392 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



also on the east slope of Garnet Hill (2160). All of these patches are 

 undoubtedly portions of one continuous sheet, which has been almost com- 

 pletely eroded. 



Remnants of the rhyolite sheet lie high up the slopes of Saddle 

 Mountain, even near its summit. One area is on the west spur of the peak 

 southwest of Saddle Mountain, at an altitude of from 9,200 to 9,500 feet. 

 Near the bottom contact the rhyolite is dense and dark colored ; higher up 

 in the mass it is somewhat fissile, forming thick slabs. It is lithoidal, with 

 traces of black glass, and is dark colored and carries small phenocrysts 

 and lithophysa? (2141). It rests directly upon basaltic breccia. 



On the northwest spur of the peak of Saddle Mountain, about 700 feet 

 below the summit, there is a small patch of rhyolite forming a nearly 

 horizontal sheet not more than 150 feet in length. It rests upon basaltic 

 breccia, which is plainly exposed in the steep faces of the narrow ridge. 

 At the bottom of the rhyolite is white rhyolitic tuff; over this is fissile, 

 light-gray, lithoidal rhyolite with small phenocrysts (2137). This passes 

 up into dark-colored spherulitic and glassy rhyolite with lithophysee and 

 small phenocrysts (2138), similar to the rhyolite on the spur to the west. 

 There are patches of rhyolite at various altitudes on the northwest spur, 

 down to 8,000 feet. At about this altitude in the gulch south of this spur 

 and of Miller Creek there is a small group of fumaroles in rhyolite that is 

 partly dense and light gray, and is jointed in rectangular plates and small 

 straight prisms, and is partly perlitic and glassy (2145 to 2149). 



Rhyolite covers the southern portion of the flat-topped spur north of 

 Miller Creek, and patches of it occur on the ridge near Parker Peak, at 

 9,500 feet, and on the south slope of the divide at the head of Lamar River, 

 at 9,800 feet. In the last two places the rock is partly lithoidal, with small 

 phenocrysts, and partly glassy. Some of it is fissile and some of it massive, 

 carrying lithophysse. None of it has been found on the north side of the 

 divide, in the drainage basin of Crandall Creek. Scattered patches of it 

 occur on both sides of Cache Creek, and a long' narrow tongue extends 

 along the west side for 4 miles, being situated between altitudes of 7,000 

 to 8,000 feet, In all these cases the structure of the rhyolite masses is that 

 of surface flows, resting on an uneven surface of older rock, and nowhere 

 in this vicinity was any of it exposed in the form of a dike or other intrusive 

 body. 



