208 The Philippine Journal of Science 1913 



Taking up the physiography of the Islands in detail, there is 

 in the northern part, on Luzon, to the eastward, first a cluster 

 of volcanoes containing Bulusan, Mayon, Isarog, and Iriga ; then 

 the narrow Albay Plain ; west of this a belt of Tertiary sediments ; 

 and farther west a trough occupied by Ragay Gulf. The anti- 

 clinal, marked by Tayabas Peninsula, consisting of folded Ter- 

 tiary sediments, lies west of this; then the volcanic group 

 consisting of Banahao, Cristobal, and Arayat; west of these, the 

 central plain of Luzon, partly a tuff plain and partly made up 

 of alluvium ; and farthest to the west a line of extinct volcanoes 

 marked by the Zambales Cordillera. In northern Luzon a zonal 

 arrangement exists, but is not so marked. Recent volcanoes are 

 few, Caua in Isabela Province being the only one we know of in 

 that section. 



In the Visayan Islands there is this same general zonal arrange- 

 ment of formations, but the evidence of vulcanism is not so 

 marked as in Luzon. The islands are marked by anticlines and 

 the narrow straits by synclines. 



This is likewise true of Mindanao. Along the eastern coast 

 there are recent and Tertiary sediments flanking a core of 

 igneous rocks on the immediate west, and dipping eastward 

 toward the "fossa," or trough, just east of Mindanao; then the 

 great Agusan trough ; west of this an interrupted volcanic chain 

 marked by such prominent points as Matutan and Apo; the 

 great alluvial flat occupied by the Rio Grande de Mindanao; 

 the regular zone of volcanic peaks marked by Malindang and the 

 old craters south of Lake Lanao and those in the unexplored 

 region south of the Rio Grande; farther west a belt of folded 

 Tertiary sediments; then the synclinal marked by Sibuguey 

 Bay ; and last, the anticlinal marked by Zamboanga Peninsula. 



Cross folding, probably with faulting, has broken up the 

 Philippine block in such a way as to produce numberless islands 

 in line with the main tectonic lines running through the larger 

 islands. 



Types of mountains. — Of the various kinds of mountains such 

 as folded, block, and volcanic mountains, the two dominant 

 types in the Philippines are the first and the third. We suspect 

 the existence of mountains of the block type in some places 

 where we have not sufficient data to prove it. It might seem at 

 first sight that the majority of the mountains of the Philippines 

 are volcanic, but these are merely superimposed upon the folded 

 mountains or are located along other lines of weakness. While 



