206 GEOLOGY 



times causes its gases to escape, and so causes the deposition of 

 mineral matter held in solution. 



The deposition of material held in solution is most notable at 

 two zones, one below that of most active solution, and the other 

 at the surface, where evaporation is active. Under proper con- 

 ditions, however, deposition may take place at any level reached 

 by water. 



Mechanical Work 



The mechanical work of ground-water is relatively unimportant. 

 Where it flows in definite streams, the channels through which it 

 flows are likely to be increased by mechanical erosion as well as 

 by solution. Either beneath the surface, or after the streams 

 issue, the mechanical sediment carried will be deposited. 



RESULTS OF THE WORK OF GROUND-WATER 



Weathering. Where the solvent work of ground-water is slight 

 and equally distributed, the result is to make the rock porous. 

 If, for example, some of the cement of sandstone is dissolved, the 

 texture of the rock becomes more open; but if all the cement is 

 removed, the rock is changed from sandstone to sand. If a com- 

 plex crystalline rock contains among its many minerals some one 

 which is more soluble than the others, that one may be dissolved. 

 This has the effect of breaking up the rock, since each mineral acts 

 as a binder for the rest. It might happen that no one of the minerals 

 is dissolved completely, but that some one of them is decomposed 

 by water, and certain of its constitutents removed. Such change 

 would be likely to cause the mineral so affected to crumble, and 

 with its crumbling, if it is an important constituent of the rock, 

 the integrity of the rock is destroyed. The increase in volume 

 attendant on hydration, etc., sometimes leads to the disruption of 

 rock. These are phases of weathering. 



Caverns. 1 Where local solution is very great, results of anol her 

 sort may be effected. In formations like limestone, which are rela- 

 tively soluble, considerable quantities of material are frequently 



1 For a racy and interesting account of caverns see Shaler's Aspects of 

 the Earth. 



