THE WORK OF RUNNING WATER. 



139 



The fall of the Niagara ^ (PL IX) is one of the most remarkable 

 knomi, both because of its large volume of water and its great descent, 



Fig. 119. — A group of pot-holes. (Turner, U. S. Geol. Surv.) 



between 160 and 170 feet. The rate at which the fall is receding is 

 a matter of interest not only in itself, but because, once determined, 

 it may be made to serve as a unit of measurement for certain important 

 events in geological history. It was formerly conjectured that this 

 fall was receding at the rate of one to three feet per century, but it 

 was not until recent years that its actual rate of recession was ap- 



1 For a brief account of this fall see Gilbert in Physiography of the United States. 



