232 ^^'^ Philippine Journal of Science lois 



A large proportion of the flowing wells in the Philippines will 

 cease to flow if the casing is continued even a few meters above 

 the ground surface; that is to say, if such wells had been drilled 

 from a slightly higher elevation, it would have been necessary 

 to pump them. It follows that, other things being equal, the 

 lower of two possible sites is preferable. However, the possi- 

 bility of surface contamination should be kept in mind in this 

 connection. A well on low ground is more often contaminated 

 by surface waters than one on higher ground. It is true also 

 that a well should not be located too near the seashore in an 

 attempt to get it on low ground. Too often in such a location 

 the well encounters brackish water. 



Through the cooperation of the Bureau of Public Works the 

 Bureau of Science has had access to drillers' logs for about 700 

 artesian wells. Samples of the drill cuttings have been sub- 

 mitted for examination in the cases of about half of these wells. 

 These data, and a knowledge of the general geology of the Phil- 

 ippines, are made the basis of the following discussion. 



For their consideration in relation to artesian waters the geo- 

 logic formations of the Philippines may be classed and sum- 

 marized as shown in Table I. 



LITTORAL AND ALLUVIAL DEPOSITS 



A large proportion of the population of the Philippines lives 

 in regions in which the land is made up of littoral and alluvial 

 deposits. The important areas of alluvium are found in great 

 structural valleys like the Central Plain of Luzon, the Cagayan 

 Valley in northern Luzon, the Bicol Valley in southeastern Luzon, 

 the Iloilo Plain in Panay, and the Cotabato and Agusan Valleys 

 in Mindanao. The littoral deposits make up the coastal plains 

 which fringe many of the islands, like the plain upon which 

 the town of Cebu is located. The structural valleys have 

 been filled up by loose clays, sands, and gravels carried down 

 from the adjacent highlands by shifting streams and deposited 

 along an ever-advancing shore line. The coastal plains have 

 been built up in the same way; in many cases they rest upon a 

 base of coral reefs, which have grown up offshore from the 

 various land masses. Both the filled structural valleys and the 

 coastal plains combine alluvial and littoral deposits in their 

 structure, since the alluvium carried by streams was deposited 

 largely at the seashore where part of it was worked over again 

 by wave action. In either situation the littoral and alluvial 

 deposits are surprisingly thick; in very few cases have wells 

 passed through them into the underlying formations. 



