298 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Basic breccia overlies the earlier breccia on the spur northeast of the 

 forks of Mountain Creek, and forms the mountains to the northeast. It is 

 distinctly bedded in places in horizontal layers, with more tuff than in the 

 breccia of Eagle Peak. It is composed of pyroxene-andesites, some with 

 abundant small phenocrysts of feldspar and pyroxene, others with almost 

 no noticeable phenocrysts. Much of it contains olivine. Only a very 

 little carries hornblende (1679 to 1683). Similar dark-colored breccia 

 forms the main mass of The Trident, where it occurs in nearly horizontal 

 layers or beds. These horizontally bedded breccias extend south to 

 Thorofare Creek, and west across the canyon of the upper Yellowstone 

 River, and form Two Ocean Plateau. 



two ocean plateau. — The mass of Two Ocean Plateau above the valley of 

 the upper Yellowstone River consists of dark-colored andesitic breccia in 

 nearly horizontal layers. The bedding appears much more regular at a 

 distance than upon close inspection, when it is often found to be very 

 irregular. In the vicinity of Two Ocean Pass the layers of bedding dip 

 about 5° NE. This dipping proves that there has been a change of position 

 since the breccias were accumulated, and indicates a depression or faulting 

 toward the east, or in the Absaroka Range. 



Near the mouth of Mink Creek, and south of it, the breccias are dark 

 colored and basic and are associated with flows of massive basalt with 

 porphyritical feldspar and augite, resembling those at Two Ocean Pass. 

 Over the limestone near the head of Pacific Creek there is basic breccia in 

 the stream channel, while on both sides of the valley at higher altitudes are 

 flows of basalt, which crOss the valley just west of the divide. These sheets 

 of basalt form a distinct ledge on both sides of the valley, which varies in 

 thickness from 200 to 400 feet, and is rudely columnar. Owing to the 

 peculiar composition of these basalts they have been classed with the sho- 

 shonites from other parts of the Park, and are described in connection 

 with them. Above the western end of the basalt ledge, in the assorted 

 andesitic breccia, there is a layer of unassorted light-colored tuff with 

 large phenocrysts of sanidine and small biotites (1723-1725). There are 

 clay -like lumps through it, and many fragments of basic andesite or basalt. 

 It is overlain by basic andesitic breccia. The same kind of tuff is exposed 

 on the southern side of Two Ocean Pass, in a layer about 30 feet thick. 

 Here it passes upward into fine-grained tuff, which is gray and bedded. 



