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.

67. Streams. Streams usually carry mud and sand along with them;
this is particularly well seen after a storm when rivers and brooks
are muddy. The puddles which collect at the foot of a hill after a
storm are muddy because of the particles of soil gathered by the water
as it runs down the hill. The particles are not dissolved in the
water, but are held there in suspension, as we call it technically.
The river made muddy after a storm by suspended particles usually
becomes clear and transparent after it has traveled onward for miles,
because, as it travels, the particles drop to the bottom and are
deposited there. Hence, materials suspended in the water are borne
along and deposited at various places (Fig. 34). The amount of