154 Niagara. April, 
should be a trigonometrical re-measurement of the rocks 
at the fall. This, compared with that made thirty-three 
years ago—one-third of a century—could not fail to afford 
data for calculating the present rate of retrocession, and 
would be a fitting compliment to the veteran geologist under 
whose auspices the first survey was made, and to whom the 
whole scientific world is a debtor for a lifetime spent in 
geological research. 
Whenever that survey be made, I believe it will decide 
that the present river is cutting back the gorge much more 
slowly than Lyell estimated; that, instead of one foot 
yearly, the retrocession is not more than, if it is as much, 
as one foot in ten years, and that, allowing for the compa- 
rative softness of the rocks below the whirlpool, we shall 
have to put back the occurrence of the glacial period to at 
least 200,000 years ago, if we conclude that the whole of the 
gorge, from the falls to Queenstown, has been excavated 
since that time. But if the conclusion at which I have 
arrived is correct, that the gorge, from the whirlpool to the 
falls is pre-glacial, and that the present river has only cut 
through the softer beds between Queenstown and the whirl- 
pool, and above the latter point merely cleared out the pre- 
glacial gorge in the harder rocks,—20,000 years, or even less, 
is amply sufficient for the work done, and the occurrence of 
the glacial epoch, as so measured, will be brought within 
the shorter period that, from other considerations, I have 
argued has elapsed since it was at its height. 
Simply looked at from a geological point of view, the time 
occupied may not seem important, and it has been usual for 
geologists to ask for an unlimited duration, though, even 
from that standpoint, it is difficult to reconcile the small 
amount of denudation that glacial moraines exhibit with the 
remote antiquity that some physicists assign tothem. In 
Ohio and Illinois, the mounds of the old Indians do not look 
more recent than the ridges and gravel hills of glacial origin, 
and in some parts cannot be distinguished from them until 
excavations are made into them. In England I know we 
have a school of geologists who have taught that the river 
valleys of the south of England have been excavated since 
the glacial period; but wherever we find undoubted glacial 
deposits, as in the north of England and in Scotland, we 
find them scarcely altered from the time when they were 
laid down. 
But the student of the succession of changes in the or- 
ganic world will have a serious difficulty removed, if it be 
proved that the glacial period occurred not more than twenty 
