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The diorite—No. 1: Section of gneissie phase of diorite from one-half 
mile below Camp Cronk, pueblo of San Pedro, Loboo Mountains. 
Megascopic.—In the hand specimen, the rock is distinetly banded, the white 
bands consisting of plagioclase-feldspar; the dark ones of amphibole. It has a 
decided granitic texture. Striations on the plagioclases are easily seen. The 
average amphibole crystal measures from 2 to 4 millimeters in length. 
The rock breaks with a rough fracture, but parts easily although somewhat 
unevenly, along the zone of the amphiboles. 
Microscopic.—The rock, when examined in thin section, shows idiomorphice 
crystals of plagioclase by Michel-Levy’s method of symmetrical extinction, 
determined mainly as labradorite, and actinolite (the grass green amphibole). 
The labradorite belongs to that portion of the series of plagioclase-feldspars which 
is characterized by the predominance of lime, and would be expressed by the 
formula Ab.,An,. 
It is very plainly seen in a number of places in the slide that there has been 
secondary granulation and cementation. This minute quartz filling is undoubtedly 
a part of the general and larger quartz cementation indicated more plainly in 
the quartz fissure veins of the region. 
Although the nature of our work does not, at present, permit of petrographic 
study in great detail, a very rough attempt was made to classify this rock accord- 
ing to the quantitative classification given by Cross, Iddings, Pirsson, and Wash- 
ington.’ By this method, the proportion of salic to ferric minerals was found 
to be 84.32 to 15.70 per cent, and from further inspection of the mineral con- 
stituents, the writer has provisionally placed it in the subrang hessase of the 
Dosalane class, but measurements on several more slides and a chemical analysis 
would be necessary to make this classification entirely trustworthy. For the 
present and for the purposes of this paper the term diorite is sufficient. 
The sedimentaries.—Above the igneous basement and best seen in 
canyons near the mountains, is a basal conglomerate of varying thickness 
with, in places, a shaly matrix, in others, coarse sandstone around pebbles 
and even large bowlders of dioritic rock. Because of limited sections, a 
thickness of more than 50 feet was nowhere to be seen, but if the observa- 
tions of the writer in other parts of Luzon can be taken as a guide, it 
undoubtedly greatly exceeds this figure. This conglomerate we believe 
to be equivalent to that of the Agno and Bued river beds of Benguet, in 
those places in which the writer has studied it with A. J. Eveland, 
geologist, who has been working in that district. 
A dark, coarse, sandy shale containing a suite of fossils having a 
decided Miocene aspect lies above the basal conglomerate. One of the 
latter is, in fact, Vicarya callosa, which, it may be said, represents one of 
the milestones of Philippine paleontology. This one characteristic form, 
together with the accompanying fossils, makes it almost certain that all 
the sedimentaries lying stratigraphically above it are at least younger 
than the Eocene. These questions will be considered more at length in 
the discussion of the paleontology which is to follow. 
As erosion to a great extent has removed these beds and as the 
excessive formation of talus due to weathering has concealed a great part 
* Quantitative Classification of Igneous Rocks. University Press, Chicago, 1903. 
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