226 



GEOLOGY, 



flow, and while each portion of the water entering a crevice or porous 

 bed may be able to keep its own mineral matter in solution, their 

 minghng may involve chemical changes resulting in the formation of 

 insoluble compounds, and therefore in deposition. This principle has 

 probably been involved in the filling of many fissures and crevices, 

 converting them into veins. (6) The escape of gases from water, whether 

 from increase of temperature or by the disturbance of water, some- 

 times 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 sur- 

 face, where evaporation is active. Under proper conditions, however, 

 deposition may take place at any level reached by water. 



Mechanical Work, 



The mechanical work of ground-water is relatively unimportant. 

 Wherever it is organized into 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. 



Fig. 202. — Diagram to illustrate the general form and relations of caves developed 

 by solution. The black portions represent the cavern spaces. Some limestone 

 sinks are represented on the surface above. 



RESULTS OF THE WORK OF GROUND-WATER. 



Weathering. — ^Where the solution efl'ected by ground-water in any 

 locahty 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 be 

 removed the rock is changed from sandstone to sand. If a complex 

 crystalline rock contains among its many minerals some one which is 



