﻿Vol. 64.] EPOCH IN NOBTH AMERICA. 151 



that might necessitate an extension of the Author's estimate of the 

 rate of erosion of Plum Creek: the old oxbow that had been utilized 

 for the construction of the reservoir was the expression of a tendency 

 of the stream to meander, and this tendency still remained under 

 the artificial condition now set up — with the result that the stream, 

 instead of taking a straight course through the new channel, had 

 swung from side to side and executed a larger amount of lateral 

 erosion than it would have done in the old channel to which it had 

 adjusted itself. Again, the deformation of the high-level lake-beach 

 implied an uplift to the north that had increased the gradient of the 

 southward-flowing streams. 



The AuTHOE, in reply, said that direct evidence of the brevity of 

 the continuance of the Matawan diversion from Niagara of the 

 drainage of the Great-Lake Basin was adduced by Dr. Warren 

 TJpham from the warped upper shore-line of glacial Lake Agassiz. 

 There the beach rises towards the north, until it is 400 feet above 

 the latest beach. This differential elevation occurred during the 

 period of the recession of the ice from the United States border to 

 Hudson's Bay. From the smallness of the delta deposited by the 

 Cheyenne and other rivers at the level of the upper beach, and 

 from the limited accumulation of dunes at the southern end of 

 Lake Agassiz, compared with that at the southern end of Lake 

 Michigan, it was clear that Lake Agassiz continued only for 

 1000 or 2000 years, showing that the northerly elevation of 

 400 feet proceeded at a rate which would have closed the Matawa 

 outlet after about that length of time. As the latitudes corre- 

 sponded, this rate of elevation was altogether probable. 



