GEOLOGICAL AND NATURAL HISTORY SURUEY. 13 



Gilbert not yet published in full. Lake Algonquin represents a 

 stage where an eastward outlet, from Georgian Bay by Trent 

 River, to Lake Iroquois, was open, the outlet to the St. Clair being 

 for a time above lake level. At that time the emergence of the 

 south end of the Lake Michigan basin, discussed in the text, may 

 have occurred. For the use of this map plate we are indfedted 

 to the Inland Educator Company of Terra Haute, Indiana. 



ALBEBTAN, OB OLDEST BECOGNIZED DEIET SHEET. 



Dr. George M. Dawson has recently called attention to a very 

 aged drift sheet found by him in the province of Alberta, east of 

 the Rocky Mountains, in Southern Canada (i). Its relation to 

 drift sheets in the United States is not definitely worked out. It is 

 overlain by a sheet that is probably either the sub-Aftonian (for- 

 merly Kansan) of Professor Chamberlin, or the somewhat later 

 sheet to which the Iowa geologists have applied the name Kan- 

 san. Should the lower sheet prove to be the one that overlies the 

 Albertan drift it will be necessary to place that drift in a still earlier 

 stage than the sub-Aftonian of Professor Chamberlin, for it is de- 

 scribed to be of markedly greater age than the drift sheet which 

 overlies it. 



AETONIAN INTEEVAIi OF EECESSION. 



The interval of recession or deglaciation, following the depo- 

 sition of the lower till sheet of Southern Iowa, and named by Pro- 

 fessor Chamberlin the Aftonian, from the type locality near Afton, 

 Iowa, is characterized by soil and weathering of such a inarked 

 character that it no doubt will be recognized widely as the detailed 

 study progresses. In the present stage of investigation it scarcely 

 can be separated from soils of other horizons, except in the dis- 

 trict which lies outside the Illinoian and the lowan drift sheets. 

 This district has not as yet received much study, hence the extent 

 to which the soil remains intact is not known. 



In Southeastern Iowa and Western Illinois several instances 

 of the occurrence of a black soil between beds of till, and thought 

 to be the Aftonian, have come to my notice. There is an element 

 of uncertainty from the fact that it is near the border of the drift 

 sheet, where minor oscillations of the ice margin are liable to have 

 occurred, which may ^have caused the burial of soils whose extent 

 is limited. It is not yet made certain that they mark a prolonged 

 interval, such as Professor Chamberlin finds to be exhibited in the 

 type locality near Afton. 



(1) Bulletin Geological Society of America. VoJ. 7, pp. 31-66. Issued Nov. 1895 



