TIME RELATIONS OR GLACIAL SUCCESSION. 23 



descriptions of the weathered zone between this till sheet and the overlying 

 Kansan. The extent of weathering- appears to him scarcely sufficient to 

 warrant a separation into a distinct glacial stage. He, however, suggested 

 that the sub-Aftonian sheet may have been formed during one of the 

 hypothetical minor advances of the ice sheet preceding its maximum 

 extension. 1 



The portion of the Labradorian ice field to which the name Illinois Lobe 

 is applied, reached its farthest limits on much, if not all, of its border at the 

 Illinoian stage of glaciation. The Illinoian drift sheet, as indicated in 

 detail farther on, passed some distance into territory which had been occu- 

 pied by the southern extension of the Keewatin ice sheet at the Kansan stage. 

 Since the Illinois ice lobe reached its farthest limits on much of its border at 

 the Illinoian stage of glaciation, any earlier deposit made by this ice lobe must 

 be largely concealed by the deposits of this stage. Attention is called below 

 to deposits in Illinois which may prove to be sufficiently older than the 

 Illinoian to be referable to a distinct stage, though the evidence as yet is 

 rather fragmentary. Concerning this matter Professor Chamberlin has sug- 

 gested 2 that the two great ice fields may have alternated in their invasions 

 in such manner that the sub-Aftonian preceded the sub-Illinoian and the 

 latter preceded the Kansan, much as the Kansan is known to have preceded 

 the Illinoian. There is, however, as yet no decisive evidence of such a rela- 

 tionship. Possibly the sub-Illinoian will prove to be of about the same 

 age as the Kansan. With these preliminary statements we pass at once to 

 the discussion of the Illinoian stage of glaciation. 



' Proc. Iowa Acad. Sci., Vol. V, 1898, pp. 86-101. 

 -Communicated to the writer. 



