338 The Philippine Journal of Science 1913 



horizontal Canguinsa sandstone. Similar relations are to be 

 observed along the western base of Mount Cambabaco. 



Overlap on the part of the Canguinsa sandstone, which would 

 be expected if this member were laid down upon the truncated 

 edges of the Vigo shale, is not very extensive. At a number of 

 places the Canguinsa sandstone is encountered above approxi- 

 mately 'the same horizon in the Bacau stage of the Vigo shale. 

 Elsewhere, as is shown in the section on Dumalog Creek, page 

 333, and the section on Guinhalinan River, page 322, more or 

 less sandstone or sandstone and fine conglomerate, bedded con- 

 formably with the Vigo shale, intervene between the Bacau 

 stage and the Canguinsa sandstone. 



The volcanic agglomerate might be taken as evidence of a 

 distinct break in the process of sedimentation if it occurred 

 uniformly between the discordant members, but the outcrops 

 do not appear to lie along the unconformity nor to be confined 

 to a single horizon. It is possible, however, that more detailed 

 work would show that the agglomerate does occur along the 

 unconformity, and thus indicate a decided break between the 

 Vigo shale and the overlying formations. In the data at hand, 

 however, the overlap of the Canguinsa sandstone and the steeper 

 dips in the Vigo shale are the principal evidences of unconformity. 



If the steeper angle of dip in the Vigo shale, as compared 

 with the overlying formations, is accounted for by assuming that 

 the later rocks were laid down on an eroded surface of previously 

 folded Vigo shale and were themselves thrown into folds sub- 

 sequently, two periods of folding are involved with a remarkable 

 coincidence in the position and trend of the later folds along 

 the axes previously established in the Vigo shale. A theory of 

 a single period of folding is simpler and is in accord with the 

 observations in the field, if mechanical unconformity, resulting 

 from the different frangibility of the Vigo shale and the over- 

 lying formations, be admitted as adequate to account for the 

 overlap of the Canguinsa sandstone and the lesser inclination 

 in the beds above the Vigo. On the other hand, there is evidence 

 in ' the sandstones and conglomerates immediately above the 

 Bacau stage that the seas became very shallow before the dep- 

 osition of the Canguinsa sandstone began, and it is possible 

 that the Vigo shale emerged from the sea and became subject 

 to erosion, although positive evidence of erosion is lacking. The 

 fossils in the two formations indicate that there was no great 

 interval of time between them. 



Whether the unconformity is one of erosion or of mechanical 

 discordance only, the Canguinsa sandstone appears to overlap 



