SLODIES FOR STUDENTS. 95 
one time and another over all that part of the land surface which 
was submerged during the drift period. If the amount of sub- 
mergence during the drift period could be determined, we should 
have the maximum measure of the extent of the opération of pan- 
ice. . Pan-ice might produce results closely simulating certain 
results produced by glacier ice. On the glacier-pan-ice combina- 
tion hypothesis, too, it is possible that any region affected by 
pan-ice at one time may have been affected by glacier ice at 
an earlier time, and that the effects of the glacier ice were not 
wholly obliterated by the pan-ice. On the other hand it is con- 
ceivable that a zone affected by pan-ice at one time was subse- 
quently elevated and covered by glacier ice which may have 
partly or even wholly obliterated the effects which the pan-ice 
had produced. 
The results which shore and pan-ice acting alone can effect, 
have already been studied. That both were operative about the 
shores of the land which the glacier ice covered during the drift 
period cannot be doubted, any more than can the existence of 
icebergs. The question of the relative importance of pan-ice and 
glacier ice in the production of the drift is a question concerning 
which there is much difference of opinion. 
Except along the coast lines and along the shores of lakes 
pan-ice could not have been operative. Away from the coasts, 
therefore, little can be ascribed to it. This removes the larger 
part of the drift area of the United States from the zone where 
shore or floating ice in any form can have been long effective. 
Along the southern part of the drift-covered coast of the United 
States there is no conclusive evidence of subsidence during the 
drift period. Further north, subsidence seems to have been a 
fact, and shore ice was doubtless a more considerable factor. 
It is not without significance that the Canadian geologists attrib- 
ute much more importance to pan-ice than do the geologists of 
the United States. Some of them ascribe to it a work compar- 
able in importance to that which the glacier ice effected.*. But 
the difference in views is perhaps one of degree rather than of 
7 Sir J. WILLIAM Dawson. The Canadian Ice Age. 1893. 
