372 GEOLOGY OP THE YELLOWSTONE NATIONAL PAEK. 



and blue mottled rock which on a smooth jointing surface exhibits a curious 

 pattern of curved blue lines and spots (1892). The numerous gulches and 

 small canyons around the east and southeast corner of the Lower Geyser 

 Basin afford excellent opportunities for studying the character of the rhyo- 

 litic lava in this neighborhood. Fine exposures of perlite and obsidian, 

 more or less spherulitic and usually associated with lithoidal rhyolite, are 

 found in many places. 



On the north side of the mouth of the canyon south of that draining 

 into Hot Lakes spherulitic rhyolite is exposed in which spherulites from 

 the size of large shot to an inch in diameter constitute almost the entire 

 rock mass. In places where they are very closely packed together the 

 exposures when seen from a short distance have the appearance of even- 

 grained conglomerate or bits of pebbles or coarse gravel. Exposures of 

 such character occur many feet in vertical thickness and many yards in 

 horizontal extent. Farther up this canyon vesicular rhyolite and fine gray 

 perlite are exposed (1916, 1920). 



Similar gray perlite with very perfect perlitic structure forms the bluff 

 on the east side of the Firehole River opposite Excelsior Geyser. It is full 

 of small phenocrysts and is slightly vesicular (1904, 1905). In places it is 

 lithoidal. It is separated into nearly horizontal layers by joints which curve 

 down to the northward at a somewhat steeper pitch than the slope of the 

 hill, indicating the descent of the lava into a basin-like depression to the 

 north and the subsequent erosion of the top of the hill. There is a tendency 

 to prismatic jointing. The slope of the bluff farther south is covered with 

 a fine sand of perlitic grains. Porphyritic rhyolite of the same variable 

 character forms the spurs around the drainage of Rabbit Creek. 



The spur of the plateau southwest of the Lower Geyser Basin has a 

 precipitous wall on the north and east sides, which exposes lithoidal por- 

 phyritic rhyolite, the top of the plateau being almost wholly glassy perlite 

 and cracked obsidian, which form the canyon of the Little Firehole River 

 near its falls (1922, 1923). 



UPPER GEYSER BASIN. 



Though the plateaus on all sides of the Upper Geyser Basin are 

 normal rhyolites like the various forms just described, the rock in the imme- 

 diate vicinity of the great geysers of this basin is abnormal. It is a dull 



