1503, INUIT VAINID GeQIOGIC. WuUMes,, 
ASSUMING the correctness of the ice-sheet or glacier theory 
of the origin of the drift which, according to one supposed to 
be of high geological authority,’ ‘‘has passed from the region of 
hypothesis to that of demonstration, and should form the basis 
b 
of all reasoning on the subject,” there are still many problems 
that are open to discussion and in regard to which writers widely 
vary in their opinions and statements. One of these is the lapse 
of time between the beginning and the end of the period, that 
portion of geologic time required to prepare for and lay down 
the drift deposits and to return to present climatic and physical 
conditions. Of late years there has been a tendency in some 
quarters to reduce the estimated length of this period. Accord- 
ing to Prestwich? 25,000 to 35,000 years would suffice for the 
whole period of formation and retreat of the ice-sheet. Pro- 
fessor Wright endorses this estimate,3 and Warren Upham‘ even 
abbreviates it a little, allowing only 20,000 or 30,000 years for 
the actual glacial period, and 6000 to 10,000 years for the since 
intervening time. Becker,> figuring on astronomical data, thinks 
that conditions favoring the glaciation of the territory covered 
by the drift may have existed within 40,000 years, and presum- 
ably gives that as the probable outside figure for their occur- 
Re meee 
On the other hand, perhaps the larger number of glacialists 
have allowed in their estimates very much longer periods. To 
say nothing of Croll, Ramsay, Geikie, and earlier writers, we 
™Geol. Mag., new ser., Decade IV, Vol. IV, 75, February 1897. (Review of 
Croll’s Life and Work.”) 
2“ Geology,” p. 534, 1888. 
3 Man and the Glacial Period, p. 364, 1895. 
4Bull. Am. Geol. Soc., V, 99, 1894. 
5 Am. Jour. Sci., 3d ser., XLVII, 95-113. 
730 
