entary
origin, accumulated originally at the bottoms of ponds, lakes, and in
the sea.

Tripoli and rottenstone are light, porous, siliceous rocks which have
resulted from the leaching of calcareous materials from various
siliceous limestones or calcareous cherts in the process of weathering.

Grinding pebbles are derived from the erosion of limestone or chalk
formations which contain concretions of extremely fine-grained and dense
chert. Under stream and wave action they are rounded and polished. The
principal sources are ocean beaches.

Corundum as an abrasive is the mineral of this name--made up of
anhydrous aluminum oxide. Emery is an intimate mechanical mixture of
corundum, magnetite, and sometimes spinel. Corundum is a product of
contact metamorphism and also a result of direct crystallization from
molten magma. Canadian corundum occurs as a constituent of syenite and
nepheline-syenite in Lower Ontario. In North Carolina and Georgia, the
corundum occurs in vein-like bodies at the contact of peridotite with
gneisses and schists, and also in part in the peridotite itself. In New
York the emery deposits are segregations of aluminum and iron oxides in
norite (a basic igneous rock). The emery of Greece and Turkey occurs as
lenses or pockets in crystalline limestones, and is the result of
contact metamorphism by intrusive granites.

Garnets result mainly from contact metamorphism, and commonly occur
either in schists and gneisses or in marble. The principal American
occurrences are of this type. Being heavy and resistant to weathering,
they are also concentrated in placers. The Spanish garnets are reported
to be obtained by washing the sands of certain streams.

Pumice is solidified rock froth formed by escape of gases from molten
igneous rocks at the surface. It is often closely associated with
volcanic ash, which is also used for abrasive purposes.

In general, the geologic processes entering into the formation of
abrasives cover almost the full range from primary igneous processes to
surface alterations and sedimentation.

ASBESTOS

ECONOMIC FEATURES

The principal uses of asbestos are in high-pressure packing in heat
engines, in thermal and electrical insulation, in fire-proofing, and in
brake-band linings.

The largest producers of asbestos are Canada (Quebec) and, to a
considerably less extent, Russia. United States interests have financial
control of about a fourth of the Canadian production, and