GROUND-WATER 205 



Deposition of mineral matter from solution. The deposition of 

 material from solution is effected in several ways. (1) It is some- 

 times deposited by evaporation. This is well shown where water 

 seeps out on arid lands. (2) Reduction of temperature often occa- 

 sions deposition. In general, hot water is a better solvent of mineral 

 matter than cold/ and if it issues with abundant mineral matter 

 in solution, the precipitation of some of it is likely to take place. 

 (3) Plants sometimes cause the precipitation of mineral matter from 

 solution. About some hot springs, even where the temperature 

 of the water is very high, small plants of low type (algse) grow in 

 profusion. In ways which are not perfectly understood, these 

 algse extract the mineral matter from the hot water. They are 

 now thought to be a chief factor in the deposits about the hot 

 springs of the Yellowstone Park. 2 The influence of organisms on 

 precipitation from solution is not confined to the waters of hot 

 springs. (4) A fourth factor involved in the deposition of mineral 

 matter from solution is relief of pressure. Pressure increases 

 the solvent power of water. It also increases the amount of gas 

 which may be dissolved, and the gas affects the solvent power of 

 the water. As water charged with gas comes to the surface, the 

 pressure is relieved and some of the gas escapes. Such mineral 

 matter as was held in solution by the help of the gas which escapes, 

 is then precipitated. (5) Precipitation is also sometimes effected 

 by the mingling of waters containing different mineral substances 

 in solution. Such mingling of solutions is most common along 

 lines of ready subterranean flow, and while each portion of the 

 water entering a crevice or porous bed might have been able to 

 keep its own mineral matter in solution, their mingling may involve 

 chemical changes, resulting in the formation of insoluble compounds, 

 and therefore in deposition. This principle has probably been 

 involved in the filling of many fissures and crevices, converting 

 them into veins. (6) The disturbance of water, as by waves, some- 



1 This is not true in the case of minerals, such as lime carbonate, dissolved 

 under the influence of gases in solution in the water. 



2 Weed. The Formation of Hot Springs Deposits; Excursion to the 

 Rocky Mountains, and Ninth Ann. Kept. U. S. Geol. Surv., pp. 613-76; and 

 B. M. Davis. Science, Vol. VI, pp. 145-57, 1897. 



