﻿238 SMITH. 



BENGUBT NO. 4. — QUARTZ DIORITE. 



The primary minerals present are plagioclase, amphibole, quartz, magnetite. 



Feldspars. — The plagioclases correspond to oligoclase, as all the ex- 

 tinction angles are almost without an exception low, 8° or very close 

 to that figure ; the index of refraction is also lower than that of quartz, 

 showing that these feldspars could not be either labradorite or anorthite. 

 The)' occur in icliomorphic crystals showing zonal growth in many cases 

 and much polysynthetic twinning according to the albite law. No good 

 Carlsbad twins could be made out, but this form of twinning does occur. 

 The average diameter of the feldspar sections varies from 0.5 to 1.5, and 

 2 millimeters is the extreme observed. 



Quartz.- — Occurs as an interstitial filling. It does not form a con- 

 spicuous constituent of any of the sections from this rock. 



Amphibole. — Some ragged sections parallel to prism and fragments 

 of basal sections form a small portion of the section. The amphibole is 

 grass green, extinction angle about 17°. 



BENGUET NO. 12. — QUARTZ DIORITE. 



This is a section of a typical quartz diorite. It is quite fresh, showing 

 little or no alteration. The texture is granitic, the holocrystalline, 

 fabric liypidiomorphic-granular. The minerals of the rock are plagio- 

 clase and orthoclase feldspar, hornblende (actinolite) quartz, magnetite, 

 and accessory apatite. The feldspars for the most part are plagiocalse, 

 exhibiting both Carlsbad and albite twinning. From the extinction 

 angles which were taken on a section cut normal to the albite twinning 

 I made out this piece at least to belong somewhere near the middle of 

 the series, oligoclase or labradorite. Some orthoclase is present, as- 

 sociated more or less with the quartz. It is decidedly not the dominant 

 feldspar. The amphibole is the pleochroie, grass-green variety known 

 as actinolite with pleochroism as follows: 5=colorless to yellow; c=dark 

 green. Quartz occurs wholly as interstitial material. Magnetite is 

 found as minute, rounded grains inclosed by hornblende. 



A series of measurements on this slide establish the following ap- 

 proximate proportions of the various minerals, from which was calculated 

 the following analysis. 



Constituent. Per cent. 



Quartz 2.04 



Plagioclase 67.09 



Hornblende 22.93 



Magnetite 6.28 



Olivine .73 



Magnetite is largely confined to the region of the amphiboles, either 

 as inclusions or in juxtaposition. 



Accessory minerals an almost entirely lacking or so inconspicuous as 

 to warrant no lengthy search for them. 



