formation of these
deposits are pretty well understood. Certain constituents of the
original rock are leached out and carried away, leaving other
constituents, as oxides and hydrates, in sufficiently large percentage
in the mass to be commercially available. The accumulation of large
deposits depends on the existence of climatic and erosional conditions
which determine that the residual deposit shall remain in place rather
than be carried off by erosion as fast as made. In the glaciated parts
of the world, deposits of this nature have usually been removed and
dispersed in the glacial drift.

When the minerals of these deposits are eroded, transported, and
redeposited in concentrated form, they come under the class of placer or
sedimentary deposits described under the following heading. There are of
course many intermediate stages, where the residual deposit is only
locally moved and where the distinction between this class of deposits
and that next described is an arbitrary one.

MINERAL DEPOSITS FORMED DIRECTLY AS PLACERS AND SEDIMENTS

Mineral deposits of this class are of large value, including as they do
salt, gypsum, potash, sulphur, phosphates, nitrates, and important
fractions of the ores of iron, manganese, gold, tin, tungsten, platinum,
and precious stones; also many common rocks of commercial importance.
The minerals of these deposits are derived from the weathering and
erosion of land surfaces, either igneous or sedimentary. They are
deposited both under air and under water, both mechanically and
chemically (in part by the aid of organisms). These deposits form the
principal type of _syngenetic_ deposits (p. 32); the term _sedigenetic_
deposits has also been applied to them.

MECHANICALLY DEPOSITED MINERALS

Mechanical erosion of preexisting mineral deposits or rocks and their
transportation, sorting, and deposition are responsible for the placers
of gold, tin, tungsten, platinum, and various precious stones, and for
certain iron sands and conglomerates. Sands, sandstones, shales, and
certain clays and bauxites also belong in this group. These deposits may
be formed under air or under water, and under various climatic and
topographic conditions. During the process of formation the minerals of
differing density are more or less sorted out and tend to become
segregated in layers. The process is not unlike the artificial process
of mechanical concentration where ores are crushed, shaken up, and
treate