SURFACE GEOLOGY. 4AQ 
period was of immense duration, and the great northern currents, with 
their floating icebergs, with loads of debris from northern regions, would 
in time be able to cover the bottom of the shallow sea with the materials 
we now find, and arranged as we now find them. All geologists agree 
in the belief in a submergence of the land, the only difference of opinion 
being in regard to the question whether prior to such submergence there 
had been spread over the whole North a vast and continuous glacier. To 
such a vast continental glacier is attributed by some the mighty work 
of giving shape and configuration to all the surface within its range, 
reducing mountains and hills to plains, and digging out the basins of 
lakes sometimes to depths even below the level of the ocean. We 
have already seen that in the Second District there is no evidence that 
the pre-glacial or ante-drift surface was essentially different from what 
it now is. If there were a climate so arctic in character as to allow of 
the extension of a sheet of ice immensely thick almost to the Ohio River, 
we should expect that the same cold climate would necessitate glaciation 
in the Alleglany Mountains but a short distance south of the Ohio, where 
no traces of glaciers have been found. The average altitude of the Alle- 
ghany range is 3,000 feet. If, on the other hand, the cold were produced 
by marine currents coming down from the arctic region, it would have 
the sharp limitations characteristic of such currents at the present day. 
It is doubted by many geologists of high authority whether we have, 
even in Greenland and in the antarctic regions, any thing now corre- 
sponding to such a widely-extending glacier. There are glaciers in these 
polar regions occupying the valleys sloping to the sea, but not one uni- 
versal glacier. If this is true, there is no analogy for such a vast glacier 
as is claimed, except such as may be found in local glaciers like those of 
the Alps, and such local glaciers are freely admitted to have existed on 
the higher grounds adjacent to the icy northern currents. 
If, again, the cold were so great and so wide-spread as the whole gla- 
ciation of so large a part of the northern continent would call for, whence 
the heat for evaporating the moisture to be condensed by the cold into 
the snow and ice of the great glacial mantle? Prof. Tyndall has forcibly 
suggested this difficulty. 
Again, what force or vis a tergo could have been exerted to impel the vast 
glacier across the great valley of the lakes, and up and over the high ground 
tothe south? In all recorded movements of glaciers the ice is carried down 
slopes, so that gravity, if not positively aiding, could not retard the move- 
ment. Ifa glacial sheet extended into southern Ohio, it must have passed 
over the vast distance from the high lands (now reported to be only 1,500: 
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