SLUIDIES, JRO SS AIDISIN TS. 93 
earth’s surface is subject to movement. Considerable areas are 
known to undergo slow elevation, while other adjacent or dis- 
tant areas are suffering subsidence. If, during the drift period, 
glaciers formed somewhat extensively on the higher lands now 
covered by the drift, it might be conceived that a moderate 
subsidence would allow ice to float out from the glaciers of the 
higher lands over the waters covering the intervening lower 
areas, thus distributing the drift over them. That is, the drift 
might be thought to be the joint product of glaciers and ice- 
bergs, glaciers working on the higher lands and icebergs over the 
lower. Conjecturally, the relative importance of these two 
agents might vary greatly, but the glaciers must have remained 
SUTCIEMI?, SLEMSIVES WO LHS TWISS Wo tne MCS Ol Whe iceloveress, 
But unless it attributed the chief work to glaciers, such a com- 
bination hypothesis as this seems open to fatal objections. We 
have already seen what icebergs can do, and that some of their 
results may simulate those of glacier ice. 
On the hypothesis that icebergs were an important agent in 
the production of the drift of the lower lands, it would be expected 
that the distinctively glacial marks would be absent from the 
lower drift-covered lands, especially near the outer borders of the 
drift. But this is not the fact. Marks of one sort or another 
which seem to be distinctively glacial, occur down to the level of 
the sea, near the southern border of the drift. Among such 
markings, the strie on the trap rock south of Jersey City may be 
mentioned. At various other points too, as in Southern Illinois, 
at or very near the extreme margin of the drift, striae are found 
beneath deposits which appear to be strictly glacial. While 
admitting that berg deposits may locally closely simulate those 
of glaciers, we cannot admit that icebergs can produce such strie 
as are found near the margin of the drift at many points, even 
at altitudes scarcely above the level of the sea. 
On the hypothesis which we are here considering, too, it would 
be expected that the position of the southern border of the drift 
was determined by icebergs, not by glacier ice. We have already 
seen that both the topographic relations of its terminus, as well 
