DRAINAGE CONTROL BY JOINTING IN ANGAT DISTRICT, 
BULACAN PROVINCE, PHILIPPINE ISLANDS 
By HupBert G. SCHENCK 
Of the Division of Mines, Bureau of Science, Manila 
TWO PLATES 
In making the trip north from Montalban, Rizal Province, 
to Angat River, Bulacan Province, Luzon, one is impressed from 
a distance by the nearly uniform elevation of the ridges and 
hilltops, and upon his first sight of Angat River the observer 
is apt to conclude that here is an old-age stream rejuvenated, 
judging from what appear to be numerous intrenched meanders, 
and that the country was once peneplained. A study of the 
topographic map of the region (Plate 1) apparently confirms this 
first field impression, for the hills vary from 300 to 400 meters 
in height, and winding rivers are evidenced. However, field 
work tends to disprove any theory of peneplanation that might 
be set forth and indicates instead that the drainage,’in a limited 
area at least, is controlled by a definite system of joints in 
igneous rocks. It is the purpose of this paper to give reasons 
for this interpretation. 
The field work upon which this paper is based was limited 
to an area between Montalban on the south and Angat River 
on the north (chiefly from Paila to Papaya on Angat River), 
while the writer was a member of a geologic party investigating 
a proposed dam site for the Metropolitan Water District of - 
Manila. The region is essentially an igneous one, although some 
Miocene limestone outcrops in Ipu River, at Papaya, near Pinag- 
kamalejan Barrio (Pinagkamaligan) between Ipu and Paila, and 
near Sicao. The principal rocks, however, are andesites, ande- 
sitic agglomerate, and basalts, though diorite and _ silicified 
' intrusive masses are known. Most of the igneous formations 
are younger than the Miocene limestone. 
The immediate area under discussion is not one of high relief, 
few peaks reaching a height of more than 400 meters, while the 
lowest valleys have elevations ranging around 100 meters above 
sea level. To the east rise the high peaks of the Eastern Cor- 
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