EELATION OF THE EAETU'S CKUST AND INTEEIOE. 493 



sinking- of the crust boneath the vast weight of the ice-sheet and a reeleva- 

 tion ■\\'hen that weight was removed, and, second, oscillations which may 

 have occurred without dependence on the glaciation. For the discrimination 

 of these movements it will be very instructive to notice the changes of 

 elevation that have been going forward at the same time in other parts 

 of the North American and European glaciated regions, and also in vari- 

 ous areas which were never thus ice-laden. If Lake Agassiz is found to 

 be an instance where nearly all these changes are apparently referable to 

 glaciation, there will be no lack of o])portunity for comparing it with other 

 regions where the effects due to glaciation are combined with independent 

 crustal movements. 



DLscitssio)/ of the relationsJiq) of the eartJis crust to tlic interior. — My 

 former i-eference of the northward ascent of the beaches of Lake Agassiz 

 to ice attraction, with the assumption that the earth was so rigid that its 

 form would not be changed by the load of the ice-sheet nor by its removal, 

 seemed more probable because of the well-known physical and mathe- 

 matical researches of Hopkins, Thomson, Pratt, and Prof. Gr. H. Darwin, 

 who conclude that the earth is probably solid, with not less rigidity than 

 that of glass or of steel. In deference to their investigations, this con- 

 clusion is accepted and taught in recent text-books of geology by A. 

 Geikie and Le Conte ; ' but in similarly recent text-l)ooks Prestwich and 

 Dana teach that the earth probably consists of a comparatively thin crust, 

 underlain by a molten interior, which may change within a moderate depth 

 to a great nucleal solid mass. Among other geologists and physicists who 

 have discussed the conditions of the earth's interior. King,- Slialer, ^ and 



' Since the publication of Le Conte's Elements of Geology, revised second edition, 1882, this 

 eminent geologist has abandoned the opinion here noted, and now believes "that the general 

 structure of the earth is that of a solid nucleus constituting nearly its whole mass, a solid crust of 

 :ncou.sideral>le comparative thickness, and a subcrust liijuid layer, either universal or over large 

 areas, separating the one from the other. * • * Also that the crust rests upon the subcrust liquid 

 as a floating body." American Geologist, Vol. IV, pp. 38-44, July, 1889; Am. Jour. Sci. (3), Vol. 

 XXXVIII, pp. 257-263, Oct., 1887; Elements of Geology, third edition, 1891, pp. 84-87, 264. 



'^U. S. Geological Exploration of the Fortieth Parallel, Vol. I, Systematic Geology, 1878, pp. 117, 

 696-725. 



'Proc, Boston Soc. Nat. Hist., 1866, Vol. XI, pp. 8-15; 1868, Vol. XII, pp. 128-136; 1874, Vol. 

 XVII, pp. 288-292. Memoirs, Boston Soc. Nat. Hist., 1874, Vol. II, pp. 320-340. Scribner's Magazine, 

 Vol. Ill, pp. 201-226, Feb., 1888. 



