VICINITY OF UPPER GEYSEE BASIN. 373 



lithoidal rock, slate color speckled with light gray, carrying abundant phe- 

 nocrysts of feldspar, most of which arc white or yellow and exhibit brilliant 

 striated cleavage planes. There are none of quartz. This rock resembles 

 that forming the bluff at the southeast corner of Geyser Meadow. It is 

 exposed in the rounded hills back of the Splendid Geyser (1924), and in 

 tabular masses back of the Grand (1926), and forms the west bank of the 

 Firehole River east of Old Faithful Geyser. The microscopical features will 

 be described in another place. Its extent and its relation to the normal 

 rhyolite of the neighborhood were not made out. Perlite with large 

 spherulites occurs on the bank of the Firehole River a short distance above 

 Old Faithful Geyser (2166, 2167). 



Above the Upper Geyser Basin the road along the Firehole River 

 traverses rhyolite like that of the plateau, for the most part lithoidal, and 

 weathering into crumbling sand. Through it the river has cut a narrow 

 rocky channel which is filled with great masses of rhyolite that have fallen 

 from the steep sides. Just below Keplers Cascade the river passes through 

 a narrow gate in a channel not more than 2 feet wide. The cut exposes 

 vertical layers of spherulitic rhyolite, mostly lithoidal (1933), the layers 

 crossing the stream at right angles. 



Following the southeast branch of the Firehole River to the pass of the 

 old trail near the north end of Shoshone Lake, one traverses country covered 

 with lithoidal and glassy rhyolite. For long distances the road lies in a fine 

 glassy sand derived from the disintegration of perlite. The cliff that stretches 

 for 3 miles along the south side of the road presents a variety of phases of 

 rhyolite. At its western end it is lithoidal and very porous and vesicular, 

 with here and there patches and streaks of glassy rock. Farther east the 

 glassy rock predominates, with smaller masses of lithoidal rhyolite. Near 

 the pass south of the road well-banded lithoidal rock abounds, and at the 

 pass glassy and spherulitic modifications occur, as varied as those of Obsidian 

 Cliff. The vesicular lithoidal rock at the west end of the cliff is light 

 bluish gray, discolored by brown and yellow stains. It carries numerous 

 phenocrysts, which are so transparent as to be easily overlooked. The small 

 irregular vesicles are not distributed uniformly through the rock, as is usually 

 the case in basalt, but are very unequally scattered, being abundant in 

 some spots and almost absent from others. This is a very common mode 

 of occurrence in the lithoidal rhyolite of this region (1934, 1935). The 



