430 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



INTERMINGLED EHYOLITE AND BASALT. 



Iii several localities, chiefly on the plateau west of Beaver Lake and 

 on the south bank of Gardiner River south of Bunsen Peak, there are lavas 

 whose abnormal appearance is striking. At the first-mentioned locality the 

 rock is partly lithoidal and dark gray, with phenocrysts of feldspar and 

 quartz, partly purplish gray to reddish, and finally porous or vesicular. 

 The latter contains small phenocrysts of feldspar, generally lath-shaped or 

 tabular, besides quartz grains, and occasionally olivine. The vesicular 

 cavities have minute white pellets of tridymite. Other parts of the same 

 rock are dark gray and lithoidal or glassy, with dull gray sphernlites, 8 or 

 10 mm. in diameter and smaller. These are distinctly radially fibrous and 

 slightly porous, in some parts of the rock passing into hollow sphernlites 

 (1804, 1806). These various modifications of the rock grade into one 

 another and intermingle in streaks without any distinct line of demarca- 

 tion between them. The appearance is that of a completely fused mass of 

 somewhat differently constituted materials. In places in this vicinity the 

 spherulitic glassy rhyolite incloses distinct fragments of basalt (1802). 



The microscopical characteristics are somewhat different from those of 

 normal rhyolite. The glassy portion in thin section is in places colorless 

 and sometimes perlitic. The number of microlites varies; in some places 

 they are scarce, in others abundant. They consist of rectangular feldspars, 

 and some with pointed corners. There are microlites of quartz and micro- 

 prisms of augite, with a few of hornblende, and in some instances minute 

 hexagonal plates of hematite or biotite. In places the colorless glass grades 

 into brown along the contact with basalt. The brown glass contains crvstals 

 from the basalt, and appears to have been produced by a fusion of the 

 latter, larger crystals of feldspar occasionally projecting into the brown 

 glass. The colorless glass has penetrated vesicular cavities in the basalt 

 and has partly fused the latter. The brown glass grades into globulitic 

 and trichitic colorless glass by the concentration of the coloring material. 

 By a more distinct crystallization there arise augite prisms and needles 

 with minute grains of magnetite attached to their surface; also thin curved 

 needles or "hairs" of augite, to which are attached clusters and clouds of 

 minute specks of magnetite. The extremely delicate hair-like needles 

 of augite are straight or curved and sometimes curled up at the end. 

 Occasionally the stouter needles are broken into lines of "margarites" whose 



