472 JAMES WALTER GOLDTHWAIT 



of 0.75 feet per mile, diminishing to less than 0.50 feet per mile near 

 Beaver Island, to Onekama, where it becomes horizontal and seems 

 to unite with the Algonquin plane to form the single 596-foot plane 

 already mentioned. While the data for the intermediate planes do not 

 permit an unqualified statement, they seem to indicate that the lowest 

 of the Algonquins, the Battlefield, and the Fort Brady beaches all con- 

 verge to the same point, Onekama, instead of being overlapped, 

 one after another, by the Nipissing. The Algoma plane seems to 

 constitute another member of this split series; but its exact position 

 south of Petoskey is somewhat in doubt. 



It is perhaps possible that the highest Algonquin beach becomes 

 horizontal at about 24 feet above Lake Michigan, near Herring Lake. 

 This is suggested by the work of the present writer in eastern Wis- 

 consin, and by the data thus far collected by those who have worked 

 in the Huron basin. If so, the 14-foot "Nipissing shore line" seems 

 to have very generally destroyed the 24-foot "Algonquin shore fine" 

 along the east side of Lake Michigan, south of Herring Lake. The 

 evidence here seems rather to indicate that the Algonquin and 

 Nipissing shore lines coincide to form the single 14-foot shore line. 

 Figures i, 3, and 5, and Plate I, embody this idea. 



SIGNIFICANCE OF THE FAN-LIKE PROFILE 



The steepness of inclination of these water planes, and their con- 

 vergence to a single point, affords a basis for choosing between differ- 

 ential uplifts and ice attraction, to explain their present condition. 

 At the Straits of Mackinac the calculated rate of inclination for the 

 highest Algonquin is 3.73 feet per mile; for the lowest of the Algon- 

 quin group, 3.00 feet; for the Battlefield, 2.10 feet; for the highest 

 Fort Brady, 1.29 feet; for the Nipissing, 0.75 feet; and for the Algoma, 

 0.33 feet. All those above the Nipissing slant too steeply to be 

 accounted for by ice attraction. They must be explained by a series 

 of earth movements which repeatedly raised the shore line of this 

 region out of water. The Nipissing and the Algoma shore lines are 

 inclined no more steeply than the surface of the lake close to the ice 

 border might be inclined by ice attraction; but they are probably 

 inclined more steeply than they could be so far from the ice border 

 for the stages they represent. The ice front must have been at least 



