172 GBOLOGI OF THE YELLOWSTONE NATIONAL PARK. 



and extend from the open plain of Snake River Valley to the summit of the 

 ridge. At one locality just south of the Park boundary Cretaceous clays 

 are exposed in a precipitous bluff at the river, but the slopes of the hills 

 are for the most part obscured by alluvial material, followed higher up 

 by extensive deposits of glacial drift and broad areas of rhyolite, the latter 

 in places extending from the summit to the plain. An overlying sandy 

 soil nearly everywhere mantles the mountain side, and large forest areas 

 conceal the structure of beds, rendering it most difficult to follow the con- 

 tinuity of strata. Observed dips and strikes indicate that in general the 

 sandstones lie inclined eastward, toward the mountain mass, but with many 

 local displacements. 



Rhyoiite. — By reference to the map it will be seen that the rhyolite is 

 represented as forming a continuous body from the extreme northern end 

 of the mountain south to the forty-fourth parallel of latitude. This rhyolite 

 is very irregular in outline and represents a comparatively thin flow of lava. 

 Over a large part of the area covered by rhyolite it is doubtful if the lava 

 sheet is more than 100 feet in thickness. In places it has been entirely 

 removed by erosion, leaving isolated patches of sandstone exposed upon 

 the surface. In two localities it caps the westward spurs from the summit 

 to Snake River, and at other points lies high up on the mountain sides. It 

 caps the sandstone on the long ridges trending southward, and presents a 

 somewhat striking appearance with its long monotonous flows, scarcely 100 

 feet in thickness, resting upon the deeply eroded arenaceous strata. These 

 long flows stretch southward nearly to Jackson Lake. 



This rhyolite body possesses a fairly uniform appearance from one end 

 to the other; that is to say, it does not vary throughout its mass more than 

 maii} r areas of equal extent on the Park Plateau; indeed, it closely resembles 

 the rhyolite of the Park. It is in general purplish gray in color and lithoidal 

 in texture. In places it is fissile, and upon Huckleberry Mountain, and in 

 several other localities along the summit, it occurs in horizontal beds with 

 jointing planes. On North Huckleberry Mountain it is thinly bedded and 

 fissile, the debris, slopes being made up of irregular fragments of lithoidal 

 rock. 



Dacite. — Near the junction of the Cretaceous rocks with the rhyolite, due 

 west of Huckleberry Mountain, occur two or three exposures of igneous 

 rock, that rise in low obscure mounds above the general level of the sand- 



