292 WALKS AND TALKS. 



work of Mesozoic and Caenozoic time ; but the whole length 

 of that gorge, even up to the present site of the Falls only 

 about six miles is far too little work for such a river as the 

 Niagara to have performed in the million or more years which 

 we allow for that time. I will, therefore, now make the 

 statement that only a part of the water which came into the 

 basin of Lake Erie (a mere river then) found its outlet by St. 

 David's. There seems to have been, at one time, an outlet 

 through the Cuyahoga Eiver, whose bed at Cleveland is two 

 hundred feet below its present bed, and perhaps for many 

 ages, the whole stream was carried off in that direction to the 

 Ohio. There seems to have been also, at some time, an out- 

 let to the basin of Lake Ontario, from the mouth of Grand 

 River of Canada. Possibly the drainage passed through one 

 or the other of these channels during most of Mesozoic and 

 Csenozoic time. 



The recession of the Falls of St. Anthony has been studied 

 by Professor N. H. Winchell. This, he maintains, is also a 

 post-glacial erosion as far as Fort Snelling. From the study 

 of old documents he ascertains that the rate of recession from 

 1680 to 1856 was about 5.15 feet a year. As the whole dis- 

 tance is eight miles, the time required is 8,202 years. 



A thorough scientific survey of the Mississippi, by author- 

 ity of the general government, was concluded some years ago 

 by Messrs. Humphreys and Abbot. They calculated that 

 5,000 years had been required for the accumulation of the 

 Mississippi Delta. This assumes, of course, uniformity in the 

 rate of deposition, while it was probably more rapid during 

 the rainy Champlain period ; but, on the other hand, the in- 

 terval expressed reaches from the close of the glacial reign, 

 not from its "acme" as in the calculation on the Falls of St. 

 Anthony. 



Similarly the age of the Nile delta has been set down at 

 6,350 years. 



A very ingenious calculation of the length of post-glacial 

 time has been based by Dr. E. Andrews on the rate of bluff- 

 erosion and terrace formation near Chicago. North of Chicago 



