CENTEAL PLATEAU. 389 



a short distance from each of the larger crystals, all being parallel and 

 pointed in one direction. They appear as though they had resulted from 

 the resistance offered by the phenoerysts to a shearing stress. The hill south 

 of the hot springs east of Marys Lake is formed of porphyritic perlite, 

 full of small hollow spherulites and lithophysse, which have been partly 

 altered by the action of heated vapors, which come up through it and 

 deposit crystals of sulphur. The phenoerysts of feldspar and the spheru- 

 lites are much decomposed, the quartzes and groundmass remaining unal- 

 tered. The hollow spherulites are partly filled with opal, and less often 

 with sulphur (2070). 



The top of the plateau from the north side of Hay den Valley to Gib- 

 bon River and Grebe Lake is covered with porphyritic glassy rhyolite or 

 obsidian, in places spherulitic. This is the character of the rock along the 

 road from the Yellowstone Falls to the valley of the Gibbon, and along the 

 west bank of Yellowstone River from Alum Creek to Cascade Creek. At 

 the north side of the mouth of Otter Creek there is much silver-gray 

 pumice, which is beautifully fibrous, exhibiting a satin-like sheen when the 

 light is reflected from the sides of the fibers, but appearing dark gray and 

 vitreous on transverse surfaces. It is filled with phenoerysts (2084 to 2089). 

 A somewhat similar pumice, or more correctly, a highly vesicular porphy- 

 ritic perlite, occurs in a small gulch west of the Upper Falls of the Yellow- 

 stone. It forms a loosely adhering breccia of pumice and perlite. Some 

 masses of perlite are quite dense, with only small vesicles, but most of it is 

 greatly inflated, with flattened cavities. In places the perlitic structure is 

 not complete, leaving kernels of irregularly shaped obsidian, which weather 

 out and cover the ground with black sand (2080 to 2083). In the meadow 

 of Cascade Creek south of Dunraven Peak the rhyolite exposed in the 

 creek bed is finely vesicular lithoidal rhyolite, purplish gray, with many 

 phenoerysts of plagioclase and fewer of sanidine and quartz (2092, 2093). 

 The rock closely resembles that which forms the bluff at the southeast 

 corner of Geyser Meadow and also the rock of the Upper Geyser Basin. 



VICINITY OF THE GRAND CANYON OF THE YELLOWSTONE. 



Since the topographic and scenic features of the Grand Canyon, and 

 the character and condition of the rocks forming it, are described in detail 

 by Mr. Arnold Hague in Part I of this monograph, it is not necessary 



