MODIFICATION OF GREAT LAKES BY EARTH MOVEMENT. 355 



Iu a wet year more water enters the lake than leaves it, and there is 

 a net rise of the surface; in a dry year there is a net fall. A seriesof 

 wet years produce exceptionally high water and a series of dry years 

 exceptionally low, so that the entire range of water height is consider- 

 ably greater than the annual range. The recorded range for Lakes 

 Superior, Michigan, and Huron is between 5 and 6 feet; for Erie and 

 Ontario, between 4 and 5 feet. 



JAN. FEB. MAR.APL. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC 



SUPERIOR 



MICHIGAN- 

 HURON 



ERIE 



\ 



/_ 



/ 



+ 



\ 



ONTARIO 



Fig. 4. 



ANNUAL OSCILLATIONS OF THE SURFACES OF THE LAURENTIAN LAKES. 



Compiled from monthly means published by the Chief of Engineers, IT. S. A. Each vertical space rep- 

 resents 6 inches. The observations for Lake Superior cover the period 1862-1895; for Michigan- 

 Huron, 1860-1895; for Erie, 1855-1895 ; for Ontario, 1860-1895. 



The accompanying diagram (fig. 5) of the oscillations of Lake 

 Michigan illustrates the annual cycle and also the progressive changes 

 from year to year. Being compiled from monthly means of gauge 

 readings, it does not show tides and seiches nor the oscillations of 

 short period. 



These various oscillations of the water, though differing widely in 

 amplitude, rate, and cause, yet coexist, and they make the actual 

 movement of the water surface highly complex. The complexity of 

 movement seriously interferes with the use of the water plane as a 

 datum level for the measurement of earth movements, and a system of 

 observations for that purpose needs to be planned with much care. 



