204 GEOLOGY 



matter, and the water, charged with organic matter in its descent 

 through the soil, is in condition to dissolve it. At greater depths 

 the water has become saturated to some extent, and, so far forth, 

 less active. Here, too, the movement is less free. The increased 

 pressure at considerable depths, on the other hand, facilitates 

 solution, which must be understood to take place under proper 

 circumstances in any zone reached by the water. 



Calculations have been made which illustrate in a measure the 

 quantitative importance of the solution effected by ground-water, 

 for the mineral matter dissolved in streams is largely from ground- 

 water. In the case of several streams, among them the Thames 

 and the Elbe, careful estimates of the amount of dissolved mineral 

 matter have been made. The Thames drains an area only about 

 one-tenth as large as the State of New York, but it is estimated to 

 carry about 1,500 tons of mineral matter in solution to the sea 

 daily. 1 From the uppermost 20,000 square miles of its drainage 

 basin, the Elbe is estimated to carry yearly about 1,370,000 tons 

 of mineral matter in solution. 



These figures make it clear that ground-water is an effect ive 

 agent in the lowering of land surfaces. It is estimated that some- 

 thing 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. 2 



The quantitative importance of the solution effected by ground- 

 water is shown in another way. It is probable that most of the 

 salt now in the sea has been taken to it in solution by waters flowing 

 in from the land. The amount of salt is stupendous (p. 289). 

 Furthermore, most of the limestone of the earth has been extracted 

 from sea-water, whither the larger part of it was carried by streams. 

 The aggregate 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. The 

 total amount of rock which must have been decomposed to yield 

 such quantities of calcium carbonate, sodium chloride, etc., is far 

 greater than all the rock now above sea-level. 



1 Geikie. Text-book of Geology, 3d ed., p. 378. 



2 Reade. Liverpool Geol. Soc., 1876 and 1884. 



