﻿BENGUET PETROGRAPHY. 247 



Some small amounts of magnetite, more hematite and limonite can be 

 seen scattered throughout the groundmass and the muscovite phenocrysts. 

 The alteration products of the muscovite could not be made out even 

 with the highest power (oc. No. 3, obj. No. 9) ; all that could be seen was 

 a granular mass or very small, irregular, doubly refracting bodies. 



Plate IV, fig. 1, is a photomicrograph of this rock. 



Remarks. — The alteration of the feldspars is the characteristic saussuritization, 

 with calcite and epidote certainly formed. 



By testing a fragment of the less altered portion of the feldspar with a 

 standardized oil, I foimd the unaltered portions to have an index Na 1.545 to 

 Na 1.554, corresponding to that of oligoclase and andesine. 



BENGUET NO. 126. — ANDESITE PORPHYRY. 



Macroscopic. — This rock in the hand specimen has a yellowish-gray 

 color, rather fine texture, yet apparently it is granitic. Occasionally a 

 patch of finer grained and darker material is present, suggesting an 

 orbicular structure. 



Microscopic. — In thin section the most striking feature of the rock 

 is the state of alteration of practically all the minerals. The feldspars 

 are no longer clearly recognizable. They are very much altered, but 

 retain their original outlines. Their arrangement in places is almost 

 ophitic, in others trachytic. Bounded and irregular grains of what 

 was originally hornblende and magnetite are all that remain of the 

 original minerals. 



Chlorite in large patches with radial structure is the most prominent 

 secondary mineral. 



Calcite in veinlets and also as an alteration product of the feldspars 

 is quite plentiful. Large patches of radial chlorite with blebs of calcite 

 scattered through them are quite characteristic of the slide. 



Some secondary epidote is found closely associated with the feldspars. 



Remarks. — At first glance one might be inclined to call this rock detrital, but 

 an occasional feldspar with idiomorphic outlines and the structure of the rock 

 would preclude this. 



From examination of the hand specimen and the slides I should call the rock a 

 dike rock — a phase of andesite porphyry. 



BENGUET NO. 23. — FELDSPAR PORPHYRY. 



This is a porphyritic rock in which zonally built plagioclase feldspars 

 are the phenocrysts. There are also some large magnetite crystal sections, 

 more or less idiomorphic, in the slide. 



The groundmass is trachytic and made up of small, lath-shaped sec- 

 tions, and an abundance of minute magnetite particles. 



Calcite is fairly abundant, being associated with feldspars; in some 

 cases large patches of calcite can be seen inclosing a few small grains 

 of feldspar; then again the calcite occurs as irregular stringers through 



