es are soft and clayey, and grind down
to a mass which is dry and powdery, and muddy in wet weather. Basalt and
related rocks resist abrasion, and cement well. Granites and other
coarse-grained igneous rocks do not cement well and are not resistant to
abrasion. Many sandstones are very hard and brittle and resist abrasion,
but do not cement.

The application of geology on a large scale to the study of sources and
qualities of crushed stone is now being required in connection with the
great state and national projects of highway building. This work is by
no means confined to a mere testing of the physical qualities of
road-building materials found along the proposed route, but includes a
careful study of their geologic occurrence, distribution, and probable
amounts. In certain of the northern states specialists in glacial
geology are preferred for this purpose.

STONE FOR METALLURGICAL PURPOSES

The use of limestone and other rock for metallurgical fluxes is
dependent very largely on chemical composition. Comparatively few
limestones are sufficiently pure for this purpose. For furnace linings,
the quartzite or ganister must be exceptionally pure. The field search
for rocks of the necessary composition has required geologic service.

CLAY

For a variety of uses to which clay is put, it is necessary to know its
degree of plasticity, tensile strength, shrinkage (both under air and
fire), fusibility, color, specific gravity, and chemical properties. The
testing of clay for its various possible uses is a highly specialized
job, usually beyond the range of a geologist, although certain
geologists have been leaders in this type of investigation. More
commonly within the range of a geologist are questions concerning
origin, field classification, distribution, quantities, and other
geologic conditions affecting quality and production.

Clay originates from the weathering of common rocks containing
silicates, by pretty well understood weathering processes (see Chapter
II). It may remain in place above the parent rock, or may be transported
and redeposited, either on land or under water, by the agencies of air,
water, and ice. The kind of parent rock, the climatic conditions and
nature of the weathering, and the degree of sorting during
transportation, all determine the composition and texture of the
resulting clay,--with the result that a classification on the basis of
origin may indicate the broad group characteristics