Quotation from: General Science

Written by: Bertha M. Clark


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.

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