EHYOLITE OF MADISON CANYON. 367 



exposures of rhyolite. A rude columnar structure is plainly seen in the 

 rhyolite, which is evidently one thick sheet and not a succession of thin 

 sheets superimposed on one another. 



The face of the cliff on the north side of the canyon exhibits long, 

 slender, vertical columns of rhyolite, 200 feet high, with shorter ones some- 

 what inclined. The upper part of the cliff is traversed by approximately 

 horizontal joints, which arch over the tops of the vertical columns, a structure 

 similar to that in the upper part of the glassy end of Obsidian Cliff. The 

 rhyolite at this point is lithoidal, with numerous small phenocrysts, and is 

 banded dark and light purple. The lighter-colored parts are finely porous 

 (1857). West of this prominent point the rhyolite is massive, in nearly 

 horizontal layers, and weathers into rounded pinnacles, which are shown 

 in PL XLV. The rock from this mass breaks into great rounded blocks, 

 like granite, and weathers into angular sand. It is literally crowded with 

 phenocrysts of quartz and sauidine, with abundant small rusted augites, 

 and is a nevadite. In places it has a dark-purple banded groundmass and 

 is somewhat vesicular (1858). Parts of it are filled with hollow spherulites, 

 which are highly crystalline and porous, with gaping cracked hollows at 

 their centers, which are encrusted with tridymite and minute feldspars, with 

 larger crystals of quartz exhibiting the characteristic steep rhombohedral 

 faces, § R. There are also a few rusted fayalites. In some instances there 

 are indications of concentric shells (1859, 1860). The same nevadite forms 

 the summit of the highest point of the north wall of the canyon (1862). 

 From a distance it appears as though there were a thinner sheet of rhyolite 

 overlying a very thick one, the top of the lower flow weathering away and 

 leaving the bottom of the upper one dense and well defined, but this 

 appearance may be deceptive, since the rock occurring in the talus at the 

 base of the slope'is all one variety, very rich in phenocrysts. The apparent 

 difference may arise from slight variation in the physical character of the 

 rock at that place. 



MADISON PLATEAU, NORTH OF THE LOWER GEYSER BASIN. 



The northwestern arm of the rhyolite plateau, west of Obsidian 

 Creek and Gibbon River and north of the Madison River, covers all the 

 lower portion of the northwestern corner of the Yellowstone Park, reaching- 

 up the slopes of the Gallatin Mountains to altitudes of 8,000 and 8,600 feet. 



