378 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



latitude, is cut into lithoidal rhyolite, which is somewhat spherulitic and is 

 filled with small phenocrysts (1955). This rhyolite continues south beyond 

 the limits of the area explored and forms the northwestern base of the 

 foothills of the Teton Range up to an elevation of over 8,000 feet. Where 

 it has been cut across by the valley of Conant Creek its contact with the 

 underlying rocks is seen to be very steeply pitched to the west, indicating 

 how steep the preexisting surface must have been at this place. The rhyo- 

 lite near the contact is rudely columnar. It is part of the great lava flood 

 which buried the slopes of the northern foothills of the same range, and is 

 found overlying sedimentary and Archean rocks and volcanic breccias of 

 andesite which had accumulated on them. About the head of Conant and 

 Boone creeks and along Berry Creek thin tongues of the rhyolite sheet, 

 continuous with the heavy mass of Pitchstone Plateau, have been left in 

 favored places, and have escaped the erosion which must have considerably 

 modified the contour of the surface in the vicinity of this high range of 

 mountains. These portions of the lava lie at higher altitudes than the top 

 of the plateau north, and even exceed in some places the highest elevation 

 of the main body of Pitchstone Plateau. In two points north of Berry 

 Creek the altitude of the present surface of the rhyolite is 8,900 feet, and 

 just east of Forellen Peak it reaches 9,300 feet, resting in a thin sheet on 

 sedimentary rocks. 



The petrographical character of the rhyolite varies somewhat in this 

 neighborhood, but the variations are mainly due to the fact that the 

 exposures are in many cases at or near the bottom of the lava flow, where 

 it has been affected by coming in contact with underlying rocks. Thus, on 

 the west slope of the mountain north of the head of Conant Creek the 

 mass of the rhyolite is lithoidal and but slightly porphyritic, but near 

 the bottom of the flow it is in places black obsidian (1958) with a fine 

 mottling that is almost imperceptible and is more pronounced in a gray, 

 glassy form of the rock (1954) from the same locality, which is similar 

 to that at the middle falls on Falls River (1952). In places the lithoidite 

 is light bluish or purplish gray and has large flattened vesicular cavities 

 intimately related to hollow spherulites and lithophysse (1953, 1957). These 

 exhibit characteristic V-shaped cracks, and have evidently resulted from the 

 gaping open in spots of a viscous substance. They are coated with yellow- 

 stained crystals of the same minerals as those which occur in lithophysse, 



