422 Pirsson — Petrography of Tripyramid Mountain. 



entirely into solution, a point emphasized by Hildebrand* and 

 Washingtonf in the determination of this oxide. 



The iron ore is also freely distributed in grains which are 

 very small in size compared with those of the feldspar and aug- 

 ite ; some is in the form of dust in the augite, and some appears 

 secondary from olivine. One or two cases of larger grains 

 were seen, and into them augite and feldspar projected, show- 

 ing in this case a relatively late, or long-continued, period of 

 crystallization. The analysis proves that the iron ore is very 

 largely ilmenite. 



jBiotite occurs in small flakes and shreds, which sink to sub- 

 microscopic dimensions. It occurs in augite and feldspar. 

 Only one instance of a larger flake, about 0*05 mm in diameter, 

 was seen. As usual, it is apt to be attached to iron ore and 

 olivine. It is a very pale-colored variety recalling phlogopite, 

 and the pleochroism is not marked, between medium brown 

 and almost colorless. Although rather generally sprinkled 

 through the rock, the total amount is small, not more than two 

 or three per cent of the total constituents. 



The olivine is the only mineral which is not fresh and unal- 

 tered. It is mostly converted into masses of a yellowish-green 

 serpentine, in which small unchanged fragments of the original 

 mineral are still found. It was originally present in small 

 irregular masses, or lumps, without good crystal form, averag- 

 ing from O04-0-05 mm . It is judged, from the character of the 

 alteration product, and the iron ore separated out, to have been 

 a variety rich in iron. 



The augite is of a pale brown color ; the larger masses quite 

 irregular in shape, sometimes wedged between feldspars, some- 

 times indented by them, and thus showing here and there a 

 tendency to ophitic fabric. The smaller grains, however, at 

 times have a more distinct crystal form and approach short 

 prismoids with the faces 010, 100, and 110 developed in the 

 prismatic zone. It has the usual wide angle of extinction. It 

 is very impure, filled with shreds of biotite, specks of iron ore 

 and needles of apatite, and is frequently colored dark by iron 

 ore dust peppered through it in irregular blotches. It is the 

 most abundant ferromagnesian mineral. Although the masses, 

 or grains, vary in size from 0*10-0 # 05 mm and from that down, 

 no true phenocrysts of it were seen. 



The plagioclase, as previously mentioned, occurs in very 

 rare, distinct phenocrysts, columnar on the a axis and 4-5 mm 

 long. One of these was found, fortunately, to be cut perpen- 

 dicular to the o (010) face, and having both albite and Carlsbad 

 twinning, which permitted it to be determined as a labradorite 

 of the composition Ab,An r 



*Bull. U. S. Geol. Surv., 176, p. 78. 

 \ Chem. Analysis of Eocks, p. 151. 



