X, A, 4 Pratt: Location of Artesian Wells 235 



may not have been extensive enough to have removed all the 

 original salt, particularly where the formation contains clay, 

 which is not easily permeable. Salt v/ater is almost inevitably 

 encountered at depth in wells near the coast line. The ground- 

 water circulation appears to be most vigorous at depths generally 

 less than 180 meters. Consequently, if potable water is encoun- 

 tered in littoral or alluvial deposits at depths of from 60 to 150 

 meters, it is usually advisable to make arrangements to use this 

 water even though it be of limited quantity and require 

 pumping, rather than to continue drilling in the hope of obtain- 

 ing flowing water or water in greater quantity at extreme 

 depths. Occasionally, where it has been possible to case out 

 salt water, wells have been deepened and have obtained fresh 

 water at lower levels, but as a rule, fresh water has not been 

 found below salt water. 



The discussion of alluvial and littoral deposits and combina- 

 tions of these two classes of deposits may be extended and ap- 

 plied to intermingled alluvial, littoral, and fragmental volcanic 

 material as well. Volcanic tuffs are often and extensively 

 interbedded with alluvium and with littoral deposits in the 

 Philippines; less frequently volcanic breccias and agglomerates 

 alternate with alluvial or littoral material. Volcanic tuffs, as 

 a matter of fact, usually contain interbedded alluvium, and 

 similarly alluvium usually includes some volcanic tuff. The com- 

 binations of these several classes of material yield water about 

 as commonly and under about the same conditions as littoral and 

 alluvial deposits themselves. 



CORALLINE LIMESTONE 



Coralline limestone is generally dry where it occurs over 

 extensive areas and in thickness. It is so porous and so thor- 

 oughly jointed and cavernous that water percolates through 

 it with little hindrance. Only in coralline limestone that is 

 interbedded with impervious beds of clay, marl, or other material 

 is water confined so as to be available under hydrostatic pres- 

 sure. Fortunately a great deal of the recent coralline limestone 

 in the Philippines is interbedded with impervious material and, 

 therefore, can often be made to yield water. Coral reefs have 

 been found in buried littoral deposits, and in this position were 

 saturated with water under pressure. More commonly coral 

 reefs have been found in deposits of water-laid volcanic tuffs 

 in relations which made the coral reef a natural reservoir for 

 ground water. But the commonest condition under which water 

 has been obtained from coralline limestone is that of interbedded 



