162 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The material consists of basic andesites and basalts, some of which are absar- 

 okite and contain orthoclase as an essential constituent. Petrographically 

 they are like the basaltic breccias of the Absaroka Range. 



The breccias are dark colored and often weather into fantastic towers, 

 pinnacles, and cliffs, whose dark rich shades of red, brown, purple, and 

 gray render them highly picturesque. The fragments are angular and sub- 

 angular and often are several feet in diameter. The best exposures occur 

 just below the amphitheater at the head of Boone Creek. On the north 

 side of Conant Creek, where the exposure is nearly 1,000 feet high, there 

 are indications of rude bedding, dipping westward. 



The breccias were thrown upon the surface of deeply eroded and faulted 

 sedimentary rocks, and undoubtedly were considerably eroded themselves 

 before being buried beneath the rhyolite, which forms the western slopes 

 and spurs and extends beneath the later basalt sheets of the Falls River 

 Basin a long distance westward. The canyons of Boone and Conant creeks 

 cut into it 400 feet and more without reaching the underlying rocks. The 

 rhyolite is porphyritic and lithoidal as a whole, but at the bottom of the 

 sheet, in contact with the underlying rocks, it is dark-colored obsidian. 



Birch Hiiu. — The Birch Hills, whose summits rise prominently above 

 the western border of the plateau, present the most northern exposures 

 of the sedimentary rocks of the Teton uplift. Separated from the north- 

 ern spurs of that range by the southern extension of the great rhyolite 

 plateaus of the Park which so effectively conceal all the earlier rocks, this 

 small area, which recent denudation has again exposed to view, presents 

 clear evidence of the same sequence of events so clearly outlined in the 

 range itself. 



The hills consist of a group of pointed eminences, with gentle eastern 

 slope and steep western declivities. The surface, formerly heavily wooded, 

 is now densely covered with a shrubby growth of black birch, concealing 

 the fallen timber that everywhere strews the ground. 



North of Falls River, whose beautiful Rainbow and Terrace falls are 

 near by, the hills consist of dacite-porphyry, forming the two main summits. 

 This rock is light gray and compact, with phenocrysts of feldspar and quartz, 

 and small biotite plates. It is a holocrystalline, intrusive body, resembling 

 the rock of Bunsen Peak in composition and structure, but more distinctly 



