SEA LEVEL OF THE CHAMPLAIN EPOCH. 261 



town, where they occupy a vertical range of about 80 feet, the lowest being 

 the last formed, corresponding to the highest beach at Hamilton, there is a 

 fall of about 400 feet to the St. Lawrence at its outflow from Lake Ontario 

 through the Thousand Islands. These two levels, and the respective de- 

 scents of 500 and 400 feet, bring us to the sea level of the Champlaiu epoch, 

 or time of departure of the ice-sheet of the second Glacial epoch, which was 

 the barrier of these glacial lakes ; for fossiliferous marine beds overlying the 

 till extend inland along the St. Lawrence valley to Ogdeusburgh and Brock- 

 ville, close below the Thousand Islands and at the same level, within a few 

 feet, as Lake Ontario. From Lake Warren to the Champlain ocean we 

 thus have an apparent descent of 900 feet. But the first and third of the 

 levels which are thus brought into close geographic correlation, namely, 

 Lake Warren, Lake Iroquois, and the sea, are separated chronologically by 

 the time of the existence of the intermediate Lake Iroquois, and we must 

 seek to eliminate the changes of levels which occurred within that time. 



If the earliest beach of Lake Iroquois had been taken for this comparison 

 there would have been 50 feet more of fall from the level of Lake Warren 

 at the western end of Lake Ontario, and 80 feet more of fall from Lake Iro- 

 quois to the sea. The 130 feet thus found measures the differential rise of 

 the area of Lake Ontario during the early part of the time between the 

 dates of Lake Warren and of the sea at Ogdeusburgh. But this differential 

 uplifting meanwhile affected the whole lake region, extending westward over 

 the area that had been occupied by the glacial Lake Warren ; and it is prob- 

 able, as shown by the beaches of Lake Agassiz, that the greater part, indeed 

 nearly all, of the 265 feet of gradual change in levels between Chicago and 

 the eastern end of Lake Erie took place during the time of the glacial Lake 

 Iroquois, and previous to the time of the sea level in the St. Lawrence valley 

 with which Chicago and Lake Warren are compared. There was also a 

 small amount of differential rise of the Ontario basin during the latter part 

 of the time of the glacial Lake Iroquois, between the formation of its latest 

 beach with outflow by Kome to the Mohawk and the complete departure of 

 the ice on tlie area crossed by the St. Lawrence to which the ocean was then 

 extended. To carry back our comparison of Chicago and Lake Warren 

 with the sea level to the stage of the glacial recession when the Niagara 

 and the Mohawk were first uncovered from the ice, we have then to subtract 

 from the 900 feet of apparent descent an undetermined amount, which is 

 probably at least 250 feet and very likely may be fully 300 feet. The height 

 of the Chicago outlet above the sea level at the time of greatest extension 

 of Lake Warren is thus found to have been 650 or 600 feet, which differs 

 only slightly from its present height of about 595 feet. Chicago having 

 had nearly the same elevation as now, we learn from the shore lines of 



