which it is
desirable to know for commercial purposes. For instance, residual clays
from the weathering of granite may be broadly contrasted with residual
clays formed by the weathering of limestone, and both differ in group
characteristics from clays in glacial deposits. Classification according
to origin also may be useful in indicating general features of depth,
quantity, and distribution. However, a genetic classification of clays
is often not sufficient to indicate the precise characteristics which
it is necessary to know in determining their availability for narrow and
special technical requirements. Furthermore, clays suitable for certain
commercial requirements may be formed in several different ways, and
classification based on specific qualities may therefore not correspond
at all to geologic classification based on origin.

Geologists have been especially interested in the causes of plasticity
of clay and in its manner of hardening when dried. In general these
phenomena have been found to be due to content of colloidal substances
of a clayey nature, which serve not only to hold the substance together
during plastic flow but to bind it during drying. The part played by
colloids in the formation of clays, as well as of many other mineral
products, is now a question which is receiving intensive study.

The same processes which produce clay also produce, under special
conditions, iron ores, bauxites, the oxide zones of many sulphide ore
bodies, and soils, all of which are referred to on other pages.

LIMITATIONS OF GEOLOGIC FIELD IN COMMERCIAL INVESTIGATION OF COMMON ROCKS

In general the qualities of the earth materials which determine their
availability for use are only to a minor extent the qualities which the
geologist ordinarily considers for mapping and descriptive purposes. The
usual geological map and report on a district indicate the distribution
and general nature of the common rocks, and also the extent to which
they are being used as mineral resources. Seldom, however, is there
added a sufficiently precise description, for instance of a clay, to
enable the reader to determine which, if any, of the many different uses
the material might be put to. The variety of uses is so great, and the
technical requirements for different purposes are so varied and so
variable, that it is almost impossible to make a description which is
sufficiently comprehensive, and at the same time sufficiently exact, to
giv