208 The Philippine Journal of Science 1914 



The influence of the structure upon the development of the 

 topography may be detected in the general north-south alignment 

 of ridges and water courses parallel to the axes of intrusions and 

 folds in the cordillera and to the general strike of the sedimentary 

 strata. The western dip of the beds in much of the area of 

 stratified rocks is reflected in long gentle slopes on the western 

 sides of ridges and steeper eastern slopes. A belt of limestone 

 running across the region in a north-south direction forms a 

 conspicuous ridge or line of hills through which the drainage 

 passes either in deep gorges or in underground courses (caves). 



The Central Plain supports a large population, and is given 

 over largely to rice cultivation. The foothill country is partly 

 under cultivation, but the larger part of it is covered with scrub 

 timber. The cordillera is practically uninhabited, and is heavily 

 forested. Near the smelting centers which are located at the 

 ore deposits, the forest has been cut away for charcoal, and the 

 cut-over areas have become an almost impassable jungle of 

 second-growth timber, bamboo, and rattan. 



GENERAL GEOLOGIC RELATIONS 



The Eastern Cordillera as a whole is a complex of Pre-Miocene, 

 igneous rocks, folded sedimentary rocks, most of which are of 

 Miocene age, and extrusive rocks of varying age. The Central 

 Plain is made up of younger flat-lying sedimentary rocks. In 

 that part of the western range of the cordillera shown on the 

 geologic map the sedimentaries lie upon the western flank, deep- 

 seated igneous rocks occupy a central position, and effusives 

 and intrusives in a general way make up the eastern slope. 



The most conspicuous holocrystalline rock is a granite which 

 is exposed in an elongated area with its longer axis extending 

 in a north-south direction. The strata in the sedimentary for- 

 mations strike parallel to this line, and dip generally to the 

 west. They overlap directly upon both the granite and upon 

 older effusives. Fringing the granite are numerous small areas 

 of intrusive rocks from which dikes extend into the granite, 

 the sedimentaries, and the older effusives; these intrusives are 

 usually of porphyritic or of fine-grained holocrystalline tex- 

 tures. East of the granite, the rocks at the surface are usually 

 altered effusives, in part fragmental. In some of the deeper 

 canons holocrystalline rocks of related types are exposed. 



The stratigraphic sequence is indicated in fig. 2, and the gen- 

 eral structure is shown in the generalized east-west cross 

 sections through the region (fig. 3). Overlapping upon a base- 



