ates exports a small amount of crude gypsum to Canada,
principally for use in Portland cement manufacture. This exportation is
due to geographic location. The United States is the largest
manufacturer of plaster boards, insulating materials, and tile, and
exports large quantities of these products to Cuba, Australia, Japan,
and South America.

GEOLOGIC FEATURES

Gypsum is a hydrated calcium sulphate. It is frequently associated with
minor quantities of anhydrite, which is calcium sulphate without water,
and under the proper natural conditions either of these materials may be
changed into the other.

Common impurities in gypsum deposits include clay and lime carbonate,
and also magnesia, silica, and iron oxide. In the material as extracted,
impurities may range from a trace to about 25 per cent. _Gypsite_, or
gypsum dirt, is an impure mixture of gypsum with clay or sand found in
Kansas and some of the western states; it is believed to have been
produced in the soil or in shallow lakes, by spring waters carrying
calcium sulphate which was leached from gypsum deposits or from other
rocks.

Gypsum deposits, like deposits of common salt, occur in beds which are
the result of evaporation of salt water. Calcium makes up a small
percentage of the dissolved material in the sea, and when sea waters are
about 37 per cent evaporated it begins to be precipitated as calcium
sulphate. Conditions for precipitation are especially favorable in arid
climates, in arms of the sea or in enclosed basins which may or may not
once have been connected with the sea. Simultaneously with the
deposition of gypsum, there may be occasional inwashings of clay and
sand, and with slight changes of conditions organic materials of a limey
nature may be deposited. Further evaporation of the waters may result in
the deposition of common salt. Thus gypsum beds are found interbedded
with shales, sandstones, and limestones, and frequently, but not always,
they are associated with salt beds. The nature of these processes is
further discussed under the heading of salt (pp. 295-298).

The anhydrite found in gypsum deposits is formed both by direct
precipitation from salt water and by subsequent alteration of the
gypsum. The latter process involves a reduction of volume, and
consequently a shrinkage and settling of the sediments. The hydration of
anhydrite to form gypsum, on the other hand, involves an increase of
volume and may result in the doming up and