370 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



purer glass, and red pumice, with layers of Ethophysse. This is shown in 

 PI. XLVI, which represents several masses of this rock that have been eroded 

 into rude monuments situated about a mile above the lower falls. The ver- 

 tical layers are well shown in the illustration. The small monument is filled 

 with lithophysse. A few hundred yards above the falls, on the east bank, 

 there is an exposure of black perlite bearing very large lithophysse, from 

 1 to 2 feet in diameter (1870). This is shown in PL XLVIL Some are 

 compact spherulites, with little if any cavity, purplish red in color, and 

 breaking with radiating cracks. Others have distinct concentric shells, one- 

 fourth of an inch thick, with a spherical nucleus, which is shrunken and 

 cracked, and have large cavities. The radiate crystallization is very pro- 

 nounced in the more porous parts of the lithophysse, and the feldspar rays 

 can be seen with a pocket lens. They are studded with minute brilliant 

 and transparent crystals of tridymite, and the sides of the cavities are spotted 

 with pseuclomorphs of fayalite, like those at Obsidian Cliff. 



The rock in which these remarkable bodies occur is a black and gray 

 perlite, full of small spherulites and phenocrysts, some layers of the rock 

 being very dense. They stand vertical and in places are much bent. Most of 

 the perlite .is very crumbling, and consists of gray glassy shells, surround- 

 ing rounded and subangular grains of black glass, which weather out into 

 black sand. The small spherulites are beautifully banded in concentric 

 shells, being blue at the center and red outside (1868). 



Where the river near this place cuts a narrow gorge through the rhyo- 

 lite, the rock is lithoidal and banded. Farther south, at the end of the ridge 

 east of the river, about 2 miles below the mouth of Nez Perce" Creek, there 

 is a breccia of perlite and lithoidal rhyolite carrying fragments of an almost 

 fibrous variety (1871), pieces of which were found west of the road 4 miles 

 north of the Lower Geyser Basin (1872). It is lithoidal and porphyritic, 

 and is traversed by long, slender pores so close together as to produce a 

 fibrous structure, the fibers curving around the phenocrysts. The sides of 

 the elongated vesicles are coated with white pellets of tridymite, the rock 

 itself being dark slate color. 



Brecciated rhyolite occurs a short distance to the southeast, in the first 

 low ridge north of the Nez Perce" Creek. North of this the low ridges are 

 covered with light-red pumiceous glass, delicately banded with black, pro- 

 ducing the most perfect and beautiful lamination. It is rich in phenocrysts 



