490 THE GLACIAL LAKE AGASSIZ. 



added mass, as an ice-sheet, upon the eartli's surface, to disturb the levels 

 of the sea and of lakes.^ Assuming- an ice-sheet witli a radial extent of 38°, 

 or about 2,600 miles, and a central depth of 10,000 feet, from which the 

 depth decreases at first slowly and then more rapidl}' to its border, he finds 

 that the average slope within 1 degree of tlie border of the ice would be 

 about 5 inches })er mile, or less than one-third of the north-northeastward 

 ascent of the highest shore-lines of Lake Agassiz in the north part of the 

 area where they have been traced with leveling. If we coniy)are the prem- 

 ises in this problem with the probable conditions affecting this glacial lake, 

 it seems sure that the North American ice-sheet in its maximum extent 

 covered not more than about one-fourth so great an area, its extent being 

 equivalent to a spherical circle with a radius of 1,200 or 1,300 miles; but, 

 on the other hand, it is probable that the maxinmm depth of this ice- 

 sheet somewhat exceeded 10,000 feet, and that the area of this great depth 

 was a belt extending eastward from a few hundred miles north or northeast 

 of the south part of Lake Agassiz to a distance of about 1,0()0 miles east- 

 northeast, lying thus much nearer than in the assumed case of Mr. Wood- 

 ward's investigation. The smaller area and less total mass of the ice-sheet 

 attracting Lake Agassiz may have been offset by the nearer position of a 

 large part of its mass than in the assumption of the problem, so that 

 possibly its influence might be as great in producing an ascent of the lake 

 level above the level of the present time; but, if this mathematical inves- 

 tigation is reliable, gravitation of the lake toward its ice barrier could not 

 give to its highest shore a northward ascent of more than a few inches per 

 mile, at the most not so much as half a foot, whereas its observed ascent 

 within the area of ni}' leveling attains a maximum rate of 1 foot to 16 

 inches per mile, and this belongs to a north-northeastward ascent of fully 

 IJ feet per mile. A quarter part, or probably less, of the changes in the 

 levels of these beaches is therefore referable to ice attraction, while the 



'U. S. Geol. Survey, Sixth Annual Report, for 1884-85, pp. 291-300; aud Bulletin No. 48, "On the 

 form and position of the sea level," 1888, p. 88. Compare also Prof. Edward HtiU's computations, 

 "On the effect of continental lauds in altering the level of the adjoining oceans," Geol. Magazine (3), 

 Vol. V, pp. 113-115, March, 1888; "Polar ice-caps aud their influence in changing sea levels," by Sir 

 William Thomson, Trans., Geol. Society of Glasgow, Vol. VIII, 1888, pp. 322-340; and "The study of 

 the earth's figure by means of the pendulum," by E. D. Preston, Am. Jour. Sci. (3), Vol. XLI, pp. 

 445-160, June, 1891. 



