﻿BENGUET PETROGRAPHY. 245 



BENGUET NO. 107.— AUGITE ANDESITE. 



Microscopic. — This rock is almost an exact duplicate of No. 155. 



Phenocrysts of plagioclase and augite lie imbedded in a typical ground- 

 mass of small, lath-shaped feldspars arranged in an orthophyric manner, 

 making a typical andesitic groundmass. Some of the slides are thick, 

 so that this structure of the groundmass can not be seen, but one slide 

 shows it well. 



The feldspars show extinction angles varying from 10° to 31°, so that 

 the phenocrysts vary from oligoclase to labradorite. All the phenocrysts 

 are quite fresh. The augite occurs in idiomorphic sections, both basal 

 and prismatic. One prismatic section (Plate I, fig. 6) shows twinning 

 and cleavage. The twinning plane is not exactly parallel to c. 



Magnetite is very abundant, but in rather small grains in the ground- 

 mass. In one slide there are remnants of a substance indicating the 

 former presence of hornblende, but we can not be sure that the slide 

 contains any hornblende or amphibole. 



BENGUET NO. 101. — ANDESITIC TUFF. 



Macroscopic. — This is a heterogeneous mixture of particolored frag- 

 ments, which are angular pieces of lava in various states of crystalliza- 

 tion. The groundmass is uniformly fine grained and dark-red to brown- 

 ish-black in color. The fragments which we find well cemented here 

 were evidently blown out of some volcanic vent and afterward rendered 

 compact under water. Occasional large crystals of hornblende, 5 by 7 

 millimeters, can be seen. The rock fragments are for the most part horn- 

 blende andesite. The largest fragment in the hand specimen is 22 

 by 12 millimeters. 



Microscopic. — These are sections from what we may term an "eruptive 

 conglomerate," or more properly speaking, an agglomerate of eruptives. 

 All the material appears to be andesitic, there being two main kinds. 

 These two differ chiefly in the material of the groundmass and this is 

 evidenced mainly by the color, one portion, making up the larger part 

 of the slides, being dark reddish or brownish. 



Green hornblende, actinolite, and the light-colored monoclinic 

 pyroxenes are present making up about one-half the slide. 



The feldspars are plagioclase, but more than this we can not tell from 

 the slide, as alteration and thickness obscure the albite twinning. 



Epidote as an original constituent is seen in one slide. It is perfectly 

 idiomorphic and appears in outline as shown in Plate I, fig. 7. 



The marked basal parting distinguishes this from hornblende or 

 pyroxene. There is scarcely any pleochroism. 



Plate III, fig. 2, is a photomicrograph of this rock. 



