GEOLOGY OF THE PHILIPPINE ISLANDS. 309 



fringing the coast line throughout the greater part of its length, and we 

 have a coastal plain wherever these have been elevated and covered with 

 a veneer of Piedmont deposits. The coastal plains are exceedingly 

 broken, thus interrupting the easiest line of communication by land. 

 The only stretches of any great importance are from Dagupan north to 

 Laoag on the Island of Luzon, the east and west coasts of Mindoro, 

 southern Masbate, the Zamboanga plain, some narrow strips on the east 

 and west coasts of Cebu, much wider belts on Negros, the chief sugar- 

 producing island, a portion of the eastern part of Samar, and possibly 

 in this connection we should consider the Iloilo plain, although this is 

 strictly intermontane. Were there coastal plains of greater extent there 

 would undoubtedly be fewer tribes and dialects. 



THE INTERMONTANE PLAINS. 



The large, more or less flat and fertile tracts which lie between the 

 cordilleras are, with one or two notable exceptions, the sites of the chief 

 human activity in these Islands. These plains owe their origin to a 

 certain extent to elevation of the troughs between what were once islands, 

 and to subsequent upbuilding by delta deposits, alluvial fens, and erup- 

 tions of pyroclastic material. The principal plains of this nature are : 



(1) The central plain of Luzon, from Manila Bay to Lingayen Gulf. 

 (2) The Cagayan Valley (northern Luzon). (3) The central plain of 

 Panay. (4) The valley of the Agusan (eastern Mindanao). (5) The 

 Cotabato Valley (central Mindanao). (6) The Albay plain (Luzon). 



The Cagayan Valley, where the greater part of the Philippine tobacco 

 is grown, perhaps is the most productive, though not necessarily the most 

 fertile plain. The central plain of Luzon is cultivated largely for rice. 

 However, much of this land is unproductive and irrigation will go far 

 toward reclaiming it. The central plain of Panay is one of the seats of 

 the sugar industry. The Cotabato Valley is occupied largely by Maguin- 

 danao Moros and produces only a modicum of the crops of which it is 

 capable. The valley of the Agusan is as yet practically uncultivated 

 excepting for isolated hemp patches. All of these plains were formed 

 in much the same manner, and therefore the central plain of Luzon may 

 be taken as typical. 



In the beginning there was a structural basin, a deep, wide, and long 

 synclinal which was occupied by the sea, giving two islands where now 

 there is one, a large one on the east and a smaller on the west, where the 

 Zambalas Mountains now rise. Material washing down the mountain 

 slopes gradually began to fill up this trough, first in the form of alluvial 

 fans and cones and then by means of a broad delta and flood plain which 

 slowly progressed southward. In all probability there was some coral 

 close to shore, forming a substructure for the plain, but this presumably 

 did not extend very far out into the sea. 



