498 THE GLACIAL LAKE AGASSIZ. 



years later Whittlesey published a similar opinion.' In 1868 Shaler refeiTed 

 the subsidence of ice-covered areas to a supposed rise of isogeothernial lines 

 in the subjacent crust, operating, in conjunction with the ice-sheet, to pro- 

 duce downward ilexure;" but in 1874 and later he regards the depression as 

 due directly to the Aveight t)f the ice, and the reelevation as due to its 

 removal.^ The same view is advanced also b}" Chamberlin to account for 

 the basins of the Laurentian lakes, where he believes a considerable part of 

 the glacial depression to have been permanent.'' 



Tardiness in the her/inninf/ of the changes of levels of the Lake Agassiz 

 basin. — That the greater part of the changes of levels upon the area of 

 Lake Agassiz has been due to differential elevation of the earth's crust, 

 instead of ice attraction, seems to be proved by the tardiness of their 

 beginning, as shown by the relationship of the highest beach of Lake 

 Agassiz to the contiguous terminal moraines formed on the adjacent land 

 areas during the recession of the ice-sheet, of which a detailed description 

 has been given in Chapter IV. The highest beach is continuous on the 

 east from Lake Traverse about 140 miles north to Maple Lake, which is as 

 far as exact exploration of it has been carried. On the west this shore-line 

 is unbroken along an extent of about 250 miles from south to north, reach- 

 ing into Manitoba. Now the adjacent Dovre, Fergus Falls, Leaf Hills, and 

 Itasca moraines appear to have been successively accumulated during the 

 time of formation of this single highest beach, which marks, through so 

 great distances and so large a poi'tion of the glacial recession, a nearly or 

 quite unvarying stage of the lake and undisturbed repose of the earth's 

 crust. If diminishing gravitatiou of the water of the lake toward the ice- 

 sheet had been the chief cause, or even an element of large importance 

 among component causes, of the changes of levels of the beaches, the 

 surface of the lake must have fallen considerably in its northern portion 



'Proc, A. A. A. S., Vol. XVI, pp. 92-97. 



^Proc, Boston Soc. Nat. Hist., Vol. XII, pp. 128-136. 



'Proc, Boston Soc. Nat. Hist., Vol. XVII, pp. 288-'292; Memoirs, Boston Soc. Nat. Hist., Vol. II, 

 pp. 335-340. Am. Jour. Sci. (3), Vol. XXXIII, pp.220, 221, March, 1887. Scribuer's Magazine, Vol. I, 

 p. 259, March, 1887. 



^Geology of Wisconsin, Vol. I, 1883, p. 290; Proc, A. A. A. S., Vol. XXXII, 1883, p. 212. The 

 problems of ice attraction and of deformation of the earth's crust have been further discussed by 

 Professor Chamberlin before the Philosophical Society of Washington, March 13, 1886 ; and, jointly with 

 Professor Salisbury, in the Sixth Annual Report, U. S. Geol. Survey, pp. 291-304. 



