shattering of the overlying
sediments.

Gypsum is fairly soluble in ground-water, and sink-holes and solution
cavities are often developed in gypsum deposits. These may allow the
inwash of surface dirt and also may interfere with the mining.

All the important commercial gypsum deposits are believed to have been
formed by evaporation of salt water in the manner indicated. Small
quantities of gypsum are formed also when pyrite and other sulphides
oxidize to sulphuric acid and this acid acts on limestone. Thus gypsum
is found in the oxide zones of some ore bodies. These occurrences are of
no commercial significance.

MICA

ECONOMIC FEATURES

The principal use of sheet mica is for insulating purposes in the
manufacture of a large variety of electrical equipment. The highest
grades are employed particularly in making condensers for magnetos of
automobile and airplane engines and for radio equipment, and in the
manufacture of spark plugs for high tension gas engines. Sheet mica is
also used in considerable amounts for glazing, for heat insulation, and
as phonograph diaphragms. Ground mica is used in pipe and boiler
coverings, as an insulator, in patent roofing, and for lubricating and
decorative purposes.

India, Canada, and the United States are the important sheet
mica-producing countries, before the war accounting for 98 per cent of
the world's total. India has long dominated the sheet mica markets of
the world, and will probably continue to supply the standard of quality
for many years. The bulk of the Indian mica is consumed in the United
States, Great Britain, and Germany. The mica of India and the United
States is chiefly muscovite. Canada is the chief source of amber mica
(phlogopite), though other deposits of potential importance are known in
Ceylon and South Africa. Canadian mica is produced chiefly in Quebec and
Ontario, and is exported principally to the United States.

Important deposits of mica (principally muscovite) are also known in
Brazil, Argentina, and German East Africa. Large shipments were made
from the two former countries during the war, both to Europe and the
United States, and Brazil particularly should become of increasing
importance as a producer of mica. The deposits in German East Africa
were being quite extensively developed immediately before the war and
large shipments were made to Germany in 1913.

The United States is the largest consumer of sheet mica and mica
splittings, a