vni, a, 5 Pratt and Smith: Petroleum Resources 341 



shale was observed dipping 45° to the westward, the later for- 

 mations lying above it and dipping in the same general direction 

 at a lesser angle. South of Balinsog, as far as Bacau, the Can- 

 guinsa sandstone is intact across the axis of the anticline, and 

 the upper formations dip gently away to the east and west on 

 either side of Canguinsa River. The Bacau stage of the Vigo 

 shale is exposed in the river at Bacau, dipping 55° east-northeast; 

 and above this exposure, in the eastern wall of the valley, the 

 Canguinsa and Malumbang formations are encountered dipping 

 at low angles to the eastward. Up and down stream from the 

 outcrop of Vigo shale at Bacau the relations are concealed; the 

 river swings to the southwest at this point, and the next ex- 

 posures downstream are of Canguinsa sandstone dipping gently 

 westward. 



The existence of a minor cross-anticline, the axis of which 

 coincides roughly with the lower part of Canguinsa (Silonguin) 

 River, is suggested in the discussion of the geologic control of 

 the topography. In the upper part of the gorge, which Can- 

 guinsa River enters after its abrupt turn to the southwest, the 

 Canguinsa sandstone dips to the north of west, while farther 

 downstream the direction of dip has swung to south of west, 

 indicating that the river in its course to the southwest has 

 passed over the axis of a small anticline trending east and west 

 in "the general western limb of the larger fold. In a profile view 

 of a cross section of the gorge, which may be obtained from 

 Mount Anuing, the effect of this small anticline may be detected 

 in the slope of the land surface to the north and south away 

 from the rims of the gorge. The northerly dip of the beds in the 

 hills along the north edge of Malumbang Plain probably re- 

 presents the northern limb of this cross-flexure. 



It is probably due to the presence of the cross anticline at 

 Bacau that the Vigo shale is thrown up so as to appear again 

 at the surface after having been carried down by the general 

 southerly plunge of the axis, far enough to escape erosion at 

 the same elevation farther north. 



Structure in the region north of the Central anticline. — In 

 Sobo Creek, which drains a part of the western slope of Mount 

 Maclayo, strata in the upper part of the Vigo shale were ob- 

 served striking north 65° east and dipping 65° to the south. On 

 Mulanay River, above the point where the tidal influence ceases, 

 numerous outcrops of Vigo shale are to be seen in a majority of 

 which the strikes are a few degrees north of west, and the dips 

 are either vertical or steep both to the north and south. A 



