GEOLOGIC RECONNAISSANCE OF SOUTHWESTERN LUZON. 101 



Arayat, which rises as a lone peak in the central valley, probably lies 

 on the same structural line as Taal. The volcanic centers of the south - 

 Avest district are thus readily referred to a parallel system of fissures 

 which corresponds to the trend of the east and west cordilleras. The 

 location of the volcanic peaks of southeastern Luzon bear a similar rela- 

 tion to the trend of the mountain ranges of eruptive rocks and the belts 

 of folded sediments in so far as they are now known. 



The emergence of the marine conglomerates, water-laid tuff and 

 coralline limestone has already been touched upon in previous paragraphs. 

 The amount of emergence is shown to be at least 200 meters in the 

 western cordillera district and there is some evidence that it is greater 

 in the area of the eastern cordillera district, possibly amounting to as 

 much as 500 meters. It is hardly probable that such great changes of 

 level should have taken place without being accompanied by displace- 

 ments. Two probable fault lines have been mentioned, one passing 

 to the east of the agglomerate area near Point Santiago and the other 

 extending northward from the base of Mount Gonzales and passing to 

 the west of Bay Lake. The volcanic rocks and the volcanoes lie along 

 great fissures and it is probable that displacements have occurred on 

 these structural lines during the emergence of the region. 



It has been argued in this paper that the probable trend of the struc- 

 ture is northwest with the exception that in the Loboo Mountains it 

 is to the northeast. It has occurred to me that a structural line having 

 a northeast trend may pass through Balayan Bay, Taal Volcano and 

 the southeastern border of Bay Lake. If such is the case the line would 

 form the southern border of the elevated fault block which includes 

 Batulao, the Tagay-tay ridge and G-onzales. 



It is well known that southwestern Luzon has suffered severe earth- 

 quakes. There has been a tendency to refer these seismic disturbances 

 to volcanic centers. It would seem more desirable to refer them to 

 tectonic lines; but the systematic study of the earthquakes has not been 

 carried on for a sufficient length of time to make this possible. 



Origin of Taal Lake. — Father Zuniga regarded Taal Lake as originat- 

 ing in the collapse of a volcanic cone, and this theory was accepted by 

 Hochstetter, Von Drasche and Centeno. Becker says that the theory 

 of volcanic collapse "seems to imply that an empty space beneath the 

 earth's surface is formed by the eruption of lava and that the inter- 

 vening rock is too weak to bear the load put upon it. . . . I doubt 

 this theory as applied to volcanic cones excepting only when invoked 

 to account for local details of structure. ... On the other hand, it 

 is well known that craters of vast size have been formed by explosions, 

 and I can see no reason to doubt that Bombon may have been, probably 

 has been, formed in this way in spite of its large dimensions." Taal 

 Lake is evidently a caldera formed by peripheral and radial faulting 



