LAKE IllOQUOIS. 259 



occuiDy 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 descents of 500 

 and 400 feet, bring us to the sea-level of the Champlain epoch, or time of 

 departure of the ice-sheet of the Glacial period, which was the barrier of 

 these glacial lakes; for fossiliferous marine beds overlying the till extend 

 inland along the St. Lawrence Valley to Ogdensburg and Brockville, 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 

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

 time of 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 com- 

 parison, there would have been probably about 150 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 Iroquois to the sea. The 230 feet thus found meas- 

 ures the differential rise of the area of Lake Ontario during the early part 

 of the time between the dates of Lake WaiTen and of the sea at Og'dens- 

 burg. But this differential uplifting meanwhile affected the whole lake 

 region, extending westward over the area that had been occupied by the 

 glacial Lake Wan-en; and it is probable, 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 differ- 

 ential rise of the Ontario basin during the latter part of the time of the 

 glacial recession, between the formation of its latest beach with outflow by 

 Rome to the Mohawk and the complete departui'e of the ice, or more prob- 

 ably the melting of an avenue through the ice-sheet, on the area crossed 

 by the St. Lawrence, to which the ocean was then extended. To cany 



