MADISOX PLATEAU. 375 



portions of the cliff are bluish-gray lithoidal rhyolite, passing up into black 

 and red obsidian (1951) with small phenocrysts. The rock is richly spher- 

 ulitic, with large lithophysse in thick layers. The lava sheet is slaggy on 

 top, with layers that have been stretched and cracked transversely. From 

 the structure of the lava it is evident that the flow poured down a steep 

 slope westward into the valley basin of Buffalo Creek. The present 

 eastern wall is a bluff 150 to 200 feet high. Farther down Boundary 

 Creek, as far as the bluff north of Falls River Basin, the rhyolite is lithoi- 

 dal, but the bluff west of the falls of Boundary Creek, forming the edge of 

 the plateau, is black and red spherulitic and porphyritic obsidian (1950). 

 The same observation was made by Professor Penfield in traveling from 

 Madison Lake across the southern end of the plateau to Falls River Basin. 

 The whole surface of the country is black obsidian and grayish-white 

 pumice (1941, 1942), which is porphyritic with large sanidines and smaller 

 quartzes, but as the level of the basin is approached the rock grows 

 lithoidal. 



On the continental divide south of Madison Lake the black obsidian 

 is in some places finely vesicular and in others spherulitic, with small litho- 

 physse coated with tridymite pellets and containing a small amount of 

 fayalite. It incloses many angular fragments of very fine-grained basalt, 

 which is often highly vesicular and which has the petrographical characters 

 of the so-called "recent basalts." Similar inclosed fragments of basalt were 

 found in various places northwest of this locality. They indicate the 

 existence of basalt flows in this vicinity prior to the outbreak of the top 

 sheet of rhyolite, but no large body of basalt was observed. 



BECHLER CANYON. 



Bechler River cuts a fine canyon through the great rhyolite mass, thus 

 separating Madison Plateau from Pitchstone Plateau. The canyon trends 

 in a northeast-southwest direction and exhibits a very marked difference 

 between its western and eastern walls. The former is a rather persistent 

 bluff 800 feet high, while the latter is from 1,000 to 2,000 feet in height, 

 presenting the greatest thickness of rhyolite exposed within the Park. 



The rock of the country forming the northern head of Bechler River 

 has the character of that on the surface of the plateaus, the rhyolite being 

 mostly glassy and pumiceous, in some places crowded with spherulites, in 



