112 MAN AND THE GLACIAL PERIOD. 



plausible. He was very confident in referring most of 

 them, to preglacial times. His views, I suppose, will be pub- 

 lished in the long-delayed volume, now about to be issued. 



" 5. Vegetable Matter between Glacial Till and Under- 

 lying Berg Till or other Drift Deposits. — When one remem- 

 bers that the front of the great ice-sheet may have been as 

 long in reaching its southern boundary as in receding 

 from it, and with as many advance and retrograde move- 

 ments, we can easily believe that much drift material 

 would have outrun the ice and have formed deposits so far 

 ahead of it that vegetation would have grown before the 

 ice arrived to bury it. 



" 6. Preglacial Soils, etc. — I believe that this will be 

 found to include most in southern Ohio, if not in Illinois, 

 as Worthen claimed." 



The phenomena of the G-lacial period are too vast 

 either to have appeared or to have disappeared suddenly. 

 By whatever cause the great accumulation of ice was pro- 

 duced, the advance to the southward must have been slow 

 and its disappearance must have been gradual, though, as 

 we shall show a little later, the final retreat of the ice- 

 front occupied but a short time relatively to the whole 

 period which has elapsed since. As we shall show also, 

 the advent of the Ice period was probably preceded and 

 accompanied by a considerable elevation of the northern 

 part of the continent Whether this elevation was con- 

 temporaneous upon both sides of the continent is perhaps 

 an open question ; but with reference to the area east of the 

 Rocky Mountains, which is now under consideration, the 

 centre of elevation was somewhere south of Hudson Bay. 

 Putting together what we know, from the nature of the 

 case, concerning the accumulation and movement of gla- 

 cial ice, and what we know from the relics of the great 

 glacial invasion, which have enabled us to determine its 

 extent and the vigour of its action, the course of events 

 seems to have been about as follows : 



