stantly causing changes,
dissolving some mineral substances, and leaving others practically
untouched; eating out crevices of various shapes and sizes, and by
gradual solution through unnumbered years enlarging these crevices
into wonderful caves, such as the Mammoth Cave of Kentucky.

66. Constructive Action of Water. Water does not always act as a
destructive agent; what it breaks down in one place it builds up in
another. It does this by means of precipitation. Water dissolves salt,
and also dissolves lead nitrate, but if a salt solution is mixed with
a lead nitrate solution, a solid white substance is formed in the
water (Fig. 32). This formation of a solid substance from the mingling
of two liquids is called precipitation; such a process occurs daily in
the rocks beneath the surface of the earth. (See Laboratory Manual.)

[Illustration: FIG. 32.--From the mingling of two liquids a solid is
sometimes formed.]

Suppose water from different sources enters a crack in a rock,
bringing different substances in solution; then the mingling of the
waters may cause precipitation, and the solid thus formed will be
deposited in the crack and fill it up. Hence, while ground water tends
to make rock porous and weak by dissolving out of it large quantities
of mineral matter, it also tends under other conditions to make it
more compact because it deposits in cracks, crevices, and pores the
mineral matter precipitated from solution.

These two forces are constantly at work; in some places the
destructive action is more prominent, in other places the constructive
action; but always the result is to change the character of the
original substance. When the mineral matter precipitated from the
solutions is deposited in cracks, _veins_ are formed (Fig. 33), which
may consist of the ore of different metals, such as gold, silver,
copper, lead, etc. Man is almost entirely dependent upon these veins
for the supply of metal needed in the various industries, because in
the original condition of the rocks, the metallic substances are so
scattered that they cannot be profitably extracted.

[Illustration: FIG. 33.--Mineral matter precipitated from solution is
deposited in crevices and forms veins.]

Naturally, the veins themselves are not composed of one substance
alone, because several different precipitates may be formed. But there
is a decided grouping of valuable metals, and these can then be
readily separated by means of electricity.