238 'The Philippine Journal of Science 1915 



At its outcrop this limestone is corroded and jointed until it 

 is a very porous rock. The sedimentary series is usually found 

 flanking the Cordilleras and dipping away from them, so that 

 very often this basal limestone is exposed in a region of heavy 

 rainfall and lies at an angle which accelerates the percolation 

 of water along it. If the limestone in this relation were pene- 

 trated by a well, it ought to yield water copiously. The difl[i- 

 culties are that the basal limestone is thin, discontinuous, and 

 broken by faulting; that inasmuch as its porosity in surface 

 exposures is due largely to solution, the limestone may not be 

 porous below the permanent level of ground water; and finally, 

 that its stratigraphic position is such that it is commonly too 

 deeply buried, except in mountainous and consequently unin- 

 habited regions, to be accessible by drilling. The conditions 

 afford a chance, however, which should be tested when oppor- 

 tunity presents. 



MASSIVE IGNEOUS ROCKS 



Massive igneous rocks abound in all of the truly mountainous 

 portions of the Philippine Islands. Igneous rocks, wherever 

 present to the exclusion of other rocks, constitute the formation 

 least favorable to the accumulation of potable artesian water. 

 They are impermeable to water because of their dense nonporous 

 texture and the absence of bedding planes. It is generally im- 

 material in this connection whether the igneous rock is of the 

 deep-seated, holocrystalline type, such as the diorites, gabbros, 

 peridotites, and occasional granites, or is one of the surface 

 lava flows, such as the widely distributed andesites and less 

 common basalts, rhyolites, and dacites, although infrequently 

 solidified lava flows are so vesicular and porous as to be per- 

 meable to water. Very rarely do common igneous rocks yield 

 water in quantity. No Philippine wells have encountered water 

 in massive igneous rocks, although a dozen, perhaps, have been 

 drilled into them. Minute quantities of water are contained 

 along fractures and joints in igneous rocks, and often mineral- 

 ized water is encountered in the occasional veins and shear zones ; 

 otherwise the rocks are almost invariably dry. Obviously, there- 

 fore, igneous rocks are to be avoided in choosing sites for 

 artesian wells. 



METAMORPHIC ROCKS 



Metamorphic types of rocks are represented in the Philip- 

 pines principally by schists, with subordinate gneisses and mar- 

 bles. Because of their dense nature metamorphic rocks are not 



