uld be a
comparatively easy one, but usually they do not. Often they are faulted
and folded and mashed to such an extent that it is difficult to go
behind the superposed structural features to the original conditions in
order to work out the geologic history. Not only is structural study
necessary for the interpretation of geologic history, but it is often
more directly applicable to economic problems,--as when, for instance,
ore deposits have been formed in the cracks and joints of rocks, and the
ore deposits themselves have been faulted and folded. Water resources
are often located in the cracks and other openings of rocks, and are
limited in their distribution and flow because of the complex attitude
of deformed rocks. Oil and gas deposits often bear a well-defined
relation to structural features, the working out of which is almost
essential to their discovery.

It is not desirable to stop with the merely descriptive aspects of
structural geology, as is so often done; for much light can be thrown on
the economic applications of this subject by consideration of the
underlying principles of mechanics,--involving the relations of earth
stresses to rock structures. The mere field mapping and description of
faults and joints is useful, but in some cases it is necessary to go a
step further and to ascertain the mechanical conditions of their origin
in order to interpret them clearly. If, for illustration, there are
successive groups of mineralized veins in a mining camp, the later ones
cutting the earlier ones, these might be treated as separate structural
units. But if it can be shown that the several sets of veins have formed
from a single movement, that there is no sharp genetic separation
between the different sets and that they are a part of a single system,
this interpretation throws new light on exploration and development, and
even on questions of ownership and extralateral rights (Chapter XVI).

PHYSIOGRAPHY

Physiography is a phase of geology which investigates the surface
features of the earth. It has to do not only with the description and
classification of surface forms, present and past (physical geography or
geomorphology), but with the processes and history of their development.
The subject is closely related to geography, climatology, sedimentation,
and hydrology. As one of the latest phases of geology to be organized
and taught, its economic applications have been comparatively recent and
are not yet