production has been small, consisting principally of flint pebbles from
the California beaches, and artificial pebbles made from rhyolite in
Nevada and quartzite in Iowa. War experience demonstrated the
possibility of using the domestic supply in larger proportion, but the
grade is such that in normal times this supply will not compete with
importations.

Feldspar as an abrasive is used mainly in scouring soaps and
window-wash. Domestic supplies are ample. The principal use of feldspar
is in the ceramic industry and the mineral is discussed at greater
length in the chapter on common rocks (p. 86).

For the large number of abrasives produced from silica, outside of flint
pebbles, domestic sources of production are ample. Siliceous rocks are
available almost everywhere. For particular purposes, however, rocks
possessing the exact combinations of qualities which make them most
suitable are in many cases distinctly localized. _Millstones and
buhrstones_, used for grinding cereals, paint ores, cement rock,
fertilizers, etc., are produced chiefly in New York and Virginia; partly
because of trade prejudice and tradition, about a third of the American
requirements are imported from France, Belgium, and Germany.
_Grindstones and pulpstones_, used for sharpening tools, grinding
wood-pulp, etc., come mainly from Ohio and to a lesser extent from
Michigan and West Virginia; about 5 per cent of the consumption is
imported from Canada and Great Britain. _Hones_, _oilstones_, and
_whetstones_ are produced largely from a rock called "novaculite" in
Arkansas, and also in Indiana, Ohio, and New England; imports are
negligible. _Flint linings_ for tube-mills were formerly imported from
Belgium, but American products, developed during the war in
Pennsylvania, Tennessee, and Iowa, appear to be wholly satisfactory
substitutes. _Diatomaceous earth_ is produced in California, Nevada,
Connecticut, and Maryland, and _tripoli and rottenstone_ in Illinois,
Missouri, and Oklahoma; domestic sources are sufficient for all needs,
but due to questions of back-haul and cost of rail transportation there
has been some importation from England and Germany.

GEOLOGIC FEATURES

The geologic features of silica (quartz), feldspar, and diamonds are
sufficiently indicated elsewhere (Chapter II; pp. 84, 196, 86, 291-292).

Diatomaceous earth is made up of remains of minute aquatic plants. It
may be loose and powdery, or coherent like chalk. It is of sedim