32 WARREN D. SMITH 
west of Luzon, and the drowned river areas on the Camarines coast 
on the eastern side. We find extensive settlements on the western 
side of Luzon, on these coastal plains, the principal tribe being the 
Tlocanos, the members of which are perhaps the thriftiest and most 
energetic of all the tribes of the Philippines. These are the only 
people who produce all their own rice. On the eastern side of 
Luzon, the settlements are very, very scarce—in fact but little is 
known, or of the people north of Baler, except that they are not 
numerous. 
THE CENTRAL PLAIN OF LUZON 
The principal interior plain of Luzon is known as the Great 
Central Plain (Fig. 1) and is the chief place of settlement in the 
Philippine Islands. It is roughly, 120 miles long by 70 wide, 
stretching from Manila Bay on the south to Lingayen Gulf on the 
north. On the west, it is bounded by the Zambales Mountains, 
and on the east, by the Eastern Cordillera. This plain was proba- 
bly the site of an ancient arm of the sea—a fact that has been called 
attention to by a number of geologists, among them Adams* who 
has drawn a hypothetical map of the Tertiary geography of the 
central portion of Luzon. 
Composition.—In the northern and western portions this plain 
is largely composed of alluvial material, as shown by well-sections 
at Lingayen. In the southeastern part it is largely made up of 
pyroclastics, as can be seen in railroad and river cuts and numerous 
well-sections. That this pyroclastic material extended to a con- 
siderable depth is shown by the following well-section at Pasay 
near Manila: 
SECTION OF WELL AT PASAY, RIZAL PROV., LUZON, P.I. 
o to 18 ft. soil, sand, and seashells 
18 “ 83 “ gray and yellow silt with pebbles, shells, and calcareous 
concretions 
83 “ 87 “ fine to coarse basaltic pebbles and tuff 
STAM aEIS yellow-gray sand, some clay, fragments of soft tuff 
iti) 160 yellow-gray tuff 
160 180 yellow sand and tuff, small basaltic pebbles 
*G. I. Adams, ‘Geological Reconnaissance of Southwestern Luzon,” Phil. Jour. 
Sci., V, No. 2. 
