154 VR A GMENTS F SCIENCE. 



clown the river. The ice which fills the gorge in winter, 

 and which grapples with the boulders, lias been regarded 

 as the transporting agent. Probably it is so to some extent. 

 But erosion acts without ceasing on the abutting points of 

 the boulders, thus withdrawing their support and urging 

 them gradually down the river. Solution also does its por- 

 tion of the work. That solid matter is carried down is 

 proved by the difference of depth between the Niagara 

 river and Lake Ontario, where the river enters it. The 

 depth falls from 72 feet to 20 feet, in consequence of the 

 deposition of solid matter caused by the diminished mo- 

 tion of the river.* 



The annexed highly instructive map lias been reduced 

 from one published in Mr. Hall's " Geology of New York." 

 It is bused on surveys executed in 1842, by Messrs. Gibson 

 and Evershed. The ragged edge of the American Fall 

 north of Goat Island marks the amount of erosion which 

 it hits been able to accomplish, while the Horseshoe Fall 

 was cutting its way southward across the end of Goat 

 Island to its present position. The American Fall is 168 

 feet high, a precipice cut down, not by itself, but by the 

 Horseshoe Fall. The latter -in 1842 was 159 feet high, 

 and, as shown by the map, is already turning eastward, to 

 excavate its gorge along the center of the upper river, p 

 is the apex of the Horseshoe, and T marks the site of the 

 Terrapin Tower, with the promontory adjacent, round 

 which I was conducted by Conroy. Probably since 1842 

 the Horseshoe has worked back beyond the position here 

 assigned to it. 



In conclusion, we may say a word regarding the proxi- 

 mate future of Niagara. At the rate of excavation assigned 

 to it by Sir Charles Lyell, namely, a foot a year, five 

 thousand years or so will carry the Horseshoe Fall far 

 higher than Goat Island. As the gorge recedes it will 

 drain, as it has hitherto done, the banks right and left of 

 it, thus leaving a nearly level terrace between Goat Island 

 and the edge of the gorge. Higher up it will totally drain 

 the American branch of the river; the channel of which in 

 due time will become cultivable land. The American Full 



* Near the mouth of tlie gorge at Queenston, the depth, according 

 the Admiralty Uharfc, is 180 feet-' well within the gorge it is 132 



