^(S The Aineriviui Geologist. jaruuiry, is!i:j 
near caeli other. Consequently the ueoid of the marine limit 
should be found at -~'/(_ of the latter beach, if the deformation 
of both had bi'cn [)r<)poitior.at('. 
To see to what degree this has l)een the ease for the <lirterent 
beaches, I have also reckoned in perci'utages the ])ri)p()rtion be- 
tween the Koicst. llidgeway. and Maumee l)eaches and from the 
tiixures thus ()l)tained as fai" as we can judge from the material at 
present available the ditlerential ui)lift of the highest or the 
Mauniee ln'ach was somewhat greater than that of the Ridgeway 
Vx'ach. the same Ix-ing the case with the llidgeway Ijcacli in com- 
jjarison with the Forest beach, bu^t the lowest one. or the Ii*o- 
<piois Ijeach. seems to have a i)ro|)ortionatcly steeper slop<' than 
the Forest beach and to be in this respect more proi)ortionate to 
the Ridgeway Ijeach. 
As this has been explored for the greatest distance and seems 
to be the easiest of identification. 1 have thought it advisaljle to 
use it for this preliminary interpolation, without attempting to 
make any correction for the divergence from the i)roi)ortion of 
27% which may occur in the southern part of the region. 
Thus of the figures on the map indicating the interpolated 
hight of the marine limit, all those along the Iroquois beach re- 
present 17)'/( of its higlil. ami those along the, Ridgeway beach 
27% of its hight. ■•■ 
Concerning the westernmost part of the glaciated area we owe 
accurate information about the gradient of the warped beaches to 
Mr. Warren T'pham's I'xcellent investigation of lake Agassiz. 
However, until the deserteil beaches around lake ^Michigan and 
lake Sui)erior are more fully explored and the damming ice-bor- 
der is continuously traced between the different liasins. it is ditfi- 
cidt to foiin any opinion aliout the absolute amount of the up- 
heaval of the land sinci' the formation of the marini- limit. 
In the meantime we must content ourselves with the follow- 
ing facts. As I'rof. J. K. Todd and Mr. rphaiii have statetl, 
the deserted shores of lake Dakota, situated close to the south- 
west of lake Airassiz. show no or only a slight uncupial deforniation. 
■"■This method of interpolation can of course be accurate only wiien 
the change of level has l)een successive and regular, as may perhaps 
to some extent have been the case with the sea. but |)robai)ly much 
less with tlie ice-djimnied lakes. Still the present state ofuiir knowl- 
edge does not seem to allow any more satisfactory method, and rhis 
may be sutiicieiir lor the ])reseiit pnrj)ose. 
