HENRY MOUNTAIN EOCKS. 361 



position. Plagioclase in thin plates may be seeu to enter into its constitution and the 

 qnantitj' of quartz doubtless varies. It even seems probable that in extreme cases the 

 groundmass may be entirely feldspathic. 



In nine other rocks, 24, 31, 33, 5G, 01, 62, 67, 68, and 69, the groundmass is ex- 

 tremely fine grained and acts but feebly on polarized light. The granular structure is 

 preserved, and I can find no proof of the glassy or strictly microfelsitic base. The 

 varying relative quantities of groundmass and crystals are particularly marked in these 

 fine-grained rocks (see 31 and 33). 



AUGITIC ROCKS. 



The rocks included here are Nos. 28, 43, and one of those numbered 31. Hand 

 specimens of 31 and 43 were among those sent. 



Macroscopicai. — Specimen 31 is distinctly porphyritic, the greater part is dull 

 ashengray in color, and in this portion feldspar and groundmass are not clearly dis- 

 tinguishable throughout. There are a few fresh pink feldspars in tabular crystals, 

 presumably orthoclase, reaching in one case nearly 2<=™ in length. Similar feldspars 

 ■were not noticed in any of the hornblendic rocks. 



The dark basic mineral is very black and occurs in short stout crystals, mostly 

 small, which lack the luster of hornblende. A careful examination with the lens 

 shows also that the sectiou of the prism is octagonal, with alternate sides but slightly 

 developed. This mineral is not so abundant as the hornblende in preceding rocks. 

 Glistening ore particles and yellow titanite are distinct, though small. 



Microscopical — (Of No. 31.) It is rather difficult to determine the nature of the 

 dominant feldspar in this rock. I think it is plagioclase, but cannot say that I can 

 prove it from the microscopical examination alone. In the first place, this feldspar does 

 not seem to polarize light so strongly as is common. Captain Button probably referred 

 to this rock when he said that certain feldspars "had almost ceased to polarize." In 

 the second place, those crystals determinable as plagioclase are apparently oligoclase 

 of medium composition, for the direction of total extinction in the sections examined 

 does not vary far from the line of the twinning plane. It is therefore often difficult to 

 recognize the polysynthetic structure. By the aid of the quartz plate many are found 

 to be distinctly triclinic, but still so m.any remain undeterminable that it is possible 

 that orthoclase predominates in the rock as a whole. The feldspars resemble those in 

 granitic rocks in their dirty appearance, the result of incipient decomposition pro- 

 ceeding from innumerable cleavage planes. 



Inclusions of augite are rare. Glass inclusions were not noticed and fluid ones are 

 indistinct and rare. The angite is irnique in its optical behavior in that it appears as 

 bright green by ordinary light and has a pleochroism as strong as is usually found in 

 green hornblende, giving, too, almost exactly the same colors. In all other and more 

 important respects this mineral shows the characteristics of augite. Contours of 

 prism, cleavage, and maximal angle of extinction in prismatic zone (nearly 45°) all 

 indicate augite. Titanite and magnetite often penetrate the augite. 



The groundmass seems wholly crystalline, yet is unlike that common in the 

 hornblendic rocks. It seems composed of feldspar and augite, with no visible quartz. 

 The feldspar is chiefly present in tabular particles, and not in irregular grains. The 

 pale-green microlites and grains, which are quite abundant, seem to be of augite, as 



