374 CxEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



well-banded lithoidal portion near the pass is dark slate colored, striped with 

 layers of light purplish gray, often of the utmost delicacy, in this respect 

 resembling the laminated lithoidal part of Obsidian Cliff. There are occa- 

 sionally small cavities with transparent tabular crystals of sanidine and 

 some of quartz. The phenocrysts are abundant, and consist of sanidine, 

 plagioclase, and quartz, with numerous rusted crystals of a ferromagnesian 

 mineral, which has been found to be augite. The longer phenocrysts lie 

 more or less parallel to the lamination (1937). Some of the obsidian 

 contains small black spherulites, the size of small shot. Other parts of it 

 are streaked with red and brown glass (1936). The most perfect perlitic 

 structure is developed in places, together with curiously crenulated cavities 

 that traverse the rock in streaks and are coated with crystalline grains, 

 which in some instances appear to be fragments of broken phenocrysts of 

 felds]3ar (1939-1940) that have been dragged apart along the line of the 

 cavity. 



MADISON PLATEAU SOUTH OF THE GEYSER BASINS. 



The whole of this plateau is rhyolite, mostly black glassy obsidian, 

 with porphyritical crystals, in places spherulitic. This alternates with 

 pumiceous glass, which occurs in bands or layers. The alternation of dense 

 and pumiceous glass is very persistent over the whole top of the plateau, but 

 the majority of outcrops consist of the denser obsidian, since the pumiceous 

 portions of the rock have been more easily eroded. The character of the 

 country and of the rocks is very monotonous and uniform, varied only in 

 the neighborhood of Summit Lake by small areas of hot springs and fuma- 

 roles. The top of the plateau southwest of the Upper Geyser Basin is 

 glassy, but the lower part of the bluff along Iron Spring Creek is lithoidal 

 gray rhyolite, more or less porous and vesicular. The shallow drainage 

 channels, as they approach the edge of the plateau, drop into deep ravines, 

 where are large streams of water which are not met with on the top of the 

 plateau. The water comes from the edge of the plateau, out of the mass of 

 the rhyolite, the upper portion of which is porous and vesicular, while the 

 lower part is dense and compact. The smaller of the streams draining south- 

 westerly into Boundary Creek, east of Buffalo Lake, cuts a gulch 150 feet 

 in the edge of the bluff which forms the east wall of the basin of Buffalo 

 Lake. The gulch has vertical, rocky walls, exposing- a fine section of the 

 rhyolite lava, the flow of which has been greatly contorted. The lowest 



