260 THE GLACIAL LAKE AGASSIZ. 



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 probably exceeds 230 

 feet, and very likely may be .fully 300 feet. The height of the Chicago 

 outlet above the sea-level at the time of gi-eatest extension of Lake Warren 

 is thus found to have been about 650 or 600 feet, wliich differs only slightly 

 from its present height of 595 feet. Chicago having had nearly the same 

 elevation as now, we learn from the shore-lines of Lake Warren that the 

 country adjoining the eastern end of Lake Erie was at that time depressed 

 more than 200 feet, while the region north of Lake Superior was about 500 

 feet lower than now. The Rome outlet of Lake Iroquois was at first 50 or 

 100 feet above the sea-level, and it was uplifted to about 300 feet above 

 the sea while it continued to be the outlet, and to probably 350 feet, lack- 

 ing less than 100 feet of its present height, by the time of the extension of 

 the sea to Ogdensburg and Brockville. 



Before proceeding to consider the later gi'eat extension of Lake Iro- 

 quois we may glance rapidly over some of the explanations of the ancient 

 elevated shore-lines of the Laurentian lakes which have been ofi"ered by 

 successive writers. Mr. Thomas Roy, a civil engineer of Toronto, in a 

 paper communicated in 1837 by Lyell to the Geological Society of Lon- 

 don, regarded the body of water that formed the teiTaces and beach ridges 

 near Toronto as an immense lake, with surface at one time about 1,000 feet 

 above the sea, held in on all sides by formerly higher barriers of land.^ But 

 LyeU during his travels in this country in 1841-42 examined these shore- 

 lines with Mr. Roy and pronounced them to be of marine origin." In 1861 

 Prof E. J. Chapman attributed the deposition of diift in this lake region to 

 a marine submergence exceeding 1,500 feet; but he, like all subsequent 

 observers, was unable to find any marine fossils. The beach ridges he 

 referred to a very extensive fresh-water lake formed in a later epoch when 

 the land was uplifted, the lake being supposed to be held in by a greater 

 elevation of the country between the Adiroudacks and the Laurentide high- 



' Proceedings Geol. Soc. London, Vol. II, pp. 537, 538. 

 -Travels in North America, Vol. II, Chapter XX. 



