530 W. D. SMITH GEOLOGIC INFLUENCES IN THE PHILIPPINES 



swamps ; second, those formed by a local widening of a river ; third, those 

 due to a damming up of a drainage basin ; fourth, explosion crater lakes ; 

 fifth, those due to local subsidence of the land. 



Of the first type we have Lake Bato, in southeastern Luzon; Lakes 

 Liguasan and Buhian, in Mindanao. Of the second are Lake Canarem, 

 in the Central Plain of Luzon, and Lake Bito, in the eastern part of 

 Leyte. The best example of the third is Laguna de Bay, near Manila, 

 while Lake Lanao is another of this type. There probably are several 

 explosion crater lakes, but the best known is Laguna de Bombon, also 

 known as Lake Taal, situated about 35 miles due south of Manila, while 

 other examples probably are Laguna de Manjan, in the northeastern part 

 of Mindoro, and several smaller ones on the islands of Sulu and Cagayan 

 Sulu. Among those illustrating the subsidence type are the Agusan 

 lakes — Pinayat, Dagun, and Linao. 



It very probably is true that many of the^e lakes have originated 

 through the operation of more than one of the causes enumerated, as in 

 the case of Laguna de Bay, which may have been formed through sub- 

 sidence in part and also by a barrier of arched tuff beds being elevated 

 between this low strip and the sea. 



To consider in detail the examples under each type : 



Lake Liguasan. — In the dry season this is little more than a great 

 swamp, which the rainy season transforms into a large lake. It covers 

 many miles of territory just south of the point where the Eio Grande de 

 Cotabato changes its course from south to northwest. In fact, this body 

 of water is connected by a maze of shallow channels with the river, and 

 also with another but smaller lake to the south, known as Laguna de 

 Buluan. These two lakes have important Moro settlements on their 

 shores, since they afford good fishing and hunting grounds, as well as 

 cheap means of transportation. The surrounding country is low and 

 excellently adapted to rice growing. 



Lake Canarem. — On the old Jesuit maps of Luzon this lake is shown 

 as a coalescing of several tributaries, and, strangely enough, according to 

 the maps, water seems to flow into it from almost any direction, and out 

 of it either toward the Agno — north — or toward the Pampanga — south. 

 The area covered by this lake is only about 25 square miles, bnt it is a 

 good example of the extended river type. 



x^gusan Lakes. — These four lakes, situated about half way between the 

 source and the mouth of the Agusan Eiver, which flows through them, 

 are apt to coalesce into one big lake in the rainy season. According to 

 tradition, they were formed during a local subsidence of the valley. This 

 seems to be confirmed by the fact that this region is marked by the 

 greatest degree of seismicity in the archipelago. Those who have traveled 



