viii, a, 5 Pratt and Smith: Petroleum Resources 337 



adjacent geosyncline. The elevation of Bondoc Peninsula above 

 sea level is due, in part at least, to its anticlinal structure. 



In the general arch which the strata form across the width 

 of the peninsula there are minor undulations which have been 

 studied as individual folds. Among these folds the anticlines 

 are more sharply defined, and hence more easily traced, than 

 the synclines. Because of this and of their probably greater 

 importance in connection with petroleum accumulation the anti- 

 clines have received more attention than the synclines. In con- 

 nection with the discussion of the structure reference should 

 be made to the geologic sections shown on the map. 



Confusing irregularity is encountered in the strikes and dips 

 of the Vigo shale in parts of the field, particularly in the upper 

 valley of Mulanay River and in a small area north of Cabon- 

 gahan. At the latter place along small streams, outcrop after 

 outcrop was examined, the strikes and dips of which are utterly 

 at variance, even when all but the most reliable-appearing expo- 

 sures are ignored. Because of this fact, the structural relations 

 have proved locally undeterminable. 



The Vigo shale is a thinly laminated and nonfrangible for- 

 mation which would scarcely be maintained as a competent arch 

 in folding, but probably would break and move upon itself where 

 the strains were most severe, leaving the strata in confused 

 disorder at these points. In the crest region of the Central 

 anticline the shale is much disturbed, and extreme confusion in 

 the attitude of the beds is encountered near the intersection of 

 this fold with the minor cross anticlines, while the strikes are 

 regular and uniform only in the limbs or far down in the core 

 of the anticline. The squeezing and distortion, due to close 

 folding in a formation structurally incompetent, together with 

 superficial displacement and caving due to erosion, are probably 

 the main causes of the observed irregularity in the Vigo shale. 



Unconformities. — In addition to the unconformity at the base 

 of the recent deposits on Bondoc Peninsula there is considerable 

 discordance between the Canguinsa sandstone and the Vigo 

 shale. No exposures were found showing an eroded surface of 

 the Vigo shale beneath the Canguinsa sandstone, but in regions 

 where the inclination of the beds is steep an abrupt increase in 

 the angle of dip is apparent on passing from the higher to the 

 lower formation, although the strike remains more or less 

 constant. 



The change in the degree of inclination of the two formations 

 is best illustrated by the conditions on the upper part of Malipa 

 Creek where steeply dipping Vigo shale is overlain by almost 



