EN GLACIAL DRIFT OF THE MISSISSIPPI BASIN. 59 



one indicating the passive transporting action of the ice in 

 bearing from their distant homes north of the lakes the crystal- 

 line boulders and dropping them quietly on the surface, the 

 other indicating the active dynamic function of the ice in 

 rubbing, bruising and scoring the material at its base. The one 

 seems to me a clear instance of englacial and superglacial trans- 

 portation ; the other an equally clear example of subglacial 

 push, drag and kneading. 



Now if it were the habit of an ice -sheet of this kind to carry 

 material from its bottom to the surface by internal movement, it 

 would seem that the distance of 400 to 500 miles which intervened 

 between the source of the crystallines and the place of their 

 deposit would have furnished ample opportunity for its exercise, 

 and that there would have been commingled with the englacial 

 and superglacial material many derivatives from the intermediate 

 region, and these derivatives should have borne the characteristic 

 markings received by them while at the base of the ice. The 

 very conspicuous absence of such commingling, and the absence 

 or phenomenal rarity of anything that even looks like such a 

 commingling, appears to me to testify in quite unmistakable 

 terms to the distinctness of the methods of transportation. In 

 view of the great territory over which this particular belt is 

 spread, and the greater territory which is embraced in the other 

 tracts not here specially considered, there is left little ground 

 for doubt that this distinctness of englacial from basal transpor- 

 tation was a prevailing fact and not an exceptional one. This 

 is supported by concurrent evidence derived from the territor}- 

 west of Lake Michigan. This territory unfortunately does not bear 

 erratics that have equally distinct characteristics, but, so far as my 

 observation goes, the phenomena are alike throughout. I am there- 

 fore brought to the conclusion that, in the interior at least, there 

 was no habitual lifting of boulders from the base of the ice sheets 

 to the surface, nor any habitual commingling of basal with engla- 

 cial and superglacial material, except, of course, as it took place by 

 virtue of the falling of the latter through crevasses to the base, 

 and by mechanical intermixture of the two at the edge of the ice. 



