ding force of growing crystals, to
hydrostatic pressure of the solutions, and to laccolithic intrusions. On
the other hand, the uniform association of other salt and gypsum
deposits with sedimentary rocks, and the absence of igneous rocks,
suggest that these deposits may have had essentially a sedimentary
origin, and that they have been modified by subsequent deformation and
alteration. The origin is still uncertain.

Other mineral deposits formed under much the same conditions as salt are
gypsum, potash, borax, nitrates, and minerals of bromine; and in a study
of the origin of salt deposits these minerals should also be
considered.

TALC AND SOAPSTONE

ECONOMIC FEATURES

Soapstone is a rock composed mainly of the mineral talc. Popularly the
terms _talc_ and _soapstone_ are often used synonymously. The softness,
greasy feel, ease of shaping, and resistance to heat and acids of this
material make it useful for many purposes. Soapstone is cut into slabs
for laundry tubs, laboratory table tops, and other structural purposes.
Finer grades are cut into slate pencils and acetylene burners. Ground
talc or soapstone is used as a filler for paper, paint, and rubber
goods, and in electrical insulation. Fine grades are used for toilet
powder.

Pyrophyllite (hydrated aluminum silicate) resembles talc in some of its
properties and is used in much the same way. Fine English clays (p. 85)
are sometimes used interchangeably with talc as paper filler.

The United States produces nearly two-thirds of the world's talc. The
other large producers are France, Italy, Austria, and Canada (Ontario).

The United States is independent of foreign markets for the bulk of its
talc consumption, but some carefully prepared talc of high quality is
imported from Canada, Italy, and France. Italy is our chief source of
talc for pharmaceutical purposes, though recently these needs have been
largely supplied by high-grade talc from California. In the United
States, Vermont and New York are the leading producers of talc and
Virginia of soapstone slabs. Reserves are large.

GEOLOGIC FEATURES

Talc is hydrated magnesium silicate, as is also serpentine, a mineral
with which talc is closely associated. Both are common alteration
products of magnesian silicate minerals such as olivine, pyroxene, and
amphibole. Talc is also derived from the recrystallization of magnesian
carbonates.

Talc deposits consist of lenses and bands in metamorphic limes