ON THE FOCUS OF POSTGLACIAL UPLIFT NORTH OF 

 THE GREAT LAKES 



J. W. SPENCER 

 Washington, D.C. 



The first determination of the approximate location of the focus 

 of postglacial uplift, based upon the amount of rise found in the 

 beaches about Lake Ontario and Georgian Bay, appeared in a 

 volume, to which access is difficult, 1 and for this reason the passage 

 may be cited, as the revision to be given below will be found in 

 general conformity with the previous conclusions. 



If the axis of maximum elevation for the various triangles about Lake 

 Ontario and Georgian Bay be produced, they meet near latitude 51 N., and 

 longitude 74J W., a few miles west of Lake Mistassi and east of the southern 

 end of James' Bay. Although mainly radiating from the focus, the axes of 

 maximum elevation for the different triangles are not uniform, and are locally 

 modified, as along the western side of Lake Ontario, where there is found a 

 secondary axis of uplift to the east. Combining the more western axes with 

 those of the eastern end of the lake, another focus of uplift appears near the 

 "Height of Land" between Lake Ontario and Hudson Bay, in about latitude 

 48 N., and longitude 76 W. From the double foci it may be inferred that 

 the uplift reached its maximum along a line joining the foci, or that the axis 

 of maximum regional uplift was meridional and located along the eastern end 

 of Lake Ontario, increasing in amount until near the "Height of Land," and 



thence with a diminishing ratio, or even depression, towards the north 



At any rate, it is in the region southeast of Hudson Bay that the maximum 

 differential elevation of the earth's crust, which involved the Iroquois beach, 

 is to be found. 2 



Since that time (1889), Gilbert, De Geer, Taylor, and recently 

 Goldthwaite have illustrated more or less fully the rise by isobars, 

 which is only another mode of expressing the same phenomena, 

 while Coleman has redetermined some of the triangles. 



Combining additional measurements obtained from Fairchild 

 and Goldthwaite, I have recalculated the mean rise in the various 

 triangles from the present heights of the beaches about Lake 



1 Transactions of the Royal Society of Canada, VII, sec. iv, 189, read May 5, 1889. 



2 J. W. Spencer, ibid., 189. 



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