NORTHWESTERN PANGASINAN. — 271 
Bataan, Pampanga, Zambales, Tarlac, and Pangasinan Provy- 
inces, a distance of about 150 kilometers. The rock types over 
this area show great similarity and the geologic history is 
practically identical. 
Within the area, the basal igneous formation can be divided 
into rocks of two ages. The older rocks generally are coarse- 
grained and consist mainly of gabbro and diorite along with 
granite, pyroxenite, and periodite. Large areas of fine-grained 
or porphyritic rocks occur, andesites and basalts, and it is possi- 
ble that part of these may be hypabyssal phases of the same 
magma. The exact nature of the formation is a matter of spec- 
ulation owing to the complete absence, as far as known, of the 
original rock into which the magma was intruded. The wide- 
spread similarity of the plutonic rocks in the Zambales Range 
certainly suggests that the magma was of batholithic size and 
nature. 
The age of this great intrusion is almost certainly pre-Miocene, 
and the majority of writers have placed the basal igneous of the 
' Philippines in the pre-Tertiary. Becker has stated :*? 
From early Paleozoic times onward an archipelago has usually marked 
the position of these Islands. The diorites and associated massive rocks 
may have made their appearance about the close of the Paleozoic. 
The occurrence of dikes in many parts of the area shows that 
igneous activity was renewed subsequent to the solidification 
of the magma. 
Mount San Isidro in the southeast corner of the area is of vol- 
canic shape (though greatly eroded), and the felsitic to por- 
phyritic character of the surrounding rocks also indicates its 
voleanic nature. Most of the andesites of the adjacent area 
probably represent lava flows originating from this volcano, 
although these flows apparently were massive in their character 
as beds are conspicuously rare. The age of this volcanic activ- 
ity is probably late Tertiary. 
The tuffaceous sedimentaries lay upon the basal igneous and 
were deposited in the Miocene-Pliocene sea subsequent to the 
lava flows, although some distant volcanic activity must have 
been taking place during the period of deposition. The struc- 
ture of the sedimentaries, as a whole, is monoclinal with a dip 
of 0 to 10° northwestward to the China Sea. Variations in 
the strike and dip show that a slight amount of folding has 
taken place. Mashing and faulting apparently are absent. No 
' intrusions were noted in the sedimentaries, and it is very likely 
* Becker, loc. cit., 81. 
