THE CAUSE OF THE GLACIAL PERIOD. 519 



and the formation of mountain-ranges, to afford an adequate 

 explanation of glaciation. It is probable that the great up- 

 lifts which are thus supposed to have caused ice-accumulation 

 were very slow in their progress, and that their effect upon 

 extensive continental areas was so distributed that the maxi- 

 mum changes in slope on then, borders would nowhere exceed 

 twenty or thirty feet per mile, while perhaps some portions 

 of the uplifted region would receive no change of slope. And 

 the subsidence beneath the weight of accumulated ice was 

 probably equally slow and similarly distributed, no limited 

 district being greatly changed. Excepting the rare instances 

 where disturbances of mountain-building or extraordinary ris- 

 ing or sinking of mountain-ranges were associated with these 

 movements, the contour of the country, with its valleys, hills, 

 and mountains, has remained in general the same from pre- 

 glacial time through the Ice age to the present with only 

 changes of slope, small in any limited tract, which in long 

 distances allowed great upheavals and depressions. The ele- 

 vation of the central part of glaciated areas, with downward 

 slopes on all sides, would favor the outward flow of the ice-sheets 

 and their erosion and transportation of the drift. But mount- 

 ains and hills jutted upward in ridges and peaks within the 

 moving ice-sheet, as they now stand forth in bold relief above 

 the lowlands ; and the ice with its inclosed drift was pushed 

 around and over them, some portions being deflected on either 

 side, and usually a larger part being carried upward across 

 their tops. Katahdin, the White Mountains, the Green Mount- 

 ains, and the Adirondacks, stood directly in the pathway of 

 the ice outflowing southeastward from the Laurentian high- 

 lands. Its thickness in northern New England and northern 

 New York seems to be measured approximately by the eleva- 

 tion of the highest of these summits above the adjoining low- 

 lands, about one mile ; but northward the ice-sheet evidently 

 was somewhat deeper upon the valley of the St. Lawrence, and 

 Professor Dana's estimate seems still reliable, that its maxi- 

 mum depth, lying on the water-shed between this valley and 

 Hudson Bay, was probably about two miles. 



