THE GEOLOGY OF LUZON, PI. 33 
180 to 463 e light, yellow-gray tuff, partially with basaltic pebbles 
463 “ 483 “ fine dark sand, some clear grains, tuff, basaltic pebbles 
483 “ 546 “ fine grained tuff, light gray 
546 “ 570 “ dark sand, some clear grains 
570 ‘‘ 594 “ tuff, with small basaltic pebbles 
504 “ 634 “ yellow clay with small basaltic pebbles 
634 “ 690 “ dark sand 
G90, 713 fine gray tuff 
713 ‘‘ 743 “* basaltic pebbles and fragments of tuff 
Phystographic features.—At first glance, the most. striking 
physiographic features of this plain are: (1) the drainage system; 
in the north the rivers flow to the north and in the south to the 
south; this is accounted for by warping; (2) the single extinct 
volcanic cone of Arayat, standing isolated in the center of this 
great flat; (3) the two large swamps and one lake located in the 
eastern part; (4) and the vast delta region of the Pampanga River 
which bounds Manila Bay on the north. 
Farther along in this paper, I shall draw some conclusions 
regarding the human response to the physiographic and geologic 
conditions, showing how these features have controlled the human 
settlements, the intermingling of various tribes, and such like 
questions. 
THE ALBAY PLAIN 
We shall pass now to the second great interior plain, which is 
known as the Albay Plain (Southeastern Luzon). This plain is 
about as long as the one we have been considering, but is much 
narrower. It extends from Legaspi to San Miguel Bay. It is an 
old coastal plain, on the outer margin of which a volcanic cluster 
has been built up since this plain rose from the sea. The main 
drainage of this plain is to the north by way of the Bicol River. 
There are two or three small lakes which are little more than 
swamp-areas along the course of this river. To the west of this 
plain, the rocks are sedimentary, while to the east, as I have 
already said, they are recent extrusives. The material of this plain 
is largely made up of volcanic ash, bowlders, bombs, lapilli, and tuff. 
This region is the principal hemp region of the Philippine Islands, 
and I believe that the peculiar composition of this volcanic soil is 
largely responsible for this. 
