THE WORK OF GROUND-WATER. 225 



that the average amount of this material dissolved from the limestone 

 area drained by this stream is 143^ tons per square mile per year. It 

 is estimated that, on the average, something like one-third as much 

 matter is carried to the sea in solution as in the form of sediment, and 

 that by this process alone land areas would be lowered something 

 like one foot in 13,000 years. ^ 



Deposition of mineral matter from solution. — The deposition of 

 material from solution is effected in several waj^s. (1) It is some- 

 times deposited by evaporation. This is well shown where water seeps 

 out on arid lands. The same process is illustrated when water is boiled. 

 (2) Reduction of temperature often occasions deposition. In gen- 

 eral, hot water is a better solvent of mineral matter than cold,' and 

 if it issues with abundant mineral matter in solution the precipita- 

 tion 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 (alga^ 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.* The influ- 

 ence 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 pressure. Pressure increases the 

 solvent power of water with respect to minerals directly; it produces 

 the same effect indirectly by its effect on the solution of gases. As 

 water charged with gas comes to the surface, the pressure is reheved 

 and some of the gas escapes. Such mineral matter as was held in solu- 

 tion by the help of the gas which escapes is then precipitated. (5) Pre- 

 cipitation is also sometim^es effected by the mingHng of waters con- 

 taining different mineral substances in solution. Such mingHng of 

 solutions would be most common along lines of ready subterranean 



^ Prestwich, Q. J. Geol. Soc, Vol. XXVIII, p. Ixvii. 



' Reade. Liverpool Geol. Soc, 1S76 and 1884. 



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

 the influence of gases in solution in the water, 



* Weed. The Formation of Hot Springs Deposits.. Excursion to the Rocky Moun- 

 tains. Compte Rendu. Fifth Session of the International Geological Congress, p. 360, 

 and Ninth Ann. Rept. U. S. Geol. Surv., pp. 613-76. Also B. M. Davis, Science, 

 Vol. VI, pp. 145-57, 1897. 



