18 Spencer — Deformation of Algonquin Beach, 



rise much more than 900 feet above the sea. But towards the 

 northeast, the altitude of the land is everywhere high, except 

 along the depression in which Lake Xipissing lies. The bar- 

 rier there descends to 707 feet above the sea. Beaches and 

 shore lines are known to exist upon the land north of the lakes, 

 and 1 have seen such upon Manitoulin and Mackinac Island. 

 But they have not been directly connected with the more 

 southern beaches. Consequently, all deductions, in the study 

 of the lake involving that district, must be somewhat provis- 

 ional. From the character of the terrestrial rise increasing 

 towards the east, it is probable that there are no depressions 

 north of Lake Huron, lower than the plain of the Algonquin 

 Beach. This beach (by calculation from the mean rate of rise) 

 should be found in the vicinity of Lake Xipissing, at from 600 

 to 700 feet above that depression. 



Combining the Canadian series of beaches about the upper 

 lakes with corresponding series on the southern side of the 

 lakes (my survey of those in Michigan being still unpublished), 

 I find that there has been a differential elevation, since the 

 Algonquin episode, between the southern end of Lake Michi- 

 gan and the vicinity of Grand Bend, on Lake Huron, amount- 

 ing to about 290 feet. Hence, we know that the Algonquin 

 plain was down to a level, at least, of less than 300 feet above 

 the sea. By a triple series of calculations, the Algonquin plain 

 is found to have had a position somewhat less than 300 feet 

 above the Iroquois plain. The Algonquin water filled the 

 Huron basin to within a mile or two of its southern end, 

 where the beach is now submerged to about 20 feet, (calculated). 

 Hence, the waters did not How by the modern St. Clair Biver 

 to the south. At this time a considerable area of the southern 

 end of Lake Michigan was laid dry, as the beach bounding the 

 Algonquin water should now be submerged to 290 feet below 

 the waters of that modern lake. But the northern part of 

 the Michigan and Huron basins was filled to an elevation far 

 above their present surface, as the basins had not yet received 

 that great tilting which partly overflowed their southern mar- 

 gins and lowered their surfaces toward the north. 



There is a well-marked terrace and beach deposit on Mack- 

 inac Island, at about 190 feet above the lake. This is nearly 

 in the line of strike, or line along which there is no differential 

 elevation, of the lowest part of Algonquin Beach about Geor- 

 gian Bay. This old shore line, on the island, is better devel- 

 oped than any of the Huron Beaches, situated elsewhere, 

 except the Algonquin. From its position, there seems no reason 

 to doubt that the waters of Algonquin lake stood at that eleva- 

 tion in the strait between the Michigan and Huron basins. 

 Accordingly, tilting in the Michigan basins has amounted, 



