GEOLOGIC RECONNAISSANCE OF SOUTHWESTERN LUZON. 



99 



has cut its chanuel down through the ridge of limestone which forms 

 the gorge. In the accompanying profile (fig. 10) traced from a photo- 

 graph, the probable former levels of the stream are indicated by benches 

 on the walls of the gorge. The elevations of these benches were de- 

 termined with considerable accuracy by the engineer in charge of the 

 construction of the dam. The highest is at a level of 342 meters. If 

 these benches have been produced by the side cutting of the Mariquina 

 River it indicates a very considerable elevation of the locality and one 

 which is commensurate with the elevation which may have taken place 

 at Antipolo and in the southern part .of the eastern cordillera. The 

 Mariquina River, after passing the gorge, reaches the tuff formation 

 near Montalban. The tuff being water-laid indicates the former presence 

 of the sea at Montalban and at that time the river may have flowed into 

 the sea but a short distance from the gorge. Subsequently, as the land 

 was elevated, the river has cut its channel to the present grade. 



Elevation 390 m 



Dam-foundation efevation 39m — 



Fig. 10. — Profile of the Gorge of the Mariquina River at the Head Works 

 of the Manila Water Supply, Showing River Terraces Cut in Limestone. 



Coralline limestones and marls. — Dana records in discussing the 

 geology of Luzon that he was told of a bed of coral occurring at Point 

 San Diego, at an elevation of 600 feet above sea level. Later, Yon 

 Drasche in writing on the Philippines, cited Dana as authority for the 

 occurrence of raised reefs, but seems to have made a mistake in the 

 locality, since he says that they are found on Santiago Point, and on 

 his geologic map he colored a portion on Santiago Point to show the 

 presence of younger coralline limestones. Not having access to Dana's 

 paper, and accepting Von Drasche's mapping the writer made a trip 

 westward from Balayan into Santiago Peninsula to see the extent of the 

 formation. Coralline limestones and marls were found at an elevation 

 of 200 meters. This formation together with the surface soil and 

 vegetation, effectively concealed the underlying rocks. The limestones 

 have the appearance of reefs and occur at elevations intermediate between 

 the highest on the upland and the living ones in the sea. Mr. Clark, 

 who was my assistant in the field work, made an excursion south from 

 Nasugbu and passed near San Diego Point. He reported the occurrence 



