120 W. Upham — Epeirogenic Movements, etc. 



adjoining the melting and mainly receding ice were covered 

 by its drift at the times of temporary re-advance of the ice- 

 border. No better illustration of conditions favorable for the 

 bnrial of forest beds in the drift can be imagined than those 

 of the Malaspina glacier or ice-sheet, between Mt. St. Elias 

 and the ocean, explored by Eussell in 1890 and 1891 and 

 found to be covered on its attenuated border with drift which 

 supports luxuriant growing forests. Let a century of excep- 

 tional snowfall cause a thickening and re-advance of that ice- 

 sheet, and sections of its drift exposed after the glacial reces- 

 sion will show a thick forest bed of chiefly or wholly temperate 

 species. Such re-advances of the continental ice-sheets, inter- 

 rupting their retreat, are known by well marked recessional 

 moraines in both North America and Europe. Near the drift 

 boundary in the Mississippi basin some of these glacial fluctua- 

 tions have involved long stages of time, measured by years or 

 centuries, with important though minor changes in altitude, as 

 shown by the excellent analytic studies of Chamberlin, Salis- 

 bury, and Leverett ; but farther north, as in the large region 

 of the glacial Lake Agassiz, the withdrawal of the ice-sheet 

 and formation of successive moraines marking slight halts and 

 re-advances due to secular changes in temperature, humidity, 

 and snowfall, were demonstrably very rapid, the whole dura- 

 tion of this glacial lake being probably only about 1,000 years.* 

 The vicissitudes of the general glacial retreat seem to me to 

 have been due thus chiefly to variations of snowfall, some long 

 terms of years having much snow and prevailingly cool tempe- 

 rature, therefore allowing considerable glacial re-advance, while 

 for the greater part other series of years favored rapid melt- 

 ing and retreat. 



Under this view we may, I think, account for all the observa- 

 tions which have been held in America and Europe as proofs 

 of interglacial epochs, without assuming that there was either 

 any far re-advance of the ice-border or any epeirogenic move- 

 ments attending the glacial retreat of such magnitude as to 

 induce the fluctuations of which the forest beds and marginal 

 moraines bear witness. Though the whole history of the wane 

 of the ice-sheets is indeed very complex and long, as measured 

 by our familiar historical time units, it was yet, in my opinion, 

 geologically very brief, if compared with all preceding geologic 

 periods and epochs. The formerly supposed necessity of predi- 

 cating long interglacial epochs seems to me a misunderstand- 

 ing. Instead, as Dana, Wright, Hitchcock, Lamplugh, Ken- 

 dall, Falsan, Hoist, Nikitin, and many other glacialists believe, 

 the Ice age seems to me to have been essentially continuous 



*Geol. and Nat. Hist. Survey of Canada, Annual Report, New series, vol. iv, 

 for 1888-89, p. 51 E. 



