434 The Philippine Journal of Science 1923 



which were upturned when the lava was forced out, and a ring' 

 of andesitic mud flows on the edge of the sedimentary beds of 

 Knoxville-Cretaceous, Chico-Cretaceous, and Tejon-Eocene ages 

 occur as a series of smooth rounded hills forming an interrupted 

 ring 1.6 kilometers or less in width, in striking contrast to 

 the resistant core and the outer ring of firmly cemented mud 

 flows which dip in all directions from the central core at angles 

 of 4° or 5°. A glance at the geologic sketch map of this Trini- 

 dad area shows that four formations occur here; namely, allu- 

 vium, coralline limestone, andesitic tuff-breccia, and tan marls 

 and associated arkosic sandstones (fig. 3). The alluvium occu- 

 pies the round or valley portion. The south and east sides and 

 portions of the north and west sides of the valley consist of 

 coralline limestones and associated marine shales and sand- 

 stones. The dominant dip of these beds on the east side of the 

 valley is 10° to 15° north. Only that portion of the valley out 

 of which Trinidad River escapes is hard andesitic tuff-breccia 

 (Plate 10, fig. 3). If a volcanic crater exposed sedimentary 

 beds upon its walls, then the dominant dip would be away from 

 the crater in all directions and lavas would be found resting 

 upon such sedimentary beds. Such is not the case at Trinidad. 

 Von Drasche regarded Trinidad Valley as a coral atoll, because 

 coralline limestones border it. Interesting as this view would 

 be, field evidence does not offer it much support. The presence 

 of andesitic tuff-breccia on the north side in fault contact with 

 coralline limestones, the prevailing north dip of the shales, sand- 

 stones, and coralline limestones on the entire east side of the 

 valley, their much steeper dip than that of depositional coral-reef 

 material, and the alternation of sandstones, shales, and lime- 

 stones, are not characters of a coral atoll. Furthermore, the 

 coralline limestone is too largely composed of rolled coral frag- 

 ments, and no evidence was found to indicate that the corals 

 were in place. 



What then is the explanation of this most extraordinary, 

 round, alluvium-covered valley surrounded on all sides by hills 

 and steep-walled mountains? After a cursory study of the 

 valley, the writer was early impressed with the fact that it was 

 neither a volcanic crater nor an atoll, but was due to some pecu- 

 liar erosional development. As outlined above, the sharp con- 

 trast upon entering and leaving this valley is striking, even to 

 the casual visitor. Viewed from the hills above Trinidad, look- 

 ing southward up the Trinidad Gorge and to the hills of Baguio 

 in the distance, the terrace condition of the east portal of Tri- 



