GEOLOGY OF EASTERN OHIO. 589 



discussed this subject for all the lakes, in the Smithsonian Contributions for 1859, where the 

 observations for fluctuation are presented from 1790 to 1854. The greatest known change 

 of level is six feet eight inches (6 ft. 8 in.); the lowest stage having been reached in the 

 winter of 1819, and the highest in June, 1838. A temporary rise in 1854 slightly exceeded 

 that of 1838. Taking the water registers prior to 1854, which were so fully kept as to 

 give a good monthly mean for a period of seven (7) years, the greatest mean monthly differ- 

 ence was four feet nine and a half inches (4 ft. 9^ in.) ; and for a period of four years 

 during which there was a full record, the greatest difference of years was four feet one and 

 three fourths inches (4 ft. If in.). Daily observations are now going on for all the lakes, 

 as a part of the government hydrographical surveys, which will require many years for 

 completion. Until the mean fluctuations are determined more accurately, I use for the 

 mean surface of Lake Erie, the average of four surveys, made from tide water, as shown in 

 the following table. 



Elevation of the Great Lakes above the Ocean. 



To fix the elevation of the lakes, I begin at those nearest the sea, to which instrumental 

 surveys have been made. 



LAKE ONTARIO. 



By lockage in the St. Lawrence canals, above mean tide ..... 234^- feet. 



By canal surveys of New York, above mean tide ..... 232 feet. 



Mean ......... 



LAKE ERIE. 



By canal surveys in New York, 1817 . 



By Captain Williams' Report of 1834, Niagara Ship Canal .... 

 By surveys Caatskill and Portland Railway, 1828 ..... 



By locks of New York Canal ....... 



Mean .......... 



LAKE HURON. 



S. W. Higgins' Geological Report of Michigan, 1838 .... 



A. Murray, Geological Report of Canada, 1849 ..... 



Mean ......... 577^ feet. 



Lake St. Clair, Geological Report, Michigan ...... 570 feet. 



LAKE MICHIGAN. 



Southern Michigan Railway, J. II. Sargent, Engineer, survey of 1856, south end . . 583 feet. 



LAKE SUPERIOR. 



By Captain Bayfield's barometrical measurement in 1824, 627 feet, evidently too great. 



A. Murray's determination, Geological Survey of Canada, 1849, 599.41 feet, say . . 600 feet. 



Survey of Bay D'Enoch & Marquette Railroad, 1859 ..... 610 feet. 



Mean ........... 605 feet. 



There is in all these lakes a slight current towards the outlets, and therefore their sur- 

 faces are not strictly level ; which accounts for some of the discrepancies in their reported 

 elevations. At Cleveland, the high water mark of June, 1838, is well preserved, having 

 been adopted by Mr. Pillsbury as the city directrix, or the zero for city surveys. Assuming 



MEMOIRS BOST. SOC. NAT. HIST. Vol. I, I't. 4. 148 



