JRHYOL1TE NEAR MAMMOTH HOT SPRINGS. 357 



another and to show their origin, it will be necessary to describe the field 

 occurrence and megascopical characteristics in a number of localities within 

 the Park, and afterwards to treat systematically the microscopical characters. 



MEGASCOPICAL CHARACTERS. 



VICINITY OF MAMMOTH HOT SPRINGS. 



The rhyolite at the Golden Grate, where the road to the Geyser Basins 

 passes along the face of a rocky cliff at the south end of Terrace Mountain, 

 is a dense, light purplish-gray rock, separated into distinct horizontal 

 layers, and jointed by irregular vertical cracks, which cause it to weather 

 in pinnacles of angular blocks. In the lower part of the cliff the rhyolite 

 is dense and dark purple, passing up into lighter-colored and more porous 

 forms, with occasional flattened cavities and yellow spots. The rock has a 

 stony lithoidal groundmass, in which glisten small crystals of quartz and 

 sanidine (1775, 1776). 



It also forms a prominent cliff, 100 to 150 feet high, along the top of 

 the southern portion of the west escarpment of Mount Everts, which is 

 well shown in the panoramic view of this mountain, by W. H. Holmes, 

 which accompanies his report on the geology of the Yellowstone National 

 Park. 1 



Here the rhyolite is massive, with a rude columnar structure. It is 

 reddish purple and lithoidal, with many small phenocrysts of quartz and 

 feldspar. Near the northern end of the sheet, on the summit of Mount 

 Everts, it passes into a grayish-white rock, finely porous in spots (1762). 

 Beneath the rhyolite sheet there is a deposit of rhyolitic dust or ash 

 about 4 feet thick, in places more, which, as Holmes has pointed out, is 

 beautifully and delicately laminated in light and dark grays, brown, and 

 buffs. The top of the sandstone strata on which this ash rests is covered 

 with a thin layer of small fragments of the same kind of sandstone and 

 sandy soil, only a few inches thick. Upon this is a layer of rhyolitic ash 

 or sand, passing up into very fine white dust (1763), formed of microscopic 

 angular particles of glass and a small amount of crystalline grains. This is 

 thinly laminated. Over it are alternating layers of coarser rhyolitic sand 

 and finer dust, all laminated, witli the thinnest possible lines, which are 



■Twelfth Ann. Rept. U. S. Geol. and Geog. Suvv. Terr. (Hayden), for 1878; Part II, PI. XXXII. 



