532 W. D. SMITH — GEOLOGIC INFLUENCES IN THE PHILIPPINES 



Taal is located. These are particularly numerous and a prominent fea- 

 ture of the topography in the vicinity of San Pablo, Batangas. Many of 

 them are perfectly circular and of unknown depths. They are of various 

 dimensions, some being only a few yards in diameter. 



Laguna de Bay. — This is the largest lake in the archipelago and the 

 most important in the life of the people of the islands. The greatest 

 length and width of water are 25 and 21 miles respectively. It is quite 

 shallow throughout. The contour of this body of water is perhaps the 

 most characteristic thing about it, being roughly heart-shaped and having 

 three prongs or fingers projecting northward. It seems probable that 

 this lake covers a stretch of low country once occupied by the sea, the 

 latter having been cut off by the deposition of a great amount of tuff, 

 and the later gentle folding of these tuff beds forming a dam on the 

 western side of the depression. The slightly arched tuff beds can be seen 

 plainly where the Pasig River cuts through them near Fort AYilliam Mc- 

 Kinley. The high land on which the post is situated is due to the bowing 

 up of these beds. The fact that this lake formerly was occupied by an 

 arm of the sea was proved to the writer's satisfaction by the finding of 

 suggestions of marine terraces on Binangonan Peninsula. Adams was 

 unable to see these, and also cast some doubt on the evidence produced by 

 the finding of recent marine shells on some of these benches. It is ad- 

 mitted that these shells may have come from kitchen-middens, as Adams 

 suggests, but the marine terrace theory seems more correct, since the 

 latter alone will satisfactorily account for the presence and appearance 

 of the benches. In a region of excessive rainfall terraces would not be 

 as well preserved as in more arid regions. 



Around the eastern shores of the lake, on the Jala Jala Peninsula, and 

 on Talim Island, there is considerable basalt in the form of flows and 

 agglomerate. The shape of the lake suggests former well defined north 

 and south valleys, whose width, shape, and extent have been modified by 

 these flows and tuff deposits. 



Lake Lanao. — This lake, 15 miles in length by about 12 miles wide, is 

 situated at an elevation of 2,250 feet above sealevel, in the upland of 

 northern Mindanao. It is roughly bell-shaped and is supposed to be very 

 deep. The writer is of the opinion that it is not a crater lake, as many 

 have supposed, but has originated through the damming up of the head- 

 waters of the Iligan River by a basalt flow and loose material which has 

 come down from the mountains on either hand. A noteworthy feature of 

 the topography of this lake country is the Keithly escarpment, on the 

 south shore of the lake. The abrupt slope of this topographic feature is 

 away from the lake. No big cuts were found in this ridge, so no judg- 

 ment of its composition was possible; but the inference from what could 



