THE CONSTRUCTIVE ACTION OF WATER 361 



The 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 partly by means 

 of precipitation. Water dis- 

 solves salt, and also dis- 

 solves lead nitrate, but if a 

 salt solution is mixed with a 

 lead nitrate solution, a solid 

 white substance is formed in 

 the water. This formation 



FIG. 227. From the mingling of two 

 liquids a solid is sometimes formed. 



of a solid substance from the 



mingling of two liquids is 



called precipitation (Fig. 227). Such a process occurs daily 



in the rocks beneath the surface of the earth. 



Suppose water from different sources enters a crack in a 

 rock, bringing different substances in solution. 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 to make it more compact because it deposits in cracks, 

 crevices, and pores mineral matter precipitated from solutions. 

 When the mineral matter precipitated from the solutions is 

 deposited in cracks, veins are formed, which may consist of 

 the ore of different metals, such as gold, silver, copper, or lead 

 (Fig. 228). 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 ex- 

 tracted. The veins are not always composed of one substance, 

 because several different precipitates may be formed. But 

 there is a decided grouping of valuable metals. 



