Niagara Falls 



Larvs of Erosion 



1895 When erosion is considered from a theoretical point of view 



Spencer and the whole energy of the water is supposed to be expressed 



in the erosion, it varies as the mass of the water into the square 

 of the velocity (nn> 2 ). Hence for a given river increase of the 

 amount of its water or increase of the velocity along its course 

 should be expressed by greater erosion. But erosion is not the 

 only expression of the theoretical value of the energy of the 

 river. Again, it is well known that the more rapid the descent of 

 the stream the more the erosive effects are expended on the floor 

 of the channel, in deepening and forming the U-shaped valleys 

 or gorges. On the other hand, the reduction in the slope causes 

 the channel to become broader — a principle which has an 

 important bearing in this study. While the observations are 

 imperfect, owing to the variable conditions of erosion, still the 

 attempt to ascertain the duration of the different episodes is the 

 only natural sequence to the measurements of the modern recession 

 of the falls, and it gives approximate results, for without con- 

 sidering the changing episodes the rate of recession is of no 

 geological interest. But this study may lead to further detailed 

 investigations. 



Episodes of the River and the Duration of Each — Age of the 



Falls 



First Episode. — From the history of the lakes and the river 

 we learn that the early falls cascaded from the brow of the 

 escarpment to the level of the Iroqouis beach 200 feet below, 

 (with the Erie drainage only 3/11 of the total discharge of the 

 upper lakes). There is no indication that the Erie rainfall was 

 greater at that time than now. The length of the chasm exca- 

 vated during the first episode is found in the data furnished by 

 the study of Foster's flats. Their location is shown at F, figure 

 1 7, and the structures are further illustrated in figures 2 1 and 22. 



594 



