losed basin, and the peculiar climatic and
topographic conditions which caused their formation have been the
subject of much speculation. This subject is further treated in the
discussion of common salt beds (pp. 295-298).

In the United States the deposits at Searles Lake, California, have been
produced by the same processes on a smaller scale. In this case
evaporation has not been carried to completion, but the crystallization
and separation out of other salts has concentrated the potassium (with
the magnesium) in the residual brine or "mother liquor." The deposits of
this lake or marsh also contain borax (see p. 276), and differ in
proportions of salts from the Stassfurt deposits. This is due to the
fact that they were probably derived, not from ocean waters, but from
the leaching of materials from the rocks of surrounding uplands,
transportation of these materials in solution by rivers and ground
waters, and concentration in the desert basin by evaporation.

The alkali lakes of Nebraska are believed to be of very recent geologic
origin. They lie in depressions in a former sand dune area, and contain
large quantities of potash supposedly accumulated by leaching of the
ashes resulting from repeated burnings of the grass in the adjacent
country.

Of other natural mineral sources, alunite is the most important. The
principal deposits worked are at Marysville, Utah, but the mineral is a
rather common one in the western part of the United States, associated
with gold deposits, as at Goldfield, Nevada. Alunite occurs as veins and
replacement deposits, often in igneous associations, and is supposed to
be of igneous source. Its origin is referred to in connection with the
Goldfield ores (p. 230).

FOOTNOTES:

[15] Gale, Hoyt S., The potash deposits of Alsace: _Bull. 715-B, U. S.
Geol. Survey_, 1920, pp. 17-55.

[16] Gale, Hoyt S., Potash deposits in Spain: _Bull. 715-A, U. S. Geol.
Survey_, 1920, pp. 1-16.

CHAPTER VIII

THE ENERGY RESOURCES--COAL, OIL, GAS (AND ASPHALT)

COAL

ECONOMIC FEATURES

Coal overshadows all other mineral resources, except water, in
production, value, and demand. It is the greatest of the energy
sources--coal, petroleum, gas, and water power. Roughly two-thirds of
the world's coal is used for power, one-sixth for smelting and
metallurgical industries, and one-sixth for heating purposes. Coal
constitutes over one-third of the railroad tonnage of the United States
and is the