418 



GEOLOGY. 



the remainder of the year, we have a retreat of 200 feet per annum 

 (to us, an improbably high estimate). The total distance to be covered 

 by the retreat previous to the beginning of the cutting of the Niagara 



gorge is taken at some 600 miles, or 

 \ I 3,000,000 feet, and the time occupied 



on the assumption of a retreat of 

 200 feet per year is 15,000 years, 

 at 300 feet per year, 10,000 years, or 

 at 100 feet per year, 30,000 years. 

 In the opinion of some glacialists 

 even the last represents too rapid 

 a retreat. The same rates applied 

 to the retreat pre-requisite to the St. 

 Anthony recession, give the results 

 17,000 to 20,000, 12,000 to 13,000, 

 and 35,000 to 40,000 respectively. 

 As already indicated and emphasized, 

 there are no means for a close deter- 

 mination of this factor. 



If the length of the Niagara 

 gorge be divided by the average rate 

 of retreat since the successive posi- 

 tions of the Falls were located by 

 accurate surveys, the quotient is 

 about 7000. This result is, however, 

 subject to several qualifications which 

 have been well stated by Gilbert and 

 others, but which cannot be discussed 

 in detail here. The chief of these lies 

 in the belief that at the time of the be- 

 ginning of the cutting of the gorge, 

 the waters of the upper lakes flowed 

 through the Nipissing valley into the 

 Ottawa (Fig. 522), and thence to the 

 sea, leaving only the waters of the Erie basin to pass over the Falls. The 

 belief is also entertained that later, as the land to the north rose relatively, 

 an outlet was found through the Trent river, and that only at a com- 

 paratively late date were the waters of the Upper Great Lakes poured 



Fig. 52 7. — The Niagara gorge. The 

 American and the Horseshoe Falls 

 are shown on opposite sides of Goat 

 Island. (After Gilbert.) 



