WORK OF GROUND-WATER 37 



water, whither the larger part of it was carried by streams, and the 

 abrogate amount of limestone is far greater than the amount of 

 salt in the sea. Some other sorts of rock, such as gypsum, of less 

 importance quantitatively, have had a similar history. 



In general, solution is probably most effective at a relatively 

 slight distance below the surface. In the mantle rock, the materials 

 are as a rule less soluble than below, for in many places they 

 represent the residuum after the soluble parts of the formation from 

 which they originated were dissolved out. Below this zone, the 

 rock contains more soluble matter, and the water, charged with 

 organic matter in its descent through the soil, is in condition to 

 dissolve it. At still greater depths the water has become saturated 

 to some extent, and, so far forth, less active. At great depths, too, 

 the movement is less free. Increased pressure on the other hand 

 facilitates solution at great depths. 



Deposition of mineral matter from solution. Mineral matter is 

 deposited from solution under various conditions, (i) Some of it is 

 deposited by evaporation. This is shown where water seeps out on 

 arid lands. (2) Reduction of temperature may occasion deposition. In 

 general, hot water is a better solvent of mineral matter than cold, 1 

 and if hot water issues with abundant mineral matter in solution, 

 some of it is likely to be precipitated on cooling. (3) Certain 

 plants cause the precipitation of mineral matter from solution, as 

 about some hot springs in which algae grow in profusion. These 

 little plants are a chief factor in the deposits about the hot springs 

 of Yellowstone Park. 2 (4) A fourth factor involved in the deposi- 

 tion of mineral matter is relief of pressure. Pressure increases the 

 solvent power of water directly; it also increases the amount of gas 

 which may be dissolved, and this in turn increases the solvent power 

 of the water for some minerals. As water charged with gas comes 

 to the surface, pressure is lessened, and some of the gas escapes. 

 In numerous cases, mineral matter is then precipitated. (5) Pre- 

 cipitation is sometimes effected by the mingling of waters contain- 

 ing different mineral substances in solution. Such mingling of 

 solutions is most common along lines of ready subterranean flow, 



1 This is not true in the case of minerals, such as the carbonates, dissolved and 

 held in solution under the influence of gases dissolved 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. 



