At localities where impervious formations (clay, etc.) occur
at or near the surface, they hold the water and form a
superficial zone of saturation. This condition makes trench
construction and maintenance difficult, and cave shelters can
usually only be made by providing means of sinking through the
saturated zone. The surface saturated zone often dries out in
summer.

In pervious, or almost pervious rocks, the zone of saturation,
or ground-water level, lies at much lower depth, and may
permit of the construction of field works as well as cave
shelters above it.

Underground water bearing horizons and water bearing faults
should be avoided in locating field works.

Wherever there is any uncertainty about the underground water
conditions, test shafts or bore holes should always be made in
advance of the construction of extensive deep works.

EFFECT OF THE WAR ON THE SCIENCE OF ECONOMIC GEOLOGY

In general, the war required an intensive application of geology along
lines already pretty well established under peace conditions. Much was
done to make the application more direct and effective, and a vast
amount of geologic information was mobilized. The general result was a
quickened appreciation of the possibilities of the use of geology for
practical purposes. Perhaps the most important single result was a wider
recognition of the real relations of mineral resources to human
activities, and of the international phases of the problem. More
specifically, there was a most careful stock-taking of mineral resources
and a consideration of the "why" of their commercial use. Many new
resources were found, as well as new ways to utilize them.

FOOTNOTES:

[60] Now known as Economic Liaison Committee.

[61] _Military geology and topography_, Herbert E. Gregory, Editor.
Prepared and issued under the auspices of Division of Geology and
Geography, National Research Council, Yale Univ. Press, New Haven, 1918.

[62] Davis, W. M., _Handbook of Northern France_, Harvard Univ. Press,
Cambridge, 1918.

[63] For more detailed description of this subject the reader is
referred to The use of geology on the Western Front, by Alfred H.
Brooks, _Prof. Paper 128-D_, _U. S. Geol. Survey_, 1920.

CHAPTER XX

GEOLOGY AND ENGINEERING CONSTRUCTION

Economic applications of geology are by no means confined to mineral
resources (including water and