350 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



to the almost uniformly level character of the rhyolitc ridges, through 

 which the streams follow long straight valleys characteristic of this 

 part of the Park. The rock is everywhere rhyolite, which fortunately 

 exhibits a great variety of modifications. On the west side of the road, 

 as it approaches Obsidian Cliff', there is a small exposure of columnar 

 rhyolite. 



Obsidian Cliff, on the east side of the stream, near the outlet of 

 Beaver lake, is of more than ordinary interest to all tourists, but 

 especially so to the geologist, on account of its peculiarities of struc 

 tore and the development of spherulites and lithophysa* found in it. 

 The cliff rises in nearly vertical walls from L50 to 200 feet above the 

 stream. It has been formed by a surface How breaking out and run- 

 ning over the rhyolite plateau, covering an area of about 10 square 

 miles. The obsidian is a natural glass, the result of rapid coolingfrom 

 a fused mass of highly acid lava, and has much tin; same chemical 

 composition as the plateau rhyolite. On the surface, at the northern 

 end of the cliff, it grades into pumice and lithoidal rhyolite. I am 

 indebted to Mr. Joseph P. lddings, who has made a careful study of 

 Obsidian Cliff', for the following note: 



The columnar Structure at the southern end of the cliff is particularly well devel- 

 oped. The compact black obsidian passes into lithoidal rock northward along the 

 face of the cliff, where various phases of crystallization may be studied in situ? 



On the top of the plateau, to the east, the dense ohsidian passes upward into 



white pumice, which has been more or less removed by glacial action. The fresh, 



unaltered condition of rock permits the mineralogioal character of the crystalliza- 

 tion to he observed in the utmost detail. The perfect ion of the lit hopliys;e and 

 spherulites, and the richness of the microscopic spherulitic growths constitute its 

 most important petrographies] feature. 



The lamination of the lithoidal portion of the mass and the general lamellar dis- 

 trihution of different kinds of structure, as well asofthegas luil. hies in the vesicu- 

 lar and pumiceous parts of the sheet, furnish valuable evidence as to the agent most 

 active in promoting the various kinds of crystallization in this and similar rhyo- 

 litic lavas. Phis agent was undoubtedly water- vapor. 49 



A short distance south of Obsidian Cliff lies Beaver lake, across 

 which are several dams kept in repair by beavers who inhabit its 

 waters. Since the rigid protection of game in the park the beaver are 

 rapidly increasing in number, and several of their houses may be seen 

 in the lake. 



Shortly after passing Beaver lake the rhyolite begins to show the 

 effects of hydrothermal action, and numerous areas of rock decompo- 

 sition may be observed on both sides of the road. 



Four miles south of Obsidian Cliff, Roaring mountain is passed. 

 Tins is a bluff rising 600 feet above the road and one of the highest 

 [joints on the lava ridge. It takes its name from t he shrill, penetrating 

 sound of the steam constantly escaping from one or more vents located 

 near the summit, and on a ealm day, or with a favorable wind, the 



