278 GEOLOGY OF THE YELLOWSTONE NATIONAL PAKE. 



end of the flat-topped mountain east of Pyramid Peak, and form the upper 

 600 feet of this mountain, the bottom of the sheets lying at about 9,000 

 feet altitude, and resting on basaltic breccia. The character of the different 

 sheets varies somewhat, a highly vesicular and strongly porphyritic basalt 

 (1477) being found at the northern end. 



Grlaciation has left its marks upon the surface of these table-lands, 

 having planed out lake basins and deposited rounded drift. The extension 

 of the basalt sheets westward is clearly indicated by the topography of 

 Pyramid Peak. From the saddle northward there is a flat bench or series 

 of benches along the eastern base of the pyramidal peak, which broaden out 

 into a flat-topped spur between the branches of Cold Creek. The basalt 

 passes beneath the upper thousand feet of this mountain and descends steeply 

 into the valley of Cold Creek, thence across a broad spur into Mist Creek, 

 north of which it forms the basalt ledges already noticed, which are con- 

 tinuous with those of Mirror Plateau. 



The same or similar basic breccia, topped by sheets of massive basalt, 

 continues southward along the base of the mountain forming the west wall 

 of Stinkingwater Canyon,- and extends far up the valleys draining the 

 region lying east of the watershed of the Yellowstone Lake. As in the 

 vicinity of Lamar River, these breccias and lava flows represent the ancient 

 slopes of basaltic volcanoes lying east of the one hundred and tenth 

 meridian. Along Jones Creek, Crow Creek, and Middle Creek basaltic 

 breccia forms the lower portion of the eastern end of the mountain ridges. 

 It is overlain by successive sheets of porphyritic basalt, which, on the 

 northern side of Middle Creek, attain a total thickness of between 900 and 

 1,000 feet. In each of these three valleys the basalt ledges have given 

 rise to high-shouldered spurs and benches on either side of the valleys, in 

 the same manner as on Cold Creek. The surface of the basalt flows 

 descends gradually to the westward, and disappears beneath more recent 

 breccia in the heads of the valleys. The character of the basalt is similar to 

 that near Lamar River. Some of the flows are full of large phenocrysts of 

 feldspar and pyroxene (1530); others exhibit only large olivines (1527). 



This early basic breccia, with its associated basaltic lavas of peculiar 

 composition, continues beyond the divide at the head of Middle Creek, 

 through Sylvan Pass, and forms the mountains and ridge as far as the shore 

 of Yellowstone Lake at Signal Point. It also occurs in isolated patches at 



