Glacial Phenomena in the Yellowstone Park, 
207 
1881.] 
That a very profound erosion had taken place along the course 
of the canon at a very early date is proved by the fact that during 
the rhyolitic period as well as the basaltic and andesitic, there 
were canons almost as deep as the present one, into which the 
coulees cascaded. At one spot near the northern base of Mt. 
Washburn the section of a fossil river is exposed, more than 
I 
