the process of transportation and deposition there
is more or less sorting, because of differing density of the mineral
fragments, resulting in the segregation or concentration of the ore
minerals in certain layers or channels. Gold, because of its weight,
tends to work down toward bedrock, or into scoured or excavated portions
of stream channels. In a few cases it is carried in some quantity to the
sea and concentrated in beach sands. The processes are not unlike the
mechanical concentration of ores by crushing and water sorting. Seldom,
however, do the processes go far enough in nature to produce an ore
which can be used directly without some further mechanical sorting. Ore
minerals concentrated in placers are those which resist abrasion and
chemical solution during the processes of weathering and trans