154 FRAGMENTS OF SCIENCE. 
down 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 has been reduced 
from one published in Mr. Hall's " Geology of New York." 
It is based 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 has 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, 
aud 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 Couroy. 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 np it will totally drain 
the American branch of the river; the channel of which in 
due time will become cultivable land. The American Fall 
* Near the mouth of the gorge at Queenston, the depth, according 
to the Admiralty Chart, is ISO feet- well within the gorge it is 133 
feet. 
