400 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



of the final consolidation of the magma, and is not included among the 

 phenociysts. In most instances the pyroxene is augite, which occurs in 

 almost all of the thin sections examined, but is present in variable amounts. 

 Like the other phenociysts, it is sometimes idiomorphic, with the crystal 

 form usual to its occurrence in similar rocks, or it may be partially rounded, 

 or in fragments. The crystals range from 3 mm. long to microscopic ones. 

 The color is green, which is strong grass green in moderately thick sections, 

 and pale green in thin ones. Its substance is very pure, with compara- 

 tively few inclusions, which are for the most part magnetite or ilmenite, 

 zircon, and pseudobrookite, and rarely apatite. In one instance augite 

 incloses a nearly idiomorphic crystal of quartz which is almost the same 

 size. In this particular case the augite is the younger of the two min- 

 erals; it is the only case in the 315 thin sections in which j^henocrysts of 

 these two minerals may be observed in conjunction. The augite is usually 

 associated with crystals of iron oxides, zircon, and pseudobrookite. 



In many cases the augite has a narrow opaque border, or a transparent 

 red one, undoubtedly composed of iron oxide. Rarely there is a narrow 

 border of brown glass surrounding the augite, when the glass of the ground- 

 mass is colorless. Occasionally fragments of augite show that the crystal 

 was inclosed in the black shell before it was broken — that is, the corrosion 

 took place before fracturing. The iron oxide sometimes penetrates cracks 

 in the augite. In one crystal there is a set of secondary inclusions like 

 delicate needles, which are arranged along cracks and lie parallel to one 

 another and to the orthoaxis of the crystal. In rhyolite that has been 

 subjected to the action of thermal waters the augite has been completely 

 decomposed and replaced by secondary minerals. 



In a few cases an orthorhombic pyroxene is also present in small 

 amount. It is faintly pleochroic, and appears to be hypersthene, though 

 ]30ssibly enstatite. It seldom occurs independently of augite, and is gener- 

 ally inclosed within the latter with parallel orientation. 



MAGNETITE AND TITANIFEEOUS IRON OXIDE. 



Magnetite and titaniferous iron oxide are prominent porphyritical 

 constituents of these rhyolites, although they are usually of microscopic 

 proportions — that is, they stand out plainly as relatively large crystals in 

 distinction to those forming the groundmass, and show by their mode of 



