a ' 5 Dickerson: Bagnio Plateau 437 



the Vigo-Miocene fossil locality which is located about 200 meters 

 north of the stock-farm buildings. In this vicinity the gap be- 

 tween the drainage toward the China Sea and Trinidad Valley is 

 exceedingly low, and both Doctor Smith and the writer were 

 agreed that this gap had been lowered long after Trinidad River 

 was firmly intrenched in its present course. The hard, resistant, 

 andesitic tuff-breccia north of Trinidad has undoubtedly acted 

 as a dam, creating at times a well-marked, local base level. 

 This dam may have been renewed by faulting. The two ter- 

 races encirling Trinidad Valley probably represent substages 

 in the general uplift of Baguio Plateau and the very notable 

 plateau at Amsalsal and its correlative terrace 8 kilometers (5 

 miles) north are probably referable to one of these substages. 

 The present floor of Trinidad Valley consists of thick alluvium 

 which in the temporary marshlike area, now being drained 

 through the energy of the farm-school management, is decidedly 

 peaty. The writer interprets this marshy area, 0.4 kilometer 

 southwest of Trinidad, as representing the abandoned channel 

 of Trinidad River when that river had a course around the 

 western side of the valley. 



In summary, then, Trinidad Valley is neither an extinct crater 

 nor an atoll but is a peculiar erosional development due to an 

 antecedent stream which in its successive intrenchments ex- 

 humed the older formations. This valley gives additional con- 

 firmatory proof that the Baguio Plateau surface was developed 

 at elevations probably varying from sea level to 360 or 450 

 meters (1,200 or 1,500 feet), since its development can be ex- 

 plained in no way except through general regional uplift. 



MINOR MODIFICATIONS 



Since the Baguio Plateau has been uplifted, minor modifica- 

 tions of its surface have occurred, but these changes are not 

 sufficient to obliterate the record. The sources of Trinidad 

 River are cut into the original surface for depths of 60 to 90 

 meters (200 to 300 feet) . Besides these erosional changes some 

 of the local subsidences, such as the pond near the Pines Hotel 

 and the sink near Mount Mirador, are due to the solution of the 

 underlying Malumbang limestone (Plate 12, fig. 3) . Some of the 

 local earthquakes at Baguio have been ascribed to this cause, 

 and some of the minor shocks may be thus explained. The more 

 hkely explanation is that the heavier shocks are results of move- 

 ments along the Recent and late Pleistocene faults, such as those 

 of Mount Santo Tomas. Doubtless certain slides, rock falls, and 

 incidental after-adjustments have exerted their influence upon 

 Baguio Plateau in minor degrees. 



