302 ELEMENTARY GEOLOGY 
immense mass of water; and to disturb its level over 
the entire world, or even over a part of a hemisphere, 
would require changes in whose occurrence we have 
no reason to believe. Even the submergence of the 
American continent would not displace enough water 
in the ocean to account for some of the variations in 
level that have been recorded. 
Variations of Level in the Interior. —It is to the 
seashore that we look for the best evidences of changes 
in level. A rise or fall of ten feet will produce effects 
here which are readily noticeable; but in the interior, 
away from the shore, a movement of ten times this 
amount might occur without beimg observed. There 
is nothing with which to compare the change, as there 
is near the seashore. Still in one or two places, we have 
good evidences of a change of level in the interior. 
At one time, just at the close of the Glacial Period 
(p. 482), the Great Lakes were prevented from flow- 
ing out through the St. Lawrence, and were raised 
above their present level until they found an outlet 
over some low point in the rim of their basins. While 
at this level they built beaches, just as the present lakes 
are doing (Fig. 177). These were, of course, hori- 
zontal; but now they are tilted, and the tilting has 
raised the northern ends higher than the southern. 
This is exactly what has been found on the seashore, 
and so we may feel certain that this recent uplift has 
