526 W. D. SMITH GEOLOGIC INFLUENCES IN THE PHILIPPINES 



One of the outstanding factors to account for the lack of coastal plains 

 is to be found, as pointed out by Semple,^ in the deep surrounding seas — 

 that is, there is little or no shallow continental shelf on which alluvial 

 material could accumulate. 



A coastal plain more tlian 10 miles wide is an exception in the Philip- 

 pines. Attention will be invited to this topic again because some striking 

 practical conclusions may be arrived at. 



iNTEPtMONTANE PLAINS 



A type of plain more important than the coastal is exhibited by those 

 found lying between the mountains, and it is on these intermontane 

 plains that the greatest development of civilization in the islands is to be 

 found. The principal ones are : 



a. Central Plain of Luzon. 



1). Cagayan Basin, northern Luzon. 



c. Central Plain of Panay. 



d. Bicol Plain, southeastern Luzon. 



e. Agusan Valley, eastern Mindanao. 



/. Cotabato Valley, southwestern Mindanao. 



As a rule, these plains have been formed by the aggradation of depres- 

 sions between two isolated land-masses which were separated in Tertiary 

 times. They generally are much wider than the coastal plains, though 

 they have much the same origin and composition. With the exception of 

 the Cagayan, they are broad and flat bottomed, holding the largest popu- 

 lation of all equivalent areas. Cebu island is an exception, the people 

 there living mostly on the coastal plain, there being no central plain and 

 the interior having a high and rugged character. 



For details regarding these important physiographic features the reader 

 is referred to the specific descriptions in earlier papers on Luzon, Min- 

 danao, etcetera, by the writer.'^ 



EivEPt Systems 



As pointed out in the beginning of this paper, there are a few large 

 rivers in the archipelago, but in the main the streams are short and swdft. 

 This is a natural consequence of the size of the land-masses and the 

 youthful stage of the topography. The principal rivers in the order of 

 their present importance are : 



« Influence of geographic environment, p. 446. 



'' Philippine Journal of Science, Manila, vol. i ; Journal of Geology, Chicago, vol. xxi, 

 no. 1, 1913, p. 29. , 



