Rocks and minerals are not all equally soluble in water; some are so little soluble that it is years before any change becomes apparent, and the substances are said to be insoluble, yet in reality they are slowly dissolving. Other rocks, like limestone, are so readily soluble in water that from the small pores and cavities eaten out by the water, there may develop in long centuries, caves and caverns (Fig. 30). Most rock, like granite, contains several substances, some of which are readily soluble and others of which are not readily soluble; in such rocks a peculiar appearance is presented, due to the rapid disappearance of the soluble substance, and the persistence of the more resistant substance (Fig. 31). We see that the solvent power of water is constantly 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.]