44 



theless obtain very different results, as a difference of a foot or 

 two may mean the missing of a certain channel, as indicated in 

 Fig. 21. The waters in limestone are generally hard but are not 

 commonly otherwise mineralized. 



Waters of Granites, Gneisses and Schists. Granites and 

 gneisses are very dense and possess very small pore spaces, and 

 most of these rocks hold very little water. In schists, however, 

 considerable water often penetrates along the foliation planes and 



is held by the rock, 

 but such water is 

 given up very slowly 

 and is not important 

 as a source of supply. 

 It is along the 

 joints in these rocks 

 that the largest sup- 

 plies are obtained. 

 (See Fig. 22.) These 

 joints are most com- 

 mon near the surface 

 and diminish in num- 



FIG. 22. Wells in jointed rocks. 



ber and in definiteness as the depth increases. For this reason 

 the water supplies from such rocks, if obtained at all, are usually 

 found within 200 or 300 feet of the surface. It is generally use- 

 less to go deeper than 500 feet for waters in these crystalline 

 rocks, although in some places, as at Atlanta, Ga., water is said 

 to have been obtained at depths as great as 1600 feet. 



Safety of Rock Waters. The safety of the water supplies 

 used for drinking purposes when near any source of pollution, de- 

 pends principally upon the character of the openings through 

 which the water passes, and this in turn depends on the nature of 

 the materials in which the water occurs or through which it has 

 passed. 



In passing downward through sand surface waters are sub- 

 jected to natural nitration, especially in the finer varieties, and 



