more or less incidentally discussed in other chapters.
We may consider more fully in this chapter the application of geology to
the general subject of water supplies.

From the geological point of view, water is a mineral,--one of the most
important of minerals,--as well as a constituent of other minerals. It
becomes a mineral resource when directly used by man. It is ordinarily
listed as a mineral resource when shipped and sold as "mineral water,"
but there is obviously no satisfactory line between waters so named and
water supplies in general, for most of them are used for the same
purposes and none of them are free from mineral matter. Water which is
pumped and piped for municipal water supply is as much a mineral
resource as water which is bottled and sold under a trade name. Likewise
water which is used for irrigation, water power, and a wide variety of
other purposes may logically be considered a mineral resource.

Notwithstanding the immense economic importance of water as a mineral
resource its value is more or less taken for granted, and considerations
of valuation and taxation are much less in evidence than in the case of
other mineral resources. Water must be had, regardless of value, and
market considerations are to a much less extent a limiting factor.
Economic applications of geology to this resource are rather more
confined to matters of exploration, development, total supply, and
conservation, than to attempts to fix money value.

DISTRIBUTION OF UNDERGROUND WATER

Free water exists in the openings in rocks where it is sometimes called
_hygroscopic_ water. There is also a large amount of water combined
molecularly with many of the minerals of rocks, in which form it is
called _water of constitution_. This water is fixed in the rock so that
it is not available for use, though some of the processes of rock
alteration liberate it and contribute it to the free water. The
immediate source of underground water, both free and combined, is mainly
the surface or rain waters. A subordinate amount may come directly from
igneous emanations or from destruction of certain hydrous minerals.
Ultimately, as already indicated, even the surface water originates from
such sources.

The openings in rocks consist of joints and many other fractures, small
spaces between the grains of rocks (pore space), and amygdaloidal and
other openings characteristic of surface volcanic rocks. Many of these
openings are capillary and