GEOLOGICAL SURVEY OF THE TEREITOEIES. 



39 



sources of the East Fork and iu the mountains at the head of the Upper 

 Yellowstone above the lake. Some of the highest mountains in the 

 Northwest are capped Axith these volcanic breccias arranged in horizon- 

 tal strata, and showing- most clearly that the agent was water. In 

 almost all cases these stratified breccias are perfectly horizontal from 

 base to summit, thereby indicating the probability that there has been 

 no important movement of the earth's crust since their deposition. We 



Fig. 4. 



VOLCA>;iC BRFXCIA AT THE HEAD OF CANON AND ROCK CREEKS. 



must conclude, then, tbat at a comparatively modern date tbe waters so 

 covered these mountain-ranges of the Korthwest that not even the sum- 

 mits of the loftiest peaks were above the surface. It is barely possible 

 that we might make an exception in the case of the Grand Tetons. We 

 may suppose that the materials were supplied from the numberless vol- 

 canic fissures in unlimited quantities in a comparatively brief space of 

 tim£ ; but the period which would be required for the waters to arrange 



