dentification and correlation of rock formations, and
as a guide to further drilling. Failure to preserve samples has often
led to useless and expensive duplication of work.

The problem of water supply in some localities is comparatively simple
and easy. In other areas there is an infinite variety of geologic
conditions which affect the problem, and the geologist finds it
necessary to bring to bear all the scientific knowledge of any sort
which can be used,--particularly knowledge in relation to the type of
rock, the stratigraphy and the structure.

SURFACE WATER SUPPLIES

Where underground water is not abundant or not cheaply available, or
where larger amounts of water are needed, as in large cities or for
irrigation purposes, surface water is used. In general, surface waters
are more likely to be contaminated by vegetable and animal matter and
to require purification for drinking purposes.

Surface waters are also used for irrigation, water power, drainage, the
carrying of sewage, etc. This great variety of uses brings the
consideration of surface waters into many fields other than geology, but
an understanding and interpretation of the geological conditions is none
the less fundamental. This is evidenced by the inclusion of geologic
discussions in most textbooks of hydrology, and in the reports of the
Hydrographic Branch of the U. S. Geological Survey. The very fact that
this important branch of governmental investigation is in a charge of
the U. S. Geological Survey indicates its close relation to geology.

The principles of geology used in the study of surface waters relate
chiefly to physiography (see Chapter I). It is usually necessary to know
the total quantity of flow, its annual and seasonal variation, and the
possible methods of equalization or concentration; the maximum quantity
of flow, the variation during periods of flood, and the possibilities of
reduction or control; the minimum flow and its possible modification by
storage or an auxiliary supply. These questions are obviously related to
the size and shape of the catchment area, the topography, the rock
structure, the relation between underground flow or absorption and the
runoff, and other physiographic factors. Quoting from D. W. Mead:[12]

Geological conditions are frequently of great importance in
their influence on the quantity and regularity of runoff. If
the geological deposits of the drainage area are highly
impervio