488 THE GLACIAL LAKE AGASSIZ. 



Tlie order in which we shall thus examine these several parts of the 

 complex causation of the changes of levels is not, however, the order of 

 their importance or several shares in the work. The third agency, mani- 

 fested in obedience to the pressure of the ice and in resilience when relieved 

 from it, is found to have been the principal factor, producing ftir the greater 

 part of the changes of levels. Its manifestation within the area of Lake 

 Agassiz during the Glacial and Recent periods on account of the other 

 conditions and stresses mentioned appears to be only a small element in 

 the problem; though, when thus originating, it is seen to have had great 

 importance in causing such changes in other parts of the world, and even in 

 parts of North America, contemporaneously with the uplifting of the basin 

 of this lake. The first agency noted is found to be a considerable factor, 

 working in the same directions as the epeirogenic effects of the transient 

 ice weight, and contributing perhaps a fifth or a fourth as much toward the 

 changed relations of the water level and the land area. But the second 

 agency, upon investigation, proves to have been slight in its effect, and 

 within the basin of Lake Agassiz, so far as it availed, it was opposed to the 

 other two. 



GRAVITATION TOWARD THE ICE-SHEET. 



Consideration of the character of the changes in the levels of the 

 beaches, resulting in a greater ascent upon the northern part of the area 

 examined than farther south, and gradually approximating, through the 

 successive stag-es of the lake, to ])arallelism with the present geoid surface 

 of level, led me in my earlier studies to attribute these changes almost 

 wholly to gravitation of the water of the lake toward the ice-sheet. The 

 cause of the present relations of the old shore-lines seemed to be discov- 

 ered in the explanation that at first this attraction had a large effect upon 

 the lake level because of the nearness of a great depth of ice on the east 

 in northern Minnesota and on the north in British America, but that after- 

 ward it was gradually diminished to a comparatively small influence when 

 the southern portion of the ice-sheet had been melted and the attracting 

 force proceeded from the region far north between Lake Winnipeg and 

 Hudson Bay.^ Under this view the earth's crust was believed to be so 



' Geol. and Nat. Hist. Survey of Minnesota, Eleventh Annual Report, p. 152; U. S. Geol. Survey, 

 Bulletin No. 39, p. 18. 



