GEOLOGICAL HISTORY OF THE LA URENTIAN BASIN. 407 



would be drained at lower levels, old beaches would be aban- 

 doned, the lakes would contract, and finally separate lakes 

 would be formed in the lowest depression in the basins of the 

 more ancient water bodies. The shape of the retreating ice 

 front would be determined by topographic conditions and would 

 in turn determine the northern outline of tlie lakes along its 

 margin. This in brief is one hypothesis that has been proposed 

 to explain the varied history recorded by the shore records, 

 sediments, etc., within the basin. 



10. Communication with the sea. Another hypothesis which 

 assumes to account for some of the facts observed, is that the con- 

 tinent was depressed at the close of the Glacial epoch sufficiently 

 to allow the sea to have access to the Laurentian basin. This 

 hypothesis is coupled with others which do not recognize a 

 period of Pleistocene glaciation, but, as already suggested, this is 

 a matter that is considered by the great body of American 

 geologists as not being any longer open to profitable discussion. 



In the study here outlined the question whether the water 

 bodies which formerly occupied the Laurentain basin were lakes 

 or arms of the sea, should not be difficult of direct and positive 

 determination. If fossils can be found within the basin, they 

 might yield definite testimony, but even if they are absent or 

 if their evidence is inconclusive, topography can be appealed to 

 with the expectation of receiving a conclusive decision. 



If the Laurentian basin was occupied by an arm of the sea 

 during various stages in the Pleistocene elevation, then the 

 records of such a submergence should occur both within and 

 without the depression, and direct connection between the 

 two should be expected. If the waters within the basin were 

 capable of making such well-defined shore records as are now 

 found, we are justified in assuming that the true ocean beach on 

 the outer slopes of the basin would be still more conspicuous. 

 Again, the waters within the basin deposited a sheet of sediment, 

 certainly not less than one hundred feet thick ; to be sure the 

 conditions for rapid accumulation were there present, but if the 

 ocean covered the adjacent land it should have left similar de- 



