5i 



did not interpret the great lakes history as it is now accep- 

 ted. In 1894 Spencer found the total duration of Niagara 

 falls to have been 32,200 years. Later studies (1906-7) 

 led him to change this result to 39,000 years. 



In a paper published in 1898, the writer of this text 

 ventured the opinion that the total duration of the falls 

 was probably 50,000 years or more. It seems clear now, 

 however, that this estimate was based on too slow a rate 

 for those sections of the gorge which were made by the 

 cataract with small volume. A review of the data bearing 

 upon the rate of recession of Horseshoe fall has led to the 

 conclusion that the average rate of recession in the Upper 

 Great gorge has been very nearly 4.5 feet (1-37 m.) per 

 armum. This applies to the Upper Great gorge and with 

 slight modification to the Lower Great gorge, but it does 

 not apply to either the New or the Old Narrow gorges, 

 nor to the first or oldest section near Queenston. 



At the very outset an assumption has to be made which 

 involves the decision of a difficult question. What part 

 of the crest line or what point upon it shall be taken to 

 represent the rate of recession? In the long run the 

 recession of the apex or extreme re-entrant of the crest 

 line will yield the true rate, but in a relatively short period, 

 like that of the instrumental surveys (1842- 1905) this 

 method may prove quite unsatisfactory, as indeed, it has 

 done. In a period preceding 1890 the apex retreated at a 

 relatively rapid rate and became very acute on a line 

 turning to one side from the main axis of the gorge. Then 

 the apex became nearly stationary and has remained so 

 for more than 20 years. There was not a deep enough 

 and large enough caldron beneath the acute angle to 

 undermine the capping hard layers. But while recession 

 ceased at the apex, it continued on a different line passing 

 west of the apex and more nearly on the main axis of the 

 gorge produced from the north. Thus, the former apex 

 will soon cease to be the apex, a new one being established 

 farther west. It seems better to take a certain limited 

 central portion where deep water is passing over, say a width 

 of 400 or 500 feet (120 or 150 m.) and use the measured 

 mean rate of recession there for the rate of elongation of 

 the gorge. This is the method used by Mr. Gilbert. 



Again, in a broader sense the recession of the falls 

 tends to be rythmical, not exactly periodical, but re- 

 current with alternating phases in which the crest line 



35065— 4! 



