38 GROUND-WATER 



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

 bed might have been able to keep its own mineral matter in solu- 

 tion, their mingling may involve chemical changes resulting in the 

 formation of insoluble compounds, and therefore in deposition. 

 This principle probably has been involved in the making of many 

 veins of ore. 



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 greatest. 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 me- 

 chanical 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, its effect is to make the rock porous. 

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

 rock becomes more porous; but if all the cement is removed, the 



Fig. 23. Diagram to illustrate the form and relations of caverns developed by 

 solution. The black spaces represent caverns. Small limestone sinks are repre- 

 sented at the surface where the roofs of caves have fallen in. 



rock is changed to sand. If a complex crystalline rock contains 

 among its minerals some one which is more soluble than the others, 

 that one may be dissolved. This has the effect of breaking up the 



