W. Upham — Estimates of Geologic Time. 217 



and Europe make it clear in the opinions of some of the 

 geologists who believe in the duality or plurality of Quater- 

 nary glacial epochs, that not astronomic but geographic causes 

 produced the Ice age. 



Glacialists who reject Croll's ingenious and brilliant theory 

 mostly appeal to great preglacial altitude of the land as the 

 chief cause of the ice accumulation, citing as proof of such 

 altitude the fiords and submarine valleys which on the shores 

 of Scandinavia and the Atlantic, Arctic, and Pacific coasts of 

 ISTorth America, descend from 1,000 to 3,000 and even 4,000 

 feet below the sea level, testifying of former uplifts of these 

 continental areas so much above their present heights. But 

 beneath the enormous weight of their ice-sheets these lands 

 sank, so that when the ice attained its maximum area and 

 thickness and during its departure the areas on which it lay 

 were depressed somewhat lower than now, and have since 

 been re-elevated. This view to account for the observed 

 records of the Ice age is held by Dana, LeConte, Wright, 

 Jamieson, and others, including the present writer. It is 

 believed to be consistent either with the doctrine of two or 

 more glacial epochs during the Quaternary era, or with the 

 reference of all the glacial drift to a single glacial epoch, 

 which is thought by Wright, Prestwich, Lamplugh, Falsan, 

 Hoist, and others, to be more probable. To myself, though 

 formerly accepting two glacial epochs, with a long warm inter- 

 val between them, the essential continuity of the Ice age seems 

 now the better provisional hypothesis, to be held with candor 

 for weighing evidence on either side. The arguments sup- 

 porting this opinion are well stated by Professor Wright.* If 

 there was only one epoch of glaciation, with moderate tempo- 

 rary retreats and re-advances of the ice-border, sufficient to 

 allow stratified beds with the remains of animals and plants to 

 be intercalated between accumulations of till, the duration of 

 the Ice age may only have comprised a few tens of thousands 

 of years. On this point Professor Prestwich has well written 

 as follows : 



For the reasons before given I think it possible that the Glacial epoch — that is 

 to say, the epoch of extreme cold— may not have lasted longer than from 15,000 



to 25,000 years, and I would for the same reasons limit the time of the 



melting away of the ice-sheet to from 8,000 to 10,000 years or less.f . 



From these and foregoing estimates which seem to me 

 acceptable, we have the probable length of Glacial and Post- 

 glacial time together 30,000 or 40,000 years, more or less ; but 

 an equal or considerably longer preceding time, while the 



* The Ice Age in North America, 1889, chapters xix and xx. Man and the 

 Glacial Period, 1892, pp. 117-120 and chapters ix and x. " Unity of the Glacial 

 Epoch," in this Journal, Nov., 1892. f Geology, vol. ii, p. 534. 



