1892.] 
471 
[De Geer 
Consequently the geoicl of the marine limit should be found at 
27% of the latter beach, if the deformation of both had been 
proportionate. 
To see to what degree this has been the case for the different 
beaches, I have also reckoned in percentages the proportion be- 
tween Forest, Ridgeway, and Maumee beaches, and from the 
figures thus obtained as far as we can judge from the material 
at present available the differential uplift of the highest or the 
Maumee beach was somewhat greater than that of the Ridgeway 
beach, the same being the case with the Ridgeway beach in com- 
parison with the Forest beach, but the lowest one, or the Iroquois 
beach, seems to have a proportionately steeper slope than the 
Forest bead) and to be in this respect more proportionate to the 
Ridgeway beach. 
As this has been explored for the greatest distance and seems 
to be the easiest of identification, I have thought it advisable to use 
it for this preliminary interpolation, without attempting to make 
any correction for the divergence from the proportion of 27% 
which may occur in the southern part of the region. 
Thus of the figures on the map indicating the interpolated 
height of the marine limit all those along the Iroquois beach repre- 
sent 75% of its height, and those along the Ridgeway beach 27 % 
of its height. 1 
Concerning the westernmost part of the glaciated area we owe 
accurate information about the gradient of the warped beaches to 
Mr. Warren Upham’s excellent investigation of Lake Agassiz. 
However, until the deserted beaches around Lake Michigan and 
Lake Superior are more fully explored and the damming ice-border 
is continuously traced between the different basins, it is difficult 
to form any opinion about the absolute amount of the upheaval of 
the land since the formation of the marine limit. 
In the mean time we must content ourselves with the follow- 
ingfacts. As Prof. J. E. Todd and Mr. TJpham have stated, the 
deserted shores of Lake Dakota, situated close to the southwest of 
Lake Agassiz, show no or only a slight unequal deformation. As 
1 This method of interpolation can of course be accurate only when the change of 
level has been successive and regular, as may perhaps to some extent have been the 
case with the sea, but probably much less with the ice-dammed lakes. Still the present 
state of our knowledge does not seem to allow any more satisfactory method, and this 
may be sufficient for the present purpose. 
