60 The Philippine Journal of Science 1922 
ture is hindered, if not prevented, by this. For example, if the 
creek at Dam Site 2 could work headwards with ease, it might 
capture Sapang * San Lorenzo and also the drainage of the main 
river, but this is partly prevented by hard, silicified andesite 
that acts as the divide here. A hard dike turns the river near 
Paila between Dam Sites 1 and 2, as is shown on the map. As 
a rule bends in the river are actually more abrupt than shown 
on the map. 
As a third important cause of the extraordinarily abrupt bends 
of Angat River, one must record faulting and the fact that the 
faults may be in conformity with the joint system. Faulting 
probably accounts for the short change of course upstream about 
6 kilometers from Papaya, where the long southwest flow is 
changed for a short distance only, and where other evidences 
of faulting are to be noted. 
These three causes of drainage control have combined with 
one another at places to bring about the present land forms. 
At the entrance of the gorge below Paila, at Dam Site 1 (Plate 
2, figs. 1 and 2) the jointing is north 30° west, as is likewise 
the strike of beds of andesite and agglomerate, and here, working 
up a joint in the least resistant material, is a small stream, 
showing that erosion is aided by both the nature of the rock and 
the joints. At another place near Ipu River a valley is develop- 
ing along what appears to be a fault, while at still another local- 
ity (near the dam sites) a fault parallels the jointing and re- 
enforces the joint-system control. But when all is considered 
and more observations are recorded, the writer believes that 
jointing will best explain the majority of drainage features. 
These systems of jointing are by no means unusual, for it is 
well known that there is a tendency for fractures to intersect at 
right angles to one another and for all the sets to be at about 
45° to the direction of compression. It is, however, in the world 
relationships that the northwest—southeast and northeast—south- 
west jointing of Angat district is most interesting. For instance, 
Iddings* records similar drainage control in the region north of 
the Yellowstone National Park in the United States, while Hobbs * 
discusses this matter of structural control of drainage (and 
* Sapa, becoming sapang in combination, is Tagalog for “creek.” 
_ Iddings, J. P., A fracture valley system, Journ. Geol. 12 (1904) 94-105. 
* Repeating patterns in the relief and in the structure of the land, Bull. 
Geol. Soc. Am, 22 (1911) 123-176. 
