46 DOMESTIC WATER SUPPLIES FOR THE FARM 



contaminated waters penetrate them. In quartzitic rock, how- 

 ever, joint fractures may admit both water and polluting mate- 

 rials from the surface. 



Like the waters of clays and for the same reasons, the waters 

 of slates and shales suffer very little pollution. 



In the vicinity of buildings or settlements the waters of lime- 

 stone are frequently contaminated and unfit for use. This is not 

 because of the- amount of lime dissolved, but because of the fact 

 that the water falling on the surface as rain often plunges directly 

 through basins or sinks into the underground channels instead of 

 slowly filtering downward through the soil and into the rock, as 

 in most other materials. This water carries with it the impurities 

 washed or otherwise brought to the sink and bears them along 

 through underground passages to distant points. (See Figs. 8 

 and 24.) It is a common practice to dump manure, sewage and 



Sink 



Sink 



FIG. 24. Limestone passage connected with sinks. 



other refuse into these sinks, regardless of the fact that it will 

 eventually enter the underground water body. Fortunately, in 

 the United States many limestone regions are thinly inhabited, so 

 the danger is not perhaps so widespread here as elsewhere. When 

 springs which have been guarded from surface wash become 

 muddy after a rain it is safe to assume that surface impurities 

 have had access to the ground water through sinks or otherwise, 

 and such waters should be avoided. 



The joints in granitic rocks generally occur in complex systems 

 of intersecting planes, and it is possible for polluted water start- 

 ing very near the mouth of the well to pass in a zigzag course 

 downward along the joints, finally reaching the well at a depth of 

 many hundred feet (Fig. 22) ; such was the case in a well at 



