associated with Glaciation. 119 



very impressive troughs which are now found on these sub- 

 marine continental slopes. But the duration of the epeiro- 

 genic uplift of these areas on the border of the glaciation for 

 the Hudson, beyond it for the California!! rivers, and near the 

 equator in western Africa, can scarcely be compared in its 

 brevity with the prolonged high altitude held during late Ter- 

 tiary and early Quaternary time by the Scandinavian peninsula 

 and by all the northern coasts of North America from Maine 

 and Puget Sound to the great Arctic archipelago and Green- 

 land. The abundant long and branching fjords of these 

 northern regions, and the wide and deep channels dividing the 

 many large and small islands north of this continent, attest a 

 very long time of preglacial high elevation there. At the 

 time of culmination of the long continued and slowly increas- 

 ing uplifts at the north, they seem to have extended during a 

 short epoch far to the south, coincident with the formation of 

 ice-sheets in high latitudes. But when these lands became 

 depressed and the ice burden of the glaciated countries was 

 removed, they in some instances, as in Great Britain and New 

 England, returned very nearly to their original levels beauti- 

 fully illustrating the natural condition of equilibrium of the 

 earth's crust, which Dutton has named isostasy, that when not 

 subjected to special and exceptional stresses it acts as if float- 

 ing on a heavier plastic and mobile interior. 



Somewhat analogous with the foregoing is the second of 

 these objections, namely, the fully proved low altitude of the 

 glaciated lands when the ice-sheets attained their maximum 

 extent and during the diversified and fluctuating history of 

 their recession. It must be recognized, however, that we have 

 in the complex series of drift deposits left for our examination 

 only a representation of the later # and closing phase of the Ice 

 age, while the land was low or near its present level. The 

 comparatively much longer early phase of high altitude attend- 

 ing the accumulation and slow extension of the ice-sheets on 

 this continent is not clearly represented by the drift and 

 numerous moraines of the glacial retreat or of the extreme 

 limit of glaciation, but by the earlier fluvial Lafayette forma- 

 tion, in which, according to Hilgard, coarse gravel from the 

 Archsean areas near the head of the Mississippi was carried 

 down by that stream quite to the shores of the Gulf of Mexico, 

 attesting, as he believes, an altitude of the Upper Mississippi 

 region at least 3,000 feet higher than now* 



The wane and departure of both the North American and 

 European ice-sheets have been marked by many stages of halt 

 and oscillation, whereby the flora, including forest trees, and 

 less frequently traces of the fauna, of the temperate areas 



*This Jouraal, III, vol. xliii, pp. 389-402, 1892. 



