VICINITY OF LOWER GEYSER BASIN. 369 



The top of the plateau south of Madison Canyon is almost entirely 

 glassy rhyolite, perlite, and obsidian. At the western edge of the top, near 

 the road, is black porphyritic obsidian, which is vesicular in layers, the 

 vesicles in places having been elongated in one direction and flattened in a 

 plane perpendicular to the layers. With it is associated a grayish-white 

 pumice of the same magma, into which it undoubtedly passes (1865, 1866). 

 The edge of the plateau north of Sentinel Creek is partly lithoidal rhyolite, 

 reddish purple, with abundant prominent sanidines and smaller quartzes 

 (1877, 1878), some of it containing beautiful white lithophysse with con- 

 centric shells (1873). But the greater part of the rock is obsidian or per- 

 lite, with the same phenocrysts as the lithoidal portion of the rock. Much of 

 it is spherulitic, having abundant small blue or red spherulites, both com- 

 pact and porous, with radial fibration and concentric zones. Through these 

 spherulites the phenocrysts are scattered indiscriminately, not appearing to 

 act as a nucleus. Larger spherulites occur, and more typical lithophysse 

 (1874 to 1876), and the obsidian in places is red and black, especially 

 in the small alcoves and arroyos along the southern edge of the plateau. 

 The eastern side of this block of table-land is a cliff from 300 to 500 feet 

 high, at the base of which runs the Firehole River. The rock is lithoidal 

 and banded, while along the east bank of the river above the falls the 

 rhyolite is glassy for the most part. 



VICINITY OF LOWER GEYSER BASIN. 



On the east bank of Firehole River east of Madison Plateau the rhyo- 

 lite occurs with marked layer structure. The layers, which are bent and 

 folded, trend almost parallel to the direction of the stream. The rock 

 exhibits great variability. Some of it is lithoidal, dense, purple, and 

 banded. Much of it is black perlite, with small blue spherulites in layers, 

 so crowded together as to leave but small patches of perlite glass; other 

 layers are microspherulitic (1869). Large spherulites are scattered through 

 the rock, together with numerous phenocrysts of sanidine and quartz, which 

 exhibit no connection with the layer structure so far as their distribution is 

 concerned, but show a tendency for the longer crystals of feldspar to lie 

 more nearly parallel to the layers than transverse to them. 



The perlite outcrops in long ridges parallel to the river, and stands in 

 nearly vertical layers of alternating characters, including spherulitic perlite, 



MON XXXII, FT II 24 



