428 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



prominent. There are some small crystals of augite and magnetite. The 

 mineral composition is not very different from that of many other rhyolites, 

 but there are minute inclusions of dark-colored rock not clearly recogniz- 

 able, probably basalt. This would explain the rather abnormal amount of 

 lime and the low silica. 



The chemical composition of the lava from the Upper Geyser Basin 

 is in accord with the mineral composition and habit of the rock, which 

 represents the least siliceous varieties occurring in the Yellowstone Park. 

 The rock is dull brownish gray, with numerous small phenocrysts, which 

 are mostly white feldspars. These are plagioclase, apparently oligoclase- 

 andesine, the presence of orthoclase being doubtful. There is no quartz. 

 Magnetite is in comparatively large grains ; augite is partly altered, and has 

 dropped out in preparing the sections. Apatite and zircon occur. The 

 groundmass is microspherulitic, with brownish spherulites and numerous 

 strongly doubly refracting needles, dotted with minute opaque grains. 

 These are altered microlites of augite with attached grains of magnetite. 

 These brownish mi crospherulites with dark needles are characteristic of this 

 variety of rhyolite, which might properly be classed as dacite. Occurring-, 

 as they do, as rather infrequent modifications of the great bodies of rhyo- 

 lite, they are here treated as dacitic facies of rhyolite. Exposures of this 

 rock occur back of the Grand Geyser and in a small hill near the Splendid 

 Geyser, in the Upper Geyser Basin. It forms the bluff west of the Paint 

 Pots, near the road in Geyser Meadow, where the finely vesicular rocks 

 resemble a basic andesite or basalt in outward appearance, but contain 

 phenocrysts of quartz, sanidine, and plagioclase. There are other localities, 

 less accessible, in which dacitic facies of the rhyolite occur. 



The chemical composition of pitchstone north of Obsidian Cliff is that 

 of a dacitic rather than of a rhyolitic glass, and yet the microscopical 

 characters indicate a closer relation to basalt, or augite-andesite. There 

 are, in fact, small inclusions of basalt scattered through the glass. The 

 character of the microscopic crystals and their similarity to those of the 

 inclosed basalt have been pointed out. The comparatively high percentage 

 of lime and magnesia, and the much greater amount of soda than of potash, 

 correspond to these mineralogical characteristics, and are not wholly due 

 to the inclusions of basalt. The different specimens of this dull obsidian 



