SURFACE GEOLOGY. 449 



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 

 to the 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. If a 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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