MICHIGAN ACADEMY OF SCIENCE. 39 



portion of Saginaw Bay and Lake Huron and fully two-tliirds 

 of the basin of Lake Michigan. The tilting affects the entire area of the 

 Superior basin. There has been about 100 feet of uplift at the head 

 of the Ottawa outlet since it was abandoned. The abandonment of this 

 outlet marks the beginning of the modern Great Lakes. The shore of 

 Lake Nipissing formed at the two outlet stage just preceding the aban- 

 donment of the Ottawa Outlet is everywhere a strong feature, and indi- 

 cates that the duration of this stage was comparatively long. It would 

 seem to require a length of some centuries if not some thousands of 

 years. Its duration was such a.s to suggest a lessening if not a cessation 

 of the differential uplift that led to the rising of the lake to the two 

 outlet stage. 



THE MODERN GREAT LAKES. 



In view of the changes which \ia\e been experienced by the bodies of 

 water in the Great Lakes basins as just outlined, it l>ecomes a matter of 

 interest to determine whether the present Great Lakes will undergo im- 

 portant changes in the future. There are of course processes at work 

 on the shores of all lakes which tend toward the tilling and extinction 

 of the water bodies. Material is brought in by streams and the deltas 

 are extended out into the lakes, while the fine material is carried on 

 to settle over the deeper portions of the beds. The material which is 

 cut by waves along the shores is also dei>osited in the deeper parts of 

 the basins as well as on the borders of the lakes. In the course of many 

 thousands of years, therefore, the lakes would become filled by these 

 ordinary processes. There will also be more or less lowering of the out- 

 lets of the lakes by erosion. In this way the level of Lakes Michigan 

 and Huron has already been lowered about 15 feet by the cutting down 

 of the St. Clair Outlet at the south end of Lake Huron. Should the 

 recession of Niagara Falls continue until Lake Erie is reached, the 

 barrier over which the water from Lake Erie now discharges would be 

 removed and the level of the lake correspondingly lowered. In ojtposi- 

 tion to the processes of erosion comes that of differential uplift and this 

 appears to be affecting the outlets of some of the lakes and it may 

 cause considerable shifting of the lines of discharge. 



Observations on the changes in level in the Great Lakes basins under- 

 taken by Gilbert, indicates that a tilting is still in progress, and that 

 the direction of the tilting is about the same as that in the Xipissing 

 shores, or in a general north-northeast direction. The portion of the 

 Michigan basin south of an isobase running east-southeast through the 

 head of the St. Clair outlet is apparently undergoing an extension of 

 the lake, but the Huron basin and the portion of the Michigan basin 

 north of this isobase show a shrinking of the lake from the present shore. 

 The poi-tion of Lake Superior south of an isobase running from the outlet 

 west-northwest and striking the north shore near the line of the United 

 States and Canada is undergoing a rise of water on the shore, while 

 that north of the line is shrinking from the shore. This shrinking has 

 exposed a beach known as the Sault beach which is submerged along the 

 south shore. In the Lake Erie basin, and also in the Lake Ontario, 

 there is an expanding of the entire shore line because the outlets are 

 at the end where ujtlift is most rapid. In the course of time the uplift 



