vni, a, 4 Saderra Maso and Smith: Seismic Disturbances 209 



there are a few active volcanoes in the Philippines, most of the 

 volcanoes are extinct, or dormant, as shown by the many worn- 

 down volcanic stocks, preserving in varying degrees their former 

 outlines. 18 



The principal folded mountains are the Central Cordillera of 

 Luzon, the Eastern Cordillera which has its southern contin- 

 uation in Tayabas Province, and the cordilleras of Cebu, 

 Leyte (?), eastern Mindanao, and various other parts of the 

 Islands. 



Block mountains are rare in the Philippine Islands, but at 

 Bambang on the main line of the Manila and Dagupan railway 

 in Luzon there are some hills which are due to the Tertiary 

 sediments being tilted up like blocks. From the railroad the 

 tilted beds are seen presenting a bold escarpment to the east, 

 undoubtedly due to a fault, but dipping gently to the southwest. 



On Panay Island the secondary ranges — those flanking 

 the Central Cordillera — are tilted blocks of Tertiary sediments. 

 The strike of these secondary ranges conforms exactly to the 

 strike of the sediments. Their eastern slopes correspond in- 

 variably to the dip of the formations. On the upstream side 

 the slopes are precipitous where the upturned edges of the 

 sandstone and the shale beds can be seen. Displacements along 

 the bedding planes of these formations are frequent and have 

 undoubtedly given rise to many local quakes. 



Kinds of rocks. — Before discussing the tectonic features of the 

 Islands it will be necessary to consider the distribution of the 

 various classes of rocks. We have the following kinds to con- 

 sider: (a) Deep-seated igneous, (b) extrusives, (c) intrusives, 

 (d) Tertiary and older sediments, (e) metamorphic rocks, (/) 

 .recent alluvial and pyroclastic deposits. 



(a) The deep-seated igneous rocks are diorite, gabbros, pyrox- 

 enite, peridotite, and syenite. These rocks have all been fully 

 described by Iddings. 19 Their distribution is the principal thing 

 of importance in this discussion. The deep-seated rocks are 



18 These various volcanoes have been taken up in other articles in more 

 or less detail. The centers of volcanic activity at the present time are 

 Taal Volcano in Batangas Province; Mount Mayon in Albay Province, 

 Luzon; Mount Canlaon in Negros; and Camiguin Island just north of 

 Misamis, Mindanao. All of these volcanoes have been in eruption within 

 the last ten years, and to them can be attributed many of the minor 

 seismic disturbances to which the Archipelago has been subjected. 



" This Journal, Sec. A (1910), 5, 155. 



