36 GROUND-WATER 



another extracted from the rock. Thus the lime carbonate of a 

 shell imbedded in rock may be removed, molecule by molecule, 

 and some other substance, such as silica, left in its place. When 

 the process is complete, the substance of the shell has been com- 

 pletely removed, though its form and structure are preserved in 

 the new material. Buried logs may be converted into stone by the 

 substitution ot mineral matter for the vegetable tissue (Fig. 22). 



3. J Materials dissolved from rock at one point may be de- 

 posited in other rock elsewhere. Thus a third type of change, 

 addition, is effected. Rock may at one time and place be rendered 

 porous by the subtraction of some of its substance, and the open- 

 ings thus formed may later become the receptacles of deposits from 

 solution. This is exemplified in the stalactitic deposits of many 

 caves. Not uncommonly cracks and fissures are filled with mineral 

 matter deposited by the waters which pass through them, making 

 veins. 



4. A further series of changes is effected by ground-water 

 when the mineral matter it contains enters into combination with 

 the mineral matter through which it passes. In the long course of 

 time, changes of this sort may be so great as to change rock com- 

 pletely. 



Importance of solution. Calculations have been made which 

 illustrate in a measure the quantitative importance of solution by 

 ground-water. Most of the mineral matter dissolved in streams 

 was contributed by ground-water (springs, etc.) flowing to them, 

 and the amount in stream water is determined readily. The 

 Thames River 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. 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. 

 Such figures make it clear that ground-water is an effective agent 

 in the lowering of land surfaces. It is estimated that something 

 like one-third as much mattei is carried to the sea in solution as in 

 sediment. 



The importance of the solution effected by ground-water is 

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

 sea has been taken to it in solution by waters flowing from the land. 

 The amount of salt is stupendous (Chapter VI). Furthermore, 

 most of the limestone of the earth has been extracted from sea- 



