long future.

GEOLOGIC FEATURES

Mineral nitrates in general, and particularly those of soda and potash,
are readily soluble at ordinary temperatures. Mineral nitrate deposits
are therefore very rare, and are found only in arid regions or other
places where they are protected from rain and ground-water. The only
large deposits known are those of northern Chile and some extensions in
adjacent parts of Peru and Bolivia. These are located on high desert
plateaus, where there is almost a total absence of rain, and form
blankets of one to six feet in thickness near the surface. The most
important mineral, the sodium nitrate or Chile saltpeter, is mingled
with various other soluble salts, including common salt, borax minerals,
and potassium nitrate, and with loose clay, sand, and gravel. The
nitrate deposits occur largely around and just above slight basin-like
depressions in the desert which contain an abundance of common salt. The
highest grade material contains 40 to 50 per cent of sodium nitrate, and
material to be of shipping grade must run at least 12 to 15 per cent.

The origin of the nitrate beds is commonly believed to be similar to
that of beds of rock salt (pp. 295-298), borax, and other saline
residues. The source of the nitrogen was probably organic matter in the
soil, such as former deposits of bird guano, bones (which are actually
found in the same desert basin), and ancient vegetable matter. By the
action of nitrifying bacteria on this organic matter, nitrate salts are
believed to have formed which were leached out by surface and ground
waters, and probably carried in solution to enclosed bodies of water.
Here they became mingled with various other salts, and all were
precipitated out as the waters of the basins evaporated. Deliquescence
and later migration of the more soluble nitrates resulted in their
accumulation around the edges of the basins. The nitrate beds are thus
essentially a product of desiccation.

While the origin just set forth is rather generally accepted, several
other theories have been advanced. It has been suggested that the
deposits were not formed in water basins, but that ground water carrying
nitrates in solution has been and is rising to the surface,--where,
under the extremely arid conditions, it evaporates rapidly, leaving the
nitrates mixed with the surface clays. One group of writers accounts for
the deposits by the fixation of atmospheric nitrogen through electrical
phenome