ESTIMATING THE AGE OF NIAGARA FALLS. 149 



about twenty feet thick, which again is underhiid by a shaly deposit 

 of seventy feet, resting upon a compact stratum of Medina sandstone 

 twenty feet thick, below which a softer sandstone, that crumbles 

 somewhat readily, extends to the level of the river. 



The present width of the river at the mouth of the gorge is 

 seven hundred and seventy feet. It is scarcely possible that the 

 original width of the gorge was here any less than this, for in the 

 narrowest places above, even where the Niagara limestone is much 

 thicker than at Lewiston, it is nowhere much less than six hundred 

 feet in width. Nor is it probable that the river has to any consider- 

 able extent enlarged its channel at the mouth of the gorge at the 



Fig. 3. — Lookiini' up tlie jrorge from near Lewiston, slitnving on the left the exposed situation 

 of the eastern face of the gorge at the extreme angle, where the measurements wei-e made. 



water level. On the contrary, it is more probable that the mouth 

 has been somewhat contracted, for the large masses of Niagara and 

 Clinton limestone- and Medina sandstone which have fallen down as 

 the shales were undermined have accumulated at the base as a talus, 

 which the present current of the river is too feeble to remove. This 

 talus of great blocks of hard stone has effectually riprapped the 

 banks, and really encroached to some extent upon the original 

 channel. 



We may therefore assume with confidence that the enlargement, 

 under subaerial agencies, of the mouth of the gorge at the top of 

 the escarpment has been no greater than the distance from the pres- 

 ent water's edge to the present line of the escarpment at the summit 

 of the Niagara limestone. This we found to be three hundred and 



