VICINITY OF LOWER GEYSER BASIN. 371 



of sanidine and smaller quartzes, the larger feldspars lying parallel to the 

 lamination (1879). This pumice passes into dark-gray porphyritic perlite, 

 full of red and blue spherulites (1882). In places it contains lithophysse 

 several inches in diameter, with wide-g-aping centers, which have been 

 crushed and dislocated to some extent, and into the cavities of which the 

 viscous matrix was in some instances forced. 



Rhyolite exhibiting variations similar to those just described — lithoidal, 

 glassy, perlitic, pumiceous, spnerulitic or with lithophysse — forms the pla- 

 teau northeast of the Lower Geyser Basin and north of Nez Perce Creek 

 (1890, 1891). " On its southern edge, about 3 miles east of the Lower Geyser 

 Basin, the bluff exposure is rhyolite, which is lithoidal and beautifully lam- 

 inated, or, perhaps more properly speaking, streaked in layers, the dark 

 purplish-gray rock being marked with lighter-colored lines or streaks, which 

 appear to be more highly crystallized, and are spotted with minute round 

 holes about the size of a pin point (1884). In a neighboring exposure, where 

 the bluff on the south side approaches the river, the banded rhyolite is light 

 reddish purple streaked with yellow (1883). It is filled with phenocrysts of 

 sanidine and smaller quartzes, which, however, are scarcely noticeable 

 except on close examination, because of their transparency and the general 

 mottling of the rock. The rock is parted in parallel plates, which appear 

 to be independent of the direction of the flow structure and stand at all 

 angles in the cliff. In one place the jointing is semicircular. 



The plateau south of Nez Perce - Creek and east of the Lower Geyser- 

 Basin is of the same character as that north. Lithoidal rhyolite alternates 

 with much glassy rhyolite, which is in many places a very fine gray perlite. 

 Southwest of the broad valley at the forks of Nez Perce" Creek there is a 

 breccia of gray perlite, some fragments of which are of large size. It is 

 both compact and vesicular to pumiceous, the vesicles being flattened and 

 elongated (1885 to 1887). The southeast branch of this stream cuts a 

 narrow gulch through black, porphyritic obsidian, which is so thoroughly 

 cracked that it weathers into a black sand. Glassy breccia occurs in 

 other places over this plateau (1889). A very remarkable form of com- 

 pactly spherulitic rhyolite is also found sparingly. It is megascopically 

 axiolitic, the spherulitic crystallization having taken place from short, curved 

 planes that cross one another at all angles, and also from most of the 

 phenocrysts which act as nuclei of small spherulites. The result is a brown 



