GRAVITATION TOWARD THE ICE-SHEET. 489 



rigid that it was not depressed by the vast weight of tlie ice nor I'aised 

 when reheved of that weight, and the changes were beheved to consist 

 chiefly in tlie ditterential subsidence of the lake level, not in the differ- 

 ential elevation of the land basin.^ The general uniformity of these 

 changes in their direction and extent, and their proljable completion during 

 the departure of the ice-sheet, seemed to accord with this hypothesis. 

 The exact comparison of the shore-lines surveyed by me, with leveling, on 

 both the east and west sides of the lake, extending for its up])er stages 140 

 miles from south to north in Minnesota and more than 300 miles from 

 south to north in North Dakota and southern Manitoba, shows no con- 

 siderable irregularity in the rates of northward and eastward ascent — that 

 is, of north-northeastward ascent — of the former lake levels, which thus 

 seem to be attributaljle to gravitation toward the waning ice-sheet, rather 

 than to a progressive elevation of the land, for that would be expected to 

 present noteworthy irregularities upon so large an area. It is probable, 

 however, that close scrutiny of the shore-lines will disclose small diverg- 

 ences, within limits of a few feet, from the uniformity of slopes which 

 they should have for agreement with this explanation; and it is to be 

 noticed that the highest shores in the \dcinity of Treherne, Brandon, and 

 Neepawa, Manitoba, have more nearly a northward tlian north-north- 

 eastward ascent; also that a slightly disproportionate increase in the ascent 

 of the highest Minnesota shore-line in the next 10 or 15 miles north of the 

 Buffalo River was ascribed to the proximity of a jiortion of the ice-sheet 

 on the east, where it was forming the Fergus Falls and Leaf Hills moraines. 

 Though it now appears true that the greater part of these changes of level 

 are due to the difterential rise of the land, the gravitation of the lake 

 toward the ice-sheet certainly operated in conjunction with that cause, 

 contributing to the full extent of its competency in producing the results 

 observed. 



Mr. R. S. Woodward, of the United States Geological Survey, has 

 worked out the mathematical problem of determining the effect of any 



' Similar oscillations in the relative heights of sea and land, associated with glaciation, have been 

 thus ascribed to ice attraction by Adhemar, in Revolutions de la Mer, 1840; by CroU, in Climate and 

 Time, 1875; and by Penck, in Schwankuugen dea Meeresspiegels, Jahrbuch der Geographischen 

 Gesellschaft zu MUuchen, bd. VII, 1882. 



