WEST OF YELLOWSTONE LAKE. 383 



others of dark and light brownish-red obsidian carrying dark-blue spheru- 

 lites with gray, brown, or purple outer shells, also completely crackled. 

 Black and red streaked obsidians alternate in layers with bands of densely 

 spherulitic material, and occasionally of porous spherulites. In places the 

 lamination is very pronounced, and thin spherulitic layers, when broken 

 from the obsidian, are covered with wart-like excrescences, which are 

 protruding spherulites, ribbed with parallel lines corresponding to the planes 

 of lamination of the rock. Parts of the rock are fissile and consist of light- 

 brown glass streaked with black and red in small blotches, and even rather 

 large lumps, which have been drawn out into lenticular shapes during the 

 flow of the rock (1994 to 1998). 



Pumiceous glassy rhyolite forms the top of the plateau in many places 

 about the West Arm of the lake. Southwest of Riddle Lake the drainage 

 exposes light-gray pumice overlying black, glassy, porphyritic rhyolite or 

 obsidian. Lower downstream, near the forks, the rhyolite is lithoidal, 

 purplish gray, and banded, and is accompanied by black glassy varieties 

 carrying spherulites, which are the commoner kinds over this part of the 



plateau. 



In the immediate vicinity of Duck Lake light and dark gray pumice 

 and perlite form a brecciated flow (1986 to 1989), while farther west the 

 rhyolite is in places lithoidal. At Rock Point porphyritic obsidian with 

 small spherulites occurs in a brecciated mass. The surface of the spheru- 

 lites and of the glass immediately in contact with them, as Avell as that of 

 the obsidian blocks, is dark red, like other portions of the body of obsidian 

 in many places. There is also black perlite with small lithophysa?; and 

 light-brown and black, streaked and blotched perlite, and silvery-gray 

 fibrous pumice (2003, 2004). 



An idea of the great diversity of the rhyolite along the west shore of 

 the lake may be gotten from several typical exposures in the neighborhood 

 of Bridge Bay, from which extensive collections have been made On the 

 south side of Bridge Creek, about a mile west of the lake, a branch stream 

 has cut into brecciated pumiceous and hyaline rhyolite. It is porphyritic 

 and partly spherulitic crackled obsidian and perlite, with many small 

 roughened cavities. The brecciated portion contains some fragments which 

 are 2 feet in diameter. A highly vesicular modification of it is almost 

 fibrous, owing to the elongation of the vesicles, which have been drawn out 



