152 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



the valley bottom, and passes through the rhyolite which adjoins the 

 breccias on the east. 



Both Conant Creek and Boone Creek head in deep amphitheater-like 

 basins, which have been cut in volcanic tuff-breccia and easily eroded sedi- 

 mentary formations, the rims of the basins being' formed of rhyolite lava 

 The lower portions of the channels are canyons cut in rhyolite. 



The mountainous portion of the district terminates abruptly on the 

 east, along the banks of the Snake River, while outside of the mountains 

 the country stretches northward as a high plateau, with ridges reaching 

 8,700 and 8,900 feet in altitude near Berry Creek, gradually descending 

 to 7,000 feet in the vicinity of Grassy Lake and Birch Hills. West of the 

 mountains long, narrow spurs descend somewhat rapidly to the level of Falls 

 River Basin at 6,500 feet. 



With this brief sketch of the topographic features in mind Ave may 

 proceed to consider the geologic structure of the region, beginning with 

 the oldest formations, which are the crystalline schists. 



CRYSTALLINE AXIS AND REGION EAST. 



The crystalline schists consist of light-pink and flesh-colored gneisses 

 and smaller amounts of mica-schists and amphibolites. Together they form 

 a steeply pitching axis, about which the overlying sedimentary formations 

 have been folded and broken. The most northerly exposure of the gneisses, 

 in Berry Creek, is not higher than 7,800 feet, while on Forellen Peak they 

 reach 9,700 feet, and are still higher in the mountain south of Owl Creek. 

 In the mountains on both sides of this creek the crystalline schists are seen 

 to have an almost vertical boundary on the west, along the fault line already 

 mentioned. On the slopes of both these mountains steep, narrow gulches 

 mark the plane of faulting and the contact between the schists and the 

 nearly vertical limestone beds. This plane passes through the summit of 

 Forellen Peak and also through the summit of the peak south. 



On the eastern side of this crystalline axis the sedimentary rocks 

 overlie it, with a gradually increasing dip toward the northeast. On the 

 south side of Owl Creek the structure is the same as that of Forellen Peak, 

 which is shown on the map. The southern side of the valley is crystalline 

 schists nearly to the summit of the' mountain, the eastern portion of which is 



