46-i J. IF. Spencer — Duration of Niagara Falls. 



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 considering the chang- 

 ing episodes the rate of recession is of no geological interest. 

 But this study may lead to further detailed investigations. 



8. 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 Iroquois beach 200 feet below, 

 with the Erie drainage only T 3 T 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 

 excavated during the first .episode is found in the data fur- 



Fig. 5. — Map of the gorge at Foster's flats; F, location of the cross section fig. 6. 



Fig. 6.— Section of the george at Foster's flats (FT, fig. 1). Platform (F) of the 

 old river floor projecting into the canon. Its section is shown in broken shading 

 but with ravines descending from both sides of it. T, rock terrace surmounted 

 by huge blocks of Niagara limestones; b, original river terrace; r, surface of 

 river: L. 0., surface of Lake Ontario. Bottom of river about 80 feet below the 

 surface of the lake. 



nished by the study of Foster's flats. Their location is shown 

 at F, figure 1, and the structures are further illustrated in 

 figures 5 and 6. 



