in a bituminous limestone, but
little seems to be known of their origin. The only other emerald
locality of commercial importance is in the Ural Mountains of Siberia.
Emeralds have been found in pegmatite dikes in North Carolina and New
England, but the production is insignificant.

Tourmaline is a complex hydrous silicate of aluminum and boron, with
varying amounts of magnesium, iron, and alkalies. It is a rather common
mineral in silicated zones in limestones near igneous contacts, but gem
tourmalines are found principally in pegmatite dikes. They have a wide
variety of colors, the red and green gems being the most prized. Maine,
California, and Connecticut are the principal American producers.

Turquoise is a hydrated copper-aluminum phosphate. It is found in
veinlets near the surface in altered granites and other igneous rocks.
It is usually associated with kaolin and frequently with quartz, and is
believed to have been formed by surface alterations. In the United
States it is produced chiefly in Nevada, Arizona, and Colorado.

In general the principal gem minerals, except pearl and turquoise, occur
as original constituents in igneous intrusives, usually of a pegmatite
or peridotite nature. Sapphire, ruby, emerald, and tourmaline result
also from contact metamorphism of sediments in the vicinity of igneous
rocks. Weathering softens the primary rocks, making it possible to
separate the gem stones from the matrix. When eroded and transported the
gems are concentrated in placers.

SALT

ECONOMIC FEATURES

The principal uses of salt are in the preserving and seasoning of foods
and in chemical industries. Chemical industries require salt for the
manufacture of many sodium compounds, and also as a source of
hydrochloric acid and chlorine. A minor use of salt is in the making of
glazes and enamel on pottery and hardware.

Because of the wide distribution of salt in continental deposits and
because of the availability of ocean and salt-lake brines as other
sources, most countries of the world either possess domestic supplies of
salt adequate for the bulk of their needs, or are able to obtain
supplies from nearby foreign countries. Certain sea salts preferred by
fish packers and other users are, however, shipped to distant points.
About a fifth of all the salt consumed in the world annually is produced
in the United States, and other large producers are Great Britain,
Germany, Russia, China, India, and France.