[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.

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 deposition by large rivers is so great that in some places channels fill up and must be dredged annually, and vessels are sometimes caught in the deposit and have to be towed away.

Running water in the form of streams and rivers, by carrying sand particles, stones, and rocks from high slopes and depositing them at lower levels, wears away land at one place and builds it up at another, and never ceases in its work of changing the nature of the earth’s surface (Fig. 35).

[Illustration:  FIG. 34.—­Deposit left by running water.]