CHAPTER VI. 

 SOURCES AND SAFETY OF UNDERGROUND SUPPLIES. 



THE water in rocks and every rock contains at least a trace 

 - either occupies perceptible cavities in the rock or occurs within 

 the minute pores. The water in the pores is given up readily 

 only by the coarser rocks, such as sandstones, the fine-grained 

 rocks yielding very little of such water when penetrated by the 

 drill. Water found in these rocks usually comes from the joint, 

 fault or foliation planes. The conditions of the occurrence of 

 water in various rocks differ widely. 



Waters of Sands and Gravels. Sands and gravels are very 

 porous. Thirty per cent of the volume of some sand or gravel 

 deposits are made up of free space between the grains. In such 

 material the whole mass below ground-water level is saturated, 

 and when penetrated by wells yields copious supplies. The 

 waters of such deposits are, as a rule, of good quality, but some 

 are mineralized, having dissolved material from the more soluble 

 fragments and particles that constitute the deposits. 



The cheapest and best method of obtaining small supplies of 

 water from sands and fine gravels is by driven wells, which can be 

 sunk quickly and at slight cost. It is difficult, however, to ex- 

 clude very fine sand or quicksand from pipes, quicksand frequently 

 penetrating the well and clogging the pipe or ruining the pump. 

 Because of the readiness with which sands and gravels yield their 

 water, wells sunk close together in such deposits may affect one 

 another, the well that draws from the sand at the lowest level tak- 

 ing the water from the higher wells. The readiness of movement 

 of the water also causes important fluctuations of the water in the 

 ground, the level often rising and falling rapidly with the begin- 

 ning and cessation of rain. To obtain permanent supplies, wells 



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