J. W. Spencer — Duration of Niagara Falls. 465 



The terrace (T) represents the former level of the river 

 (about 190 feet above Lake Ontario). It is the only feature 

 of the kind in the canon. It is about 50-60 feet above the 

 Iroquois level to which the river descended. Thus the slope 

 of the earlier and smaller streams was about half as great again 

 as the modern river over the rapids at this locality. The youth- 

 ful river was broad and shallow, like and of about the same 

 magnitude as the modern American channel and falls, acting 

 evenly over the whole breadth and receding at about the same 

 rate. The remnant of the platform shows how far the fall 

 had receded before the physical change which threw the cur- 

 rent to the eastern side of the channel. This change could be 

 effected by increasing the height of the falls which would 

 favor the deepening of the chasm at the expense of the width, 

 especially as the lower rocks are mostly shale. This change 

 of breadth from a wide and shallow to a narrow and deep 

 channel is shown along the lower part of the canon and is 

 illustrated by the contracted channel at the bottom of the 

 «anon in a section just above the end of the gorge (fig. T). 



Fig. *l.- — Section half a mile from the end of the canon (yg fig. 1); bb, terraces 

 of river at the original level ; L. 0., level of Lake Ontario; bottom of river about 

 80 feet below the surface of Lake Ontario. 



As the changing conditions were gradual, I have placed 

 the close of the first episode at the time when the falls had 

 reached the foot of the terrace (B fig. 8), which is 11,000 feet 

 from the end of the chasm. Varying the rate of recession for 

 the different conditions of height and volume, acting under a 

 general uniformity, the time needed to excavate the immature 

 canon as far as Foster's terrace is found to be 17,200 years. 



Second Episode. — The subsiding of the waters at the end 

 of the first episode, which concentrated the stream upon the 

 side of the channel amounted to 220 feet, thus increasing the 

 descent of the water to 420 feet, with the lake receding twelve 

 miles, and adding this length of shaly rocks to be removed. 

 The increased descent gave rise to new cascades over the hard 

 Clinton limestones (c and d, fig. 8) and Medina sandstones (^, fig. 

 8) at the end of the canon, after the shales between it and the 

 lake had been somewhat reduced in height. A modern repeti- 

 tion of three such cascades over the same series of rocks may 

 be seen along the Genesee River near Rochester. Under this 

 condition the upper cascade receded by itself past Foster's ter- 



