224 GEOLOGY. 



face. In the outermost zone of mantle rock the materials are usually 

 less soluble than below, for they often represent the residuum after the 

 soluble parts of the formation from which the}^ originated were dis- 

 solved 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 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 the quantitative 

 importance of the solution effected by ground-water. The springs of 

 Leuk (Switzerland) bring to the surface annually more than 2000 tons 

 of calcium sulphate (gypsum) in solution, and in the same time the 

 springs of Bath (England) bring up an amount of mineral matter in 

 solution sufficient to make a column 9 feet in diameter, and 140 feet 

 high.^ 



The amount of mineral matter in solution in streams is also sig- 

 nificant, for while stream-water is not all derived from ground-water, 

 much of it had such an origin. 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. Though the Thames drains 

 an area only about one-tenth as large as the State of New York, it is 

 estimated to carry about 1500 tons of mineral matter in solution to 

 the sea daily. ^ 



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 mat- 

 ter in solution. Estimates of the amounts of material carried to the 

 sea in solution by several rivers are given on pp. 102 and 103. Much of 

 this matter was brought to the rivers by waters which had been 

 underground before reaching the streams. 



From these figures it is clear that we have to reckon here with a 

 very considerable factor in the lowering of land surfaces. From the 

 amount of lime carbonate carried by the Thames it has been estimated 



^ Geikie. Text-book of Geology, 3d ed., p. 367. 

 2 Ibid., p. 378. 



