ions.

UNDERGROUND AND SURFACE WATERS IN RELATION TO EXCAVATION AND CONSTRUCTION

Scarcely more than a mention of this subject is necessary. In mining,
the pumping charge is one of the great factors of cost. A forecast of
the amount and flow of water to be encountered in mining is based on the
geologic conditions. The same is true in excavating tunnels, canals, and
deep foundations. Detailed study of the amount and nature of water in
the rock and soil of the Panama Canal has been vital to a knowledge of
the cause and possibilities of prevention of slides. Rock slides in
general are closely related to the amount and distribution of the water
content.

The importance of ground-water as a detriment in military operations was
shown during the recent war in trenching and other field works. At the
outset, with the possible exception of the German army, a lack of
scientific study of ground-water conditions led to much unnecessary
difficulty. It soon became necessary to study and map the water
conditions in great detail in advance of operations. Much of this work
was done by geologists (see Chapter XIX).

Geological considerations are involved in a great variety of engineering
undertakings related to river and harbor improvements, dam sites, etc.,
mentioned in Chapter XX.

FOOTNOTES:

[12] Mead, Daniel W., _Hydrology_: McGraw-Hill Book Co., New York, 1919,
pp. 447-448, 456.

CHAPTER VI

THE COMMON ROCKS AND SOILS AS MINERAL RESOURCES

ECONOMIC FEATURES OF THE COMMON ROCKS

Under the general heading of common rocks are included the ordinary
igneous, sedimentary, and "metamorphic" rocks, and the unconsolidated
clays, sands, and gravels characteristic of surface conditions, which
are mined and quarried for commercial use. Soils are closely related to
this group; but since they present special problems of their own, they
are discussed under a separate heading at the end of the chapter. Names
of the common rocks will be used with the general commercial
significance given them by the United States Geological Survey in its
mineral resource reports.

Because of their inexhaustible quantity and ready availability, the
value of the common rock products is not large per unit of weight; but
in the aggregate it ranks high among mineral products. In respect to
tonnage, common rocks constitute perhaps 10 per cent of the world annual
output of all mineral commodities (exclusive of water).

The greater tonnage of the c