INTRODUCTION 517 



travel and study in the islands, working mainly along economic lines — 

 has caused to be overlooked here many important things which would 

 occur to the trained geographer studying the problem for the first time. 

 If the reader will look on this paper as an introduction to the subject and 

 no more, he perhaps will not be too critical. It should be borne in mind, 

 too, that the writer is primarily a geologist, and therefore the viewpoint 

 of this article may differ from that of the geographer. 



The same general groups of rocks exist in the Philippines as are found 

 in other parts of the world. There are deep-seated igneous rocks, in- 

 trusions, and volcanic flows; there are metamorphic rocks, pyroclastics, 

 and sediments, both consolidated and unconsolidated. The layman usu- 

 ally thinks of the Philippine Islands as almost entirely volcanic; never- 

 theless there is a wide distribution of the sedimentary series. 



There is another essential feature of Philippine geology, namely, the 

 striking similarity which exists between the formations in the Philippines 

 and those of the west coast of America. The great basaltic and andesitic 

 flows of the Pacific northwest can be duplicated, if not in size, in petro- 

 graphic and structural characteristics, in this archipelago. Some of the 

 Pacific Coast formations contain fossil forms not greatly unlike those 

 found in the Philippines, and in many cases the lithology of the forma- 

 tions is quite similar in the two regions. The radiolarian cherts of the 

 California and Oregon stratigraphic column also have their counterpart 

 in these Islands. 



Eeference to figure 1 will show the distribution of the various forma- 

 tions. It will be seen that certain types of rocks predominate, at least 

 on the surface, in certain parts of the archipelago ; but this is due to no 

 general law that we know of. Eeduced to the simplest terms, the Philip- 

 pine terranes consist of the sediments laid down on the Asiatic continental 

 platform in Tertiary times, crumpled by crustal movements and in part 

 broken through and veneered by igneous rocks of varying ages. In addi- 

 tion there are coral debris and aerial deposits which complicate the series. 



■The deep-seated rocks are not very widely distributed on the surface 

 and usually are found only in the canyons of the central ranges. They 

 are particularly abundant in northern Luzon, throughout the central 

 Cordillera ; in Palawan Island ; the western Cordillera of Panay ; the cen- 

 tral Cordillera of Cebu and Leyte; the eastern Cordillera of Mindanao; 

 on Masbate Island; in fact, wherever the streams have been able to cut 

 through the overlying and more recent formations. 



In all parts of the Philippines there is a large amount of extrusive 

 material which forms a mantle over the deeper lying formations. Nat- 

 urally these extrusions are found around the volcanic areas and are very 



XXXIX — Bull. Geol. Soc. Am., Vol. 28, 1916 



