384 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



in thin tubes. This form of the rock is microspherulitic or lithoidal, with 

 some glassy portions, and the breccia, which is largely composed of this 

 material, is the same as that which occurs on the Firehole River a short 

 distance below the Lower Geyser Basin (2020 to 2023). A more varied 

 occurrence is crossed by the trail in a coulde about a mile south of Bridge 

 Creek. It is mostly glassy and is full of phenocrysts of quartz and feldspar. 

 One black projecting mass of rock consists of finely vesicular to pumiceous 

 perlitic obsidian. This grades into a light reddish-brown pumiceous breccia 

 inclosing fragments of white pumice with light-brown borders and small 

 fragments of light-brown pumice and pieces of black obsidian. This passes 

 into a dense red perlite mottled with black and gray, and the latter grades 

 into reddish-brown and also dark drab-colored perlite, which in turn 

 passes into spherulitic obsidian banded with minutely spherulitic or lithoi- 

 dal layers full of small roughened cavities. It is scoriaceous in places and 

 exhibits a great variety of colors on weathered surfaces (2013 to 2019). 

 All this variation of texture and color takes place within a distance of 100 

 feet. Farther south the rhyolite becomes more lithoidal in places, purplish 

 lithoidal and spherulitic bands being intermingled with obsidian and perlite. 

 It is often roughly vesicular and slag-like (201-1). The modifications of 

 rhyolite just described constantly recur over the plateau in this vicinity. 



A still more varied assortment of rhyolite is found in a low bluff on the 

 lake shore half a mile south of Bridge Bay. It consists of brecciated pumice 

 and scoria, with some massive lava, and appears to be the surface or the 

 forward end of a How of porphyritic rhyolite. The colors of the rock, all 

 of whose varieties appear to be textural modifications of one magma, range 

 from jet black through different shades of gra}^ to almost white, besides 

 reds which are dark liver colored to pink, and browns that are reddish and 

 others that approach yellow. These colors occur separately in large masses 

 or are combined in brecciated bodies in blotches and streaks, or as mottlino-s 

 and bandings in massive rock. The greater part of the rock is glassy, but 

 some of it is lithoidal. There is black crackled obsidian grading into per- 

 lite banded by delicate layers of spherulites. Some of it is more spherulitic 

 and carries hollow spherulites, which are distinctly fibrous on the inside 

 and are coated with tridymite pellets. There is dense black obsidian so 

 filled with hollow spherulites as to appear like a porous or vesicular 

 rock. Black, red, and brown obsidians occur together, with and without 

 spherulites, some of which are blue and red, while others are porous and 



