SOURCES AND SAFETY OF UNDERGROUND SUPPLIES 43 



FIG. 20. Grains 

 in sands and 

 sandstones with 

 intervening 

 pores filled with 

 mineral matter 

 preventing the 

 absorption of 

 water. 



Some conglomerates furnish considerable water, although, as 

 a rule, the absorptive power of conglomerates is not so great 

 as that of sandstones, and they are much less frequently en- 

 countered. 



Quartzite is a sandstone in which the spaces between the 

 grains have been filled by hard siliceous matter. (See Fig. 20.) 

 Because of the filling of the pores by this material 

 there is relatively little chance for the water to 

 enter, and the rocks are not commonly an import- 

 ant source of supply. Such water as they yield is 

 mainly from joints. 



Waters of Slates. Slate, like clay, is a poor 

 water bearer but may yield water from crevices or 

 along bedding joint and cleavage planes. Its most 

 important use, with reference to water supply, is 

 as a confining layer to prevent the escape of water 

 from porous sandstones which may be interbedded 

 with it. The waters in slate are reached by deep wells and are 

 generally uncontaminated but are not uncommonly mineralized. 



Waters of Limestones. Waters occur in limestone mainly in 

 open channels, caverns, etc., dissolved in the rock by the water 

 itself. The water originally probably followed joint or bedding 



planes which were gradu- 

 ally enlarged by solution 

 into the caverns that now 

 exist. 



The occurrence of cav- 

 erns and passages within 

 the limestone is very ir- 

 regular, and their location 

 can seldom be predicted. 

 Most deep wells which 

 are drilled in limestone regions, however, encounter one or more 

 such passages at a relatively slight distance below the surface. 

 Wells in limestone, even where only a few feet apart, may never- 



FIG. 21. Difference in conditions of adjacent 

 wells in limestone. 



