414 GEOLOGY. 



of the ice retreat. The time-unit is the period which has elapsed 

 since the Late Wisconsin began to be exposed to erosion: 



From the Late Wisconsin to the present 1 time-unit. 



From the Early Wisconsin to the present 2 to 2 J time-units 



From the Iowan to the present 3 to 5 



From the Illinoian to the present 7 to 9 " 



From the Kansan to the present 15 to 17 l 



From the sub-Aftonian to the present. . x " 



So far as now known, the sub-Aftonian is everywhere buried by 

 later deposits, and the method of estimate by erosion is inapplicable 

 to it. Some hints of its relative age may be gained from the growth 

 of vegetation, and the development of the fauna and flora between 

 it and the Kansan, and from the superior amount of disintegration 

 and other internal changes which its material suffered; all of which 

 imply a considerable period anterior to the Kansan. If the sub- 

 Aftonian is equivalent to the very old drift of New Jersey and Penn- 

 sylvania (the Jersej^an), the erosion measure may be applied there, 

 with the result of indicating great antiquity. 



The average of these estimates is not far from the geometrical 

 series 1, 2, 4, 8, 16. This symmetry is not presumed to have any 

 dynamic significance, but it may serve a mnemonic purpose. Subse- 

 quent studies have tended rather to increase than diminish the high 

 ratio of the earlier epochs. In particular, the studies of Calvin in 

 southwestern Iowa have strongly impressed him with the relative 

 greatness of the erosion of that region. It is not unlikely, however, 

 that this was in some measure dependent upon a more favorable topo- 

 graphic attitude, due to a relatively greater westward slope before 

 the western side of the Great Plains was lifted to its present elevation. 2 



Under full admonition as to the tentative nature of such estimates, 

 the figures above given may perhaps be taken as representative. There 

 is every presumption that they will need to be modified by further 

 researches, probably in the direction of extension. 



1 A special estimate of the amount of the erosion suffered by the Kansan and Late 

 Wisconsin, respectively, in central Iowa, where they lie side by side under condi- 

 tions favorable for the comparison, gave Bain a ratio of 17 to 1. Geology of Polk 

 County, Iowa Geol. Surv., Vol. VI. 



2 For estimates of period of time involved in certain glacial oscillations, see Taylor, 

 Jour, of Geol., Vol V, 1897. 



