CliUST DEFORMATION BY THE ICE-SHEET. 497 



mony with the exphiuatiou that they were due to the evaporation of the 

 lake aud the consequent partial restoration of equilibrium by the underflow 

 of plastic rock ; but he regards his observations as too incomplete to furnish 

 absolute proof of this hypothesis.^ A supplementary and more satisfactory 

 test is supplied by this survey of Lake Ag'assiz. Debarred from referring 

 the northward ascent of the beaches of this glacial lake chiefly to ice 

 attraction, I regard my observations of their increasing rate of ascent in 

 proceeding from south to north, the gradual approximation in the lower 

 beaches toward horizontality, and the probable completion of these changes 

 in relative elevation during the existence of Lake Agassiz and the depart- 

 ure of the ice-sheet, as all strikingly accordant with this explanation, and, 

 indeed, as demonstrative of its truth. These changes in the levels of the 

 beaches of Lake Agassiz, partly pertaining to the lake itself and in larger 

 part to the crust of the earth, are thus believed still to be wholly referable 

 to the influence of the. ice-sheet in its recession, with which they show 

 such remarkable correspondence in the direction, character, and gradual 

 decrease of the northward ascent. No irregularities of the differential 

 changes in elevation are found which seem to require other explanation, the 

 rise caused by the removal of tlie ice-sheet not being combined upon this 

 area, so far as can be determined, with independent earth movements either 

 of elevation or depression. 



Histonj of the doctrine of crust deformation hy the ice-sheet. — Jamieson 

 appears to have been the first, in 1865, to suggest this view, which I 

 receive from him, that the submergence of glaciated lands when they 

 were loaded with ice has been caused directly by this load pressing down 

 the earth's crust upon its fused interior, and that the subsequent reeleva- 

 tiou was a hydi-ostatic (or we may better say isostatic) uplifting of the 

 crust by underflow of the inner mass when the ice was melted away.^ Two 



I Am. Jour. Soi. (3), Vol. XXXI, pp. 284-299, AprD, 1886. U. S. Geol. Survey, Monograph I, Lake 

 Bouueville, 1890, pp. 379-392. 



-Quart. Jour. Geol. Soc, Vol. XXI, p. 178. Later discussions of this subject by Mr. Jamieson are 

 in the Geological Magazine (2), Vol. IX, pp. 400^07 and 457-466, Sept. and Oct., 1882; and (3), Vol. 

 IV, ijp. 344-348, Aug., 1887. In the article last cited he applies this explanation to the changes pi the 

 beaches of Lake Agassiz, which up to that time I had attributed mainly to ice attraction. The same 

 principle, however, was brouu^t forward by Sir John Herschel in 1836, and had been advocated by 

 Prof. James Hall, of New York, in 18.")9, in attributing to the weight of sediments the long-continued 

 subsidence of the areas on which they have been deposited in great thickness. 



MON XXV 32 



