﻿246 SMITH. 



Remarks. — One of the noteworthy features of this rock is the intergrowth of 

 feldspar and hornblende. (Plate I, fig. 9.) The feldspar is found in irregular 

 grains all through the hornblende with a sharp, elean-eut line between the two 

 minerals. Their appearance is decidedly not that of alteration of the hornblende, 

 for they are in a very fresh state. 



Magnetite, as usual, is quite abundant, though in small grains. 



BENGUET NO. 102. — ANDESITE TUFF. 



Macroscopic. — This apparently is a phase, though somewhat different, 

 of Nos. 101 and 103. The rock fragments are larger and somewhat more 

 rounded. The different portions of the rock vary considerably in color — 

 that is, they are almost black, the color changing through chocolate to 

 greenish-yellow — and the grain varies from exceedingly fine-grained por- 

 tions to those containing phenocrysts of considerable size. 



Microscopic. — This rock is evidently an intermediate phase between 

 Nos. 101 and 103. The mineral composition is practically the same, 

 plagioclase, pyroxene, hornblende ( ?), and magnetite. If there is any 

 quartz present, it is in very minute quantities. However, there are two 

 very noticeable features as exhibited in one slide. One of these, and 

 perhaps the most important, is the pseudo-microspherulitic structure. 

 Plate I, fig. 8, will show the relationships. 



The second feature is the fairly abrupt contact between the darker and 

 lighter portions of the rock. The darker differs merely in the possession 

 of more iron staining its groundmass. Under the microscope this line 

 appears far less regular than in the hand specimen, and the two portions 

 of the rock dovetail along this boundary. 



BENGUET NO. 132. — ANDESITE PORPHYRY. 



Macroscopic. — A coarse-grained, porphyritic rock consisting of large, 

 approximately square phenocrysts of white, dull to greasy looking feld- 

 spars set in a cryptocrystalline groundmass of a bluish black color. An 

 average phenocryst measures 7 by 9 millimeters. Minute grains of a 

 white mica are found as inclusions of the feldspar. The groundmass is 

 slightly vesicular. 



Microscopic. — On microscopic examination the groundmass, and the 

 phenocrysts as well, are seen to be cryptocrystalline. Alteration has 

 gone quite far and this is the reason for the oily appearance of the 

 phenocrysts which are of two kinds and sizes, the larger were originally 

 plagioclase. This is proved by occasional albite twinning in the unaltered 

 remnants. They are nearly quadratic in outline. 



The smaller phenocrysts occur with lath-shaped outlines, sometimes 

 in rude, pseudo-hexagonal sections but more often they are rounded and 

 badly corroded, being accompanied by a dark-greenish reaction rim, 

 probably consisting in part of ferrous iron. They are undoubtedly 

 remnants of muscovite. 



