518 W. D. SMITH — GEOLOGIC INFLUENCES IN THE PHILIPPINES 



pronounced in the Zambales Range of southwestern Luzon and in various 

 parts of the Central Cordillera, lyiiig above the old igneous rocks and 

 the Tertiary sediments. The Central Cordillera of Luzon consists of 

 great masses of andesite, probably marking Tertiary volcanoes. In the 

 Zambales Mountains there is a development of andesite marking what 

 is probably a still later period of volcanic activity. On Mount Arayat, 

 which rises isolated out of the central plain of Luzon, basalt occurs, and 

 also is found in considerable amounts around Taal Volcano and on the 

 Binangonan Peninsula. Extrusives are particularly well developed in 

 the volcanic cluster of southeastern Luzon, comprising the well known 

 peaks of Bulusan, Mayon, Isarog, etcetera. They are found overlying 

 much of Masbate, particularly in the central portion; also in western 

 Panay, a portion of Cebu, most of northern Negros, central Leyte, and 

 notably in Mindanao, there being a broad belt of extrusives running north 

 and south through the Apo and Matutum ranges. There also is a patch 

 of basaltic material around Lake Lanao, and a great volcanic mass of 

 which Mount Malindang is the center. Great areas of these extrusives 

 also cover almost the entire islands of Basilan and Jolo, and the lesser 

 islands of the Sulu Archipelago. 



As yet we know of extrusives in Palawan only in the northern part. 

 The principal mountainous mass of Mindoro — Mount Halcon— is largely 

 andesitic. 



There is one general conclusion which may be drawn from the extru- 

 sives in the Philippine Islands, namely, that the entire recent volcanic 

 activity, as far as we know, consists of basaltic ejecta, and the older stocks, 

 without exception, are andesitic. The volcanic materials of the present 

 vents are entirely fragmental. 



Small and large intrusions of diorite, granite, and basalt are innu- 

 merable throughout the islands. In the Central Cordillera of Luzon the 

 intrusions seem to be generally diorite. They cut both the Tertiary sedi- 

 ments and the overlying extrusives. In the province of Ambos Camerine, 

 in southeastern Luzon, granite intrusions can be seen cutting the diorite 

 and possibly the sediments. In the Sulu Archipelago there have been 

 found a number of small basaltic intrusions cutting some of the recent 

 sediments. Owing to the absence of an accurate base map of the Philip- 

 pines and to the fact that our work has been largely of a reconnaissance 

 nature, these intrusions have not been mapped in detail or with sufficient 

 accuracy for us to state whether or not they follow any general system of 

 jointing or earth lineaments. 



Flanking all the Cordilleras on both slopes there is a greater or less 

 development of sandstones, shales, and limestones which have been bowed 



