860 | Buffalo and Chicago, or [October, 
above their present level. To this question only one answer 
seems possible. The dam was the ridge of Queenston heights, 
now forty feet above the lake-level. Here is a barrier sufficient 
to confine the water till it reached the required height. 
But the objection naturally rises: If the crest of the Silurian 
escarpment at Lewiston is forty feet above Lake Erie, how could 
the river take its course in that direction and not over w Chi- 
H cago ridge fifteen feet lower ? 
a Three answers are possible. 
ist. The point at which the river crossed the ridge must have 
been rather lower than the adjacent portions. A stream always 
chooses the lowest ground. Now it is just possible that this 
=` point was so much below the rest of the ridge that it was lower 
than the notch at Chicago, and therefore determined the outflow 
in the northeasterly direction. This, however hardly amounts to 
_ a probability, and certainly deserves no more than mention. 
2d. Most geologists are agreed that during , the ice age the 
northern or north-eastern part of the continent was depressed 
much below its present level—in some places many hundred feet 
below it—and that as the ice age passed away it slowly recov- | 
~ ered nearly its former elevation. The presence of recent arctic d 
shells on the tops of hills, as at Mount Royal, Montreal, is decis 44 
ive proof of this. I will not now discuss the cause, it does not — 
concern the subject. But in view of the fact that the ridge at 
Queenston is far to the east and somewhat to the north of Chi- 
_ cago there is nothing improbable in the supposition that it was 
lower in depression and later in reélevation. Hence at the time 
_ in question it may have been so far below its present height as t° 
have been actually the lower of the two, and have thus deter 
- mined the outflow of the water by the ordinary laws of drainage: 
. on then the river was capable of cutting it down by erosion aS — 
fast as it rose by continental elevation, the channel once estab- 
lished would be permanently maintained. And there is no T°3- 
son | to doubt the competency of the Niagara to accompli 
i 
“3a But there -is a third and a more likely answer ar r 
ow northward retreat of the ice exposed the southern an 
ern portions of the country first, holding the north ie north- 
t covered till a later time. Hence the conclusion is unavoil on 
ta a the aha district and Central New York were 
