of the Yellowstone Park. 455 



again thin interbedded sheets of basalt are found in the upper 

 portion of the rhyolite, but in general the great body of rock 

 presents a singularly uniform mass over the entire plateau. 

 Nowhere have any aqueous deposits or accumulations of 

 detrital material between successive flows been recognized. 

 Neither have any marked evidences of erosion or long intervals 

 of comparative rest been observed during the pouring out of 

 these rhyolites. Although no plant remains or invertebrate 

 fossils have as yet been obtained associated with the rhyolites 

 of the park, they have been referred on other evidence to the 

 Pliocene period. 



Along the abrupt wall of the grand canyon of the Yellow- 

 stone, just north of Tower Creek, occurs a bed of coarse con- 

 glomerate, made up of Archean and andesitic material. On 

 the west side of the canyon the conglomerates rest directly 

 upon andesitic breccias. On both sides of the canyon the con- 

 glomerate is overlain by basaltic flows in which columnar 

 structure is well developed. In the conglomerate on the west 

 wall, and directly beneath the overlying basalt, a few vertebrate 

 remains have been collected, which, though fragmentary, were 

 sufficient to enable Prof. O. C. Marsh to determine them as 

 belonging to the skeleton of a fossil horse of Pliocene time. 

 A careful search of the canyon conglomerate failed to find any 

 pebbles of rhyolite mingled with the other material, showing 

 conclusively that none was present in the immediate region 

 when the conglomerate was deposited, as the configuration of 

 the drainage basin is such that had any rhyolite been present, 

 evidences of it would have been detected in the waterlain 

 material. The basalt on both sides of the canyon is covered 

 by rhyolite, a portion of the great rhyolite field in all proba- 

 bility belonging to the earlier flows of this period. 



After the cessation of the rhyolite came the final phase of 

 volcanic activity, the pouring out of the recent basalts. They 

 occur along the northern and western boundaries of the Park, 

 skirting the outer edges of the rhyolite mass, where in broad, 

 but comparatively thin, sheets they overlie the former rock. 

 They are admirably shown in the region of Bunsen Peak and 

 Gardiner River but are best developed in Falls River basin, 

 stretching far westward into Idaho in somber, monotonous 

 beds. Occasionally dikes of basalt penetrate the rhyolite, as 

 shown in the escarpment of Madison canyon, but in the broad 

 central portion of the Park plateau outflows of basalt are 

 entirely wanting, even near such a powerful center of eruption 

 as the Sheriden volcano. Upon the cessation of these recent 

 basaltic outflows, after a long period of activity, eruptions of 

 igneous rock in this region came to an end. That this activity 

 lasted throughout the greater part of Pliocene time seems well 



