874 EDITORIAL 



demonstrated to be equivalent to the Albertan beds of Canada. 

 The studies of Mr. Leverett have made it quite sure that the 

 Kansan ice-sheet crossed the Mississippi and invaded Illinois to 

 some moderate distance. He has also shown that the Illinois 

 ice-sheet returned the compliment and invaded Iowa. Between 

 these invasions there was a considerable interval of time, as indi- 

 cated by the greater erosion of the Kansan deposits and by the 

 prevalence of a soil horizon and of peat beds between the Kan- 

 san and Illinois till sheets where they overlap. He has shown 

 also that there was a notable interval between the invasion of 

 Iowa by the Illinois ice-sheet and the spreading of the loess 

 over its deposits, as indicated by erosion and the formation of a 

 soil horizon. This loess mantle seems to be identical with that 

 which is intimately connected with the east lowan drift sheet. 

 It thus appears that the invasion of the Illinois ice marks a dis- 

 tinguishable stage of glaciation separated by a notable interval 

 from both the earlier Kansan stage and the later lowan stage. 

 This interval appears to be of such moment as to make it inad- 

 visable to correlate the Illinois drift sheet with the lowan drift 

 sheet. As a result, the practice of designating the former the 

 Illinois sheet has already sprung up among us. The evidence 

 at present seems sufficient to justify its tentative use in the litera- 

 ture of the subject. It should of course be credited to Mr. 

 Leverett. 



The series in the Mississippi basin, as thus modified, would 

 be as follows in stratigraphic order : 



9. Wisconsin Till Sheets (earlier and later). 



8. Interglacial deposits (Toronto perhaps). 

 7. lowan Till Sheet. 



6. Interglacial deposit. 

 5. Illinois Till Sheet (Leverett). 



4. Interglacial deposit (Buchanan of Calvin). 

 3. Kansan Till Sheet. 



2. Aftonian beds, Interglacial. 

 I. Albertan Drift Sheet (Dawson). 



I 



