348 The Philippine Journal of Science 1913 



GEOLOGIC HISTORY 



The southern part of Bondoc Peninsula appears to have been 

 the site of shallow water deposition during the larger part of 

 Miocene time. Of the conditions prior to the Miocene, there is 

 little evidence. Quartz-veined diorite and schist, both older than 

 the Miocene, occur farther north on Bondoc Peninsula and prob- 

 ably continue into the region under discussion, lying beneath 

 the sedimentary strata. That rocks of this character formed a 

 part of the land mass from which the sedimentary beds in the 

 oil field were derived is proved by the presence of rounded quartz, 

 diorite, and schist fragments in the sandstone and conglomerate 

 members of the series. 



The fact that the highest beds are folded in fair accord with 

 those at the base of the stratigraphic column indicates that the 

 major part of the folding occurred after the close of the Miocene 

 and the completion of sedimentary processes in this region. 

 From the steeper dip in the eastern limbs of most of the anti- 

 clinal folds, it might be inferred that the folding stresses were 

 transmitted from the west. Not all the evidence obtained con- 

 firms this view, however, and without more data a conclusion 

 is hardly justified. 



The interbedding in thin layers of fine-grained shale, sandy 

 shale, and sandstone shows that the Vigo shale formed in mod- 

 erately shallow water. The thickness of the series and the 

 regularity of the beds imply uniform conditions over the area 

 throughout which they are distributed. Continued deposition 

 of sediment in shallow water, until a succession of strata equal 

 in thickness to the Vigo shale is built up, would appear to 

 require an accompanying gradual subsidence of the sea floor. 

 The less clearly defined bedding planes and the increased pro- 

 portion of fine sediment in the Bacau stage, as compared with 

 the lower part of the Vigo shale, suggest that deposition became 

 more constant and regular and that the water became deeper 

 before the beds in this stage were deposited; however, shallow 

 water conditions must have prevailed at the close of the period 

 when the sandstones and fine conglomerates were laid down. 

 The Vigo shale may have emerged above sea level, entirely or 

 in part, and have been subject to erosion before the succeeding 

 beds were deposited. A period of volcanic activity m or ad- 

 jacent to the region, at about this time, is attested by the 

 presence of the volcanic agglomerate near the top of the Vigo 

 shale. 



The thick massive beds of fine sediment in the Canguinsa 



