Perry.] 134 [February 28, 
According to Professor Dana, this subsidence which closed the 
‘¢ Glacial era’? was the initiatory event of the next or Champlain 
era; in other words it was a preparation for the Terrace period, the 
marine part of which is now under consideration,—a preparation for 
the laying down of beds which are sometimes called Newer Plisto- 
cene. That there should have been a slight sinking of the land, re- 
sulting from the weight of the ice, and thus perhaps a beginning of 
depression before the close of the Glacial period, is not unreasonable 
to suppose; and it may have occurred; still I find no positive evi- 
dence that such was the fact. In case a submergence were really 
initiated, the downward movement must have been slight in the 
Glacial period, while it would no doubt cease with it. That the drift 
was to a great extent established as we now find it, and that thus the 
ice had already melted above such portions of it as are now covered 
by marine strata, before their deposition, seems probable? from the 
general relations of the beds. So far as I can judge, after an atten- 
tive study of the matter in neighborhoods containing unmodified 
drift and marine deposits, the latter overlie the former without any 
intermixture. ‘There is a strong line of demarcation between them; 
there are also various indications that, in given localities, the earlier 
work was ended before the later was begun; that the main mass of 
drift occupied substantially its existing position, before the waters of 
the ocean came in and began a distinct deposition of stratified mate- 
rial. Exceptions along the seaboard are not denied; there may have 
been many such; I only claim that cases of this sort should be re- 
garded, as they really are, as exceptional, and not as constituting 
the rule. 
But there are other localities in which sedimentary marine depos- 
its of clay and sand rest, not directly on typical drift, but on other 
stratified beds which not only contain no marine remains, but lie next 
above the drift proper. Now what is the age of these interlopers— 
of these intermediate strata—which are as clearly above the unmodi- 
fied drift, as they are beneath the unmistakable marine deposits? In 
1 Paper cited, p. 66. 
2 Professor Agassiz, in a recent conversation, suggested the following view of the 
matter ;—‘‘The ice-sheet may have receded to the present outlines of the con- 
tinent, and been undermined by the ocean to the extent of the theoretical depres- 
sion. The glaciation of all our shore islands shows how much beyond the present 
outlines of the continent the ice once existed. And the Arctic explorers have 
made us familiar with the great amount of animal life that animates the sea below 
the heavy ice-covering stretching over its surface.” 
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