360 MODIFICATION OF GREAT LAKES BY EARTH MOVEMENT. 



advances or retreats as the basin is tipped. Consider, for example, 

 Lake Superior. On the map (fig. 7) a line lias been drawn through the 

 outlet at the head of St. Marys Eiver in a direction at right angles to 

 the direction of tilting. All points on this line, called the isobase of the 

 outlet, are raised or lowered equally by the tilting and are unchanged 

 with reference to one another. All points southwest of it are lowered, 

 the amount varying with their distances from the line, and all points to 

 the northeast are raised. The water, always holding its surface level 

 and always regulated in volume by the discharge at the outlet, retreats 

 from the rising northeast coasts and encroaches on the sinking south- 

 west coasts. Assuming the rate of tilting to be 0.42 foot per 100 miles 

 per century, the mean lake level is rising at Dulnth 6 inches per century 

 and falling at Heron Bay 5 inches. Where the isobase intersects the 

 northwestern shore, which happens to be at the international boundary, 

 there is no change. 



Lake Ontario lies altogether southwest of the isobase of its outlet, 

 and the water is encroaching on all its shores. The same tilting that 

 enlarged it from the area marked by the dotted line of figure 2 is still 

 increasing its extent. The estimated vertical rise at Hamilton is 6 

 inches per century. The whole coast of Lake Erie also is being sub- 

 merged, the estimated rate at Toledo and Sandusky being 8 or 9 inches 

 per century. 



The isobase of the double lake Huron-Michigan passes southwest of 

 Lake Huron and crosses Lake Michigan. All coasts of Lake Huron 

 are therefore rising as compared to the outlet, and the consequent 

 apparent lowering of the mean water surface is estimated at 6 inches 

 per century for Mackinac and at 10 inches for the mouth of the French 

 Eiver, on Georgian Bay. In Lake Michigan the line of no change 

 passes near Manistee, Mich. At Escanaba the estimated fall of the 

 water is 4 inches per century; at Milwaukee the estimated rise is 5 or 

 6 inches, and at Chicago between 9 and 10 inches. 



These slow changes of mean water level are concealed from ordinary 

 observation by the more rapid and impressive changes due to variations 

 of volume, but they are worthy of consideration in the planning of 

 engineering works of a permanent character, and there is at least one 

 place where their influence is of moment to a large community. The 

 city of Chicago is built on a smooth plain, little above the high-water 

 level of Lake Michigan. Every decade the mean level of the water is 

 an inch higher, and the margin of safety is so narrow that inches are 

 valuable. Already the older part of the city has lifted itself several 

 feet to secure better drainage, and the time will surely come when, other 

 measures of protection are imperatively demanded. 



Looking to the more distant future, we may estimate the date at 

 which the geographic revolution prophesied by Spencer will occur. 

 Near Chicago, as already mentioned, is an old channel made by the 

 outlet of a glacial lake. The bed of the channel at the summit of the 



