54 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



posure of granitic rock. That there are no such formations in the 

 whole drainage of the Ux)per Yellowstone is established by the fact of 

 the almost total absence of granite pebbles on the shores of the lake or 

 in the bed of the river. The home of this wanderer must be sought 

 elsewhere. To the north, beyond the valley of the third caiion and the 

 East Fork, lies the granite highland previously mentioned. To the 

 northwest, beyond the valley of Gardiner River, at the southern end 

 of the Gallatin Mountains, is auother exposure of granite at an eleva- 

 tion sufficient to have given origin to it. The distance in either case 

 is upwards of 20 miles. From the great falls the river descends in 

 a northerly direction until it strikes the base of the granite high- 

 land ; here it unites with the East Fork and turns to the west along the 

 south base of that highland, following the line of the great displacement, 

 until it passes the granite gateway of the second caiion (see map, plate 

 XXX). To reach its present position from the northern locality, the 

 bowlder must cross the course of the great valley of the East Fork 

 and the third caiion and ascend the river, as it now exists, a distance 

 of 20 miles, avoiding on its way, by a circuitous route, the- interven- 

 ing Washburn Range and the opposing mass of Amethyst Mountain. 

 If from the Gallatin Mountains, it must first have crossed the vallej'" of 

 the Upi>er Gardiner River and afterward a considerable spur of the Wash- 

 burn Mountains — a journey of 20 miles southeast. JSTotwithstanding 

 the fact that this pathway would, with anything like the present 

 topography, seem to present fewer obstacles to the advance of a glacier 

 than that from the north, I cannot regard it as at all probable that this 

 was its course. The mass of the Gallatin Mountains is not great. 

 Glaciers originating in its short abrupt valleys would have no great 

 longitudinal extent, and would probably advance no farther than the 

 deepest part of the valley that lies along its immediate base. 



The great ranges to the north are of sufficient extent to give birth to 

 ice rivers of the grandest proportions. The present distribution of the 

 erratic fragments of granite tends to strengthen the impression that 

 they had their origin in the north. If this be admitted, it becomes at 

 once clear that the erosion of the Grand Canon has been accomplished 

 since the close of the glacial period, or at least that a second erosion 

 has taken place if a caiion did exist prior to the glacial epoch. 



That a very profound erosion had taken place along the course of the 

 caiion at a very early date is proved by the fact that during the rhyo- 

 litic period as well as the basaltic and andesitic, there were caiions 

 almost as deep as the present one, into which the coulees cascaded. At 

 one spot near the northern base of Mount Washburn the section of a 

 fossil river is exposed more than half way down the caiion wall, the 

 bed of which has been cast in andesitic lava, and the volume of whose 

 water discharge is recorded in pumice stone. 



These events probably belong, however, to Miocene and Pliocene times, 

 and the topography of this region in those periods — the course of the 

 rivers and the configuration of the country must for the most part re- 

 main unknown. 



Topographic changes of Quaternary times are, however, much more 

 easily traced. The mass of glacial ice necessary to carry the great 

 bowlder described above to its present resting place would change the 

 whole drainage of the Park, The waters of the Upper Yellowstone 

 and of the numerous tributaries of the lake would be forced across the 

 low continental divide to the south and become tributary to Snake 

 River and the Pacific, or otherwise to some of the western branches of 

 the Missouri. 



