20 8 Glacial Phenomena in the Yellowstone Park. [March, 
half way down the canon wall, the bed of which has been cast in 
andesitic lava, and the volume of whose water discharge is re- 
corded 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 remain unknown. 
Topographic changes of quarternary times are, however, much 
more easily traced. The mass of glacial ice necessary to carry 
the great boulder 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. 
