Science, Geology and Physics 



The terrace (T) represents the former level of the river 1895 

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

 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 youthful 

 river was broad and shallow, like and of about the same mag- 

 nitude 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 current 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 canon in a section just above the end of the 

 gorge (fig. 23). 



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. 24), which is 1 1,000 feet from 

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

 different conditions of height and volume, acting under a gen- 

 eral uniformity, the time needed to excavate the immature canon 

 as far as Foster's terrace is found to be 1 7,200 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 1 2 miles, and 

 adding this length of shaly rocks to be removed. The increased 

 descent gave rise to new cascades over the hard Clinton lime- 

 stones (c and d, fig. 24) and Medina sandstones (/i, fig. 24) at 

 the end of the canon, after the shales between it and the lake 

 had been somewhat reduced in height. A modern repetition of 



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