Conclusions. 
363 
It has been shown however that in some cases an old outlet at 
a much lower level has been blocked up by glacial debris causing 
the lake to rise and find a new outlet. 
3. The great depth of the lakes has been thought to be suf¬ 
ficient proof of their glacial origin, but the fact that in some of 
of these cases the depth can be shown to be due to differential 
uplift and in others to drift obstructions at the outlet, is suffi¬ 
cient to throw doubt upon this conclusion. 
4. The fact that ice has once occupied a basin has been often 
assumed as proof of the ice-made character of the basin. If ice 
could cut out such a basin, still more easily could it enlarge 
and deepen one which is found ready made. Let us examine the¬ 
oretically, such a basin lying in the path of a large ice stream. 
Let the ice fill the basin and flow on beyond to any distance. 
What takes place in the basin? First, let us assume that the only 
check upon the rate of movement of the ice is that due to the in¬ 
crease in mass. It is like a lake in the course of a river. The 
current in the lake is inversely proportional to its size as com¬ 
pared with the river. If it is a large lake the current is imper¬ 
ceptible. In the case of the ice-filled basin, if the ice mass be 
ten times that of an equal length of the ice stream, its motion 
will be only one-tenth as rapid. This is the most favorable as¬ 
sumption that can possibly be made, yet even under this suppo¬ 
sition, differential wearing could not take place, as the added 
power due to greater thickness in the basin is more than com¬ 
pensated for by the retardation of motion, since part of the 
expansion is lateral [i. e., the ice is not ten times as thick in 
the basin as it is outside]. It seems clear that the basin could 
not be deepened by the ice although it might suffer some wear. 
But this writer is of the opinion, whether it be well founded 
or not, that the supposition just made is not the true one, and 
that the real case is even less favorable to the theory of ice ero¬ 
sion. 
The ice in the bottom is urged on by the weight of the ice on 
the lee slope, minus the friction on that slope. 
Its passage out of the basin is resisted by the weight of the 
ice on the stoss slope plus the friction on that slope. If the 
two slopes are equal the weights of ice will be equal but the re- 
