﻿BENGUET PETROGRAPHY. 243 



Hornblende. — For the most part in broken prism sections, pleochroic 

 c==dark green, a = yellowish, in thinner section it would be nearly 

 colorless, it forms the next most abundant constituent of the slide, most 

 prominent in the "Ivugel" areas, where it is as abundant or even more 

 so, than the feldspars. However, it occurs in these areas in very small, 

 irregular grains, much smaller than in the more coarse-grained portions. 

 Where good contacts can be studied it appears that the hornblende 

 erystalized first. 



Quartz. — Probably makes up 2 to 3 per cent of the slide as interstitial 

 material. 



Magnetite. — Present, hut not very abundant. Largely confined to the 

 body of the hornblende. 



Sphene. — Occasional, stray crystals of small size. 



Iron oxide stains the slide here and there. 



Plate II, fig. 2, is a photomicrograph, with polarized light, of this rock. 



Remarks. — From all the indications that can he gleaned from the slide, the 

 darker "Kugel" areas simply represent a differentiation in the original magma. 

 This part, it is my idea, was a little richer in the constituents that make up the 

 ferromagnesium minerals ; certain conditions must have prevailed more favorable 

 for the production of hornblende than pyroxene, these were conditions of pressure, 

 temperature and the composition of the magma, the intimate relations of which 

 are just beginning to be understood. The results seem to be more in the nature 

 of a concretion and the law of mass action appears to have entered largely into 

 the process. 



An examination of the boundary between the darker, fine-grained areas and 

 the rest of the rock does not reveal a sharp line, although on macroscopic exam- 

 ination it does appear so. Under high power the boundary is irregular and chrys- 

 tals from both areas penetrate each portion. Quartz seems to be more abundant 

 along the contact than anywhere else in the slide. Just at that point the 

 rock solution may have been depleted of alumina, etc., through the formation of 

 excessive amounts of hornblende in the "concretionary" part. 



BENGUET NO. 116. DACITE. 



Macroscopic. — A hypocrystalline, rather aphanitic rock, which has a 

 somewhat porphyritic fabric and is greenish-yellow. The rock has a 

 dense, bluish, fine-grained ground mass in which are numerous phenocn r sts 

 of epidote, themselves not large, rounded areas which appear to be 

 decomposed feldspars and occasional, clear, limpid, more or less rounded 

 or irregular, areas of quartz. There are very few ferromagnesian min- 

 erals visible in the hand specimen, and they are mere black specks. 



Microscopic. — The prominent features of the slide are : 



1. The saussuritized feldspar (plagioclase) phenocrysts. 



2. The comparatively clear, rounded phenocrysts of quartz. 



3. The ground mass in places quite trachytic, in others glassy, the 

 whole having a felty or hyalopilitic character. 



