252 A. 1'. COLEMAN THE EARLIEST J XT1':H-(;LA< ] AL PERIOD 



There are reasons also for thinking that tlie Aftonian beds of Oliio and 

 adjacent States are of the same age. In both cases there is only one slieel 

 of bonlder-clay beneath, while there are four, separated by layers of iiitcr- 

 glacial materials, above. All of the genera of trees mentioned in ila- 

 Aftonian occur in the Toronto formation, and probably all but oue of the 

 seven mammals found at I'oronto are of the same genera as Aftonian 

 mammals. The Ohio region has mostly been glaciated by ice-sheets com- 

 ing from the Keewatin center, and if we correlate the Toronto and Afto- 

 nian formations it implies that the ice-sheets of the two centers had 

 parallel histories, which seems highly probable. 



Length of inter-Glaciat. Time 



The total length of the earliest inter-Glacial interval must haw been 

 far greater than that of post-Glacial time. It begins, as shown in the 

 Toronto region, with a period of. river erosion comparable to that needed 

 by the Don to cut its channel since the ice departed ; is continued by the 

 deposit of delta materials to the depth of 185 feet, reqtiiring thousands 

 of years, and ends with the cutting of river valleys much more mature 

 than the postglacial valleys. Attempts have been made to estimate these 

 different processes with the general result of tripling or quadrupling post- 

 Glacial time. Tn a jiaper read by the present writer before the Geological 

 Congress it has been shown from the rate of recession of Scarboro Heights 

 through wave erosion that Lake Ontario is at least 8,000 years old. This 

 result is corroborated by the rate of building of Toronto Island, which is 

 formed of materials transported from Scarboro. The method of calcula- 

 tion is far more definite and accurate than estimates formed from the 

 dunes of southern Lake Michigan. 



The 8,000 years reqttired by Lake Ontario to give its shores their pres- 

 ent development must have been required also by Lake Iroquois, with 

 shores of equal maturity. The marine episode coming between the two 

 and certain preliminary stages of the two lakes not included in the time 

 required to form their present shores probably demand an equal amount 

 of time. The whole time since the ice left the Ontario basin can hardly 

 be less than 25,000 years. 



This estimate, which is based on definitely measured factors and is not 

 merely a guess, gives a fair idea of post-Glacial time, probably under 

 rather than over the true amount, and may be used as a measure for cer- 

 tain interglacial phenomena. The preliminary stages of the inter-Glacial, 

 including the cutting of a river valley 16 feet into shale, may be esti- 



