GEOLOGlCilL SUEVEY OF, THE TEERITOEIES. 



85 



sandstones, and pudding-stones, which reach an aggregate thickness of 

 800 to 1,000 feet. Above the Upper Falls the Yellowstone flows over a 



hard, basaltic bed for 



sixteen miles from its outlet at the lake ; there is 



then an abrupt transition from the hard basalt to the more yielding 

 breccia, so that the river easily carved out a channel through it ; the 

 vertical walls are clearly seen from below the falls, passing diagonally 

 across the rim. The Lower Falls are formed in the same way ; the entire 

 mass of the water falls into a circular basin, which has been worn into 





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the hard rock, so that the rebound is one of the magnificent fea- 

 tures of the scene. Below the Lower Falls the sides of the canon show 

 the material of which it is mostly composed. Where the river has cut 

 its channel through the hard basalt, the irregular fissures, which un- 

 doubtedly extend down, in some manner, toward the heated interior, are 

 distinctly seen. Local deposits of silica, as white as snow, sometimes 

 400 or 500 feet in thickness, are seen on both sides of the Yellowstone. 

 These also are worn into columns, which stand out boldly from the nearly 

 vertical sides in a multiplicity of picturesque forms. The basis material 



