﻿244 SMITH. 



The existence of the ferromagnesian minerals, pyroxenes, or amphi- 

 holes can only he inferred as alteration has left no recognizable remnants. 



Remarks. — The alteration of the plagioelas.e has gone so far that scarcely a 

 trace of the polysynthetic twinning can be made out; however, the outline of the 

 original mineral is still preserved. The interior of these phenoerysts is usually 

 taken up with quartz and epidote, some of which is the low, doubly-refracting 

 zoisite. 



Pyrite is abundant in some of the slides, magnetite in all of them. 



BENGUET NO. 106. — HYPERSTHENE AUGITE ANDESITE. 



Macroscopic. — An almost black aphanitic, for the most part micro- 

 cryptocrystalline rock, hut plainly porphyritic. The larger phenoerysts, 

 not over 2 millimeters in diameter, are jet-black amphiboles or pyroxenes, 

 while there are smaller, lath-shaped feldspars, which are apt to be over- 

 looked unless the light strikes them. These are about 2.5 millimeters in 

 length by 0.5 millimeter wide, and are striated. Occasional larger, 

 glassy spots of what is probably quartz are also to be seen. 



Microscopic- — This rock in thin section reveals a distinctly porphyritic 

 structure, large, square and brick-shaped feldspars, plagioclases, and 

 pyroxenes occur, also idiomorphic in a groundmass, which varies from 

 vitreous to cryptocrystalline, and also holocrystalline. 



There are a number of large, -irregular, also sometimes square sections 

 of magnetite in the slide, as well as innumerable fi ne grains of it in the 

 groundmass. 



The plagioclases show both albite twinning and zonal structure. Sec- 

 tions normal to the albite laminas give extinction angles of 26° to 28°, 

 thus corresponding to andesine. One zonally built crystal of feldspar 

 appeared to have an outer coating of anorthite. Carlsbad twins also are 

 not infrequent butthe thickness of most of the slides of this rock prevent 

 close investigation of their extinction angles. Plate I, fig. 3, shows 

 typical appearance of altered feldspar or rather of one which is about 

 three-fourths decomposed. 



The pyroxene is largely hypersthene and exhibits marked pleochroism, 

 but augite, which reveals none, is also to be seen in the slide. Plate I, 

 fig. 4, shows the habit of the hypersthene in this rock. 



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



Remarks. — In one place augite has grown around the hypersthene, having 

 crystallized later. It would also appear that the plagioelase had crystallized 

 after the hyphersthene. From evidence in the slide it would seem as if the felspar 

 and the augite cystallized out almost simultaneously. Plate I, fig. 5, shows a 

 basal section of augite twinned parallel to the macropinacoid. 



It appears, from the descrptions by Diller, Iddings and Becker, that we have 

 here a hypersthene-augite andesite very nearly identical with the recent lavas of 

 Shasta County, California, and the Comstock Lode, Nevada. See Bull. 150 

 V. 8. G. S., Diller; also Geology of the Comstock Lode, Becker; also Mon. 20, 

 U. S. G~S., 364.) 



