THE GEOLOGY OF LUZON, P.I. 39 
The western Cordillera—The western Cordillera is generally 
known as the Zambales Range, extending from Olongapo north 
into Pangasinan Province. Another part of this range consists of 
a cluster of volcanic stocks in the Province of Bataan. This range 
of mountains is not by any means a continuous one—there being 
a few isolated high peaks—but in the main the range is not very 
elevated. The highest point in this range is Mount Pinatubo. 
This has never been accurately measured, but is in the neighbor- 
hood of 6,000 feet. Very little geological work has been done in 
this Cordillera. Von Drasche has done some work in the neighbor- 
hood of Iba, and Fanning has touched it in a few points in the 
neighborhood of Agno and Alaminos in Pangasinan, and I have 
been on the second highest peak of Pinatubo, about 5,500 feet; and 
also on one of the high peaks of Mount Mariveles, which is also 
about 5,000 feet high. 
In general, the rocks of this range are volcanic extrusives, 
andesites, with marls and shales on the flanks. The Cinco Picos 
Range, however, on the western side of Subig Bay, consists of a 
totally different rock from that found on the east side, being a 
dense pyroxenite.* There are no active volcanoes along this line,? 
and the old volcanic stocks are pretty well eroded. 
There is a considerable stretch of alluvial running from Subig 
northwest to San Narciso. ‘This stretch of country here is very dry 
in certain seasons, and owing to the composition of the soil the 
water sinks in rapidly and the whole appearance of the country is 
very much like that of the desert in the western part of the United 
States—particularly on the western slope of Pinatubo. For a 
* The effect of the geology upon geodetic calculations was very effectively demon- 
strated recently in this part of Luzon. A considerable discrepancy between the 
astronomically determined points and the trigonometric stations near Olongapo was 
found to exist. The small Cinco Picos Range, which consists of pyroxenite, lies to the 
west of the stations and the great andesitic mass of the Zambales to the eastward. 
The observers expected the plumb bob to be deflected in an easterly direction owing 
to the main mountain mass being to the east, but the deflection was in the opposite 
direction toward the smaller mass. Not until an examination disclosed the denser 
rock in the Cinco Picos (to the west) could the discrepancy be explained. 
2It was reported by Mr. Snyder of the Bureau of Lands that smoke was seen 
issuing from the top of one of these peaks. 
