HOLMES.] YELLOWSTONE LAKE. 55 



Thus far I have spoken of the granite bowlders only. I do not imagine 

 that these give anything like a full record of the glacial work accomplished 

 in this region. There are indications of glaciation on a grand scale in 

 which the volcanic rocks alone were involved. In order to get at this 

 matter in a comprehensive way, let us go back to the beginning of Qua- 

 ternary times; let us try to determine the configuration of the country 

 before the ice had begun its work. The last great event of the Pliocene 

 age was the accumulation of a great series of stratified conglomerates — 

 the greatest of all the wonders of this unique region. The remnants of 

 these formations are found distributed over an area of nearly 10,000 

 square miles, and occur in such a manner as to lead us to believe that 

 they were formerly" continuous over a large part of this area. That they 

 are of Pliocene age is proved by the plentiful occurrence of the vegetable 

 remains of that age. 



The fact that this great series of formations could be laid down in 

 moderately well distributed strata from the bottoms of the lowest valleys 

 to the summits of the loftiest peaks, without inclosing, so far as observed, 

 a single granite fragment, is sufficient proof that the granitic rocks could 

 only have formed the sub-strata of the country in Pliocene times. 



At the beginning of the glacial i)eriod, therefore, the surface of the 

 Park district must have been almost exclusively volcanic. The ice 

 streams would encounter no other than volcanic rocks, and the record 

 of the earlier part of the glacial work must be looked for in the volcanic 

 rocks and in the volcanic drift of the glaciated district. 



It takes no great effort of the imagination to picture the whole Park 

 Plateau covered with a mass of snow and ice — a great neve field fed by 

 the accumulating snows from the surrounding highland, and extending 

 its numerous ice tongues far out to the south and west, forcing them 

 across the ocean divide into the Snake Eiver Valley, or into the head- 

 waters of the Missouri. 



A dense forest now covers the whole region, and the deposits of glacial 

 drift are hidden from the observer. 



The geologic formations are of a character not fitted to retain the 

 striations and the polishing accomplished by the ice. 



The only result remaining to bear witness to the great events of that 

 time is a fine group of lakes now resting in the center of the plateau. 



YELLOWSTONE LAKE. 



I may not be able to find good evidence that these lakes are of glacial 

 origin, for who can say that they do not occupy basins of subsidence, 

 or basins resulting from the building up of a rim by deposits of lava. 



In reference to these two points, it may be remarked that no evidences 

 of subsidences or displacements have been observed about the shores of 

 these lakes, besides their very irregular outHne and their long arms 

 reaching out in all directions make it difficult to believe that their basins 

 are areas of subsidence. 



In regard to the building up of the rims of the basins and the filling 

 up of outlets by streams of lava, it may be answered that so far as we 

 know tlie rhyolites which form nearly the whole of the lake shore were 

 deposited long antecedent to the period of erosion and glaciation which 

 witnessed the modeling and remodeling of the face of the whole country. 



The rhyolite lavas are probably Miocene and Pliocene, and the basin 

 into wliich they flowed may or may not have contained a lake or lakes. 



Such exposures of the bottoms of the flows as occur at the north show 



