RATE OF RECESSION 443 



920 feet, exclusive of islands, the rate of 0.45 foot observed since 1842 

 should be reduced to 0.35 foot if it be compared with the work of the 

 main cataract. At the time of my investigations, engineers generally 

 conceded to the smaller falls 20 per cent of all the water. In opposition, 

 I reduced this to 6 or 7 per cent. It has since been found by the U. S. 

 Lake Survey to be scarcely 5 per cent; so that my results show only a 

 practically negligible error. With a proportional efficiency in recession 

 (as the mean rate at the Canadian Falls is 4.2 feet a year), this would 

 give a theoretical rate of 0.21 foot for the smaller falls; but as this 

 amount of water is acting on only 920 feet of rock face in place of 1,200 

 feet, for comparative purposes the rate should be raised to 0.27 foot a 

 year, or somewhat greater than was assumed by me (0.20 foot) in "Evo- 

 lution of the Falls of Magara." 



Again, those calculations are within limits of error, for the falls have 

 decreased their height by 50 or 60 feet since the separation of the two 

 cataracts,^ which has favored the accumulation of talus at the foot of the 

 American Falls; but Peter Kalm, the naturalist, most positively asserts 

 that their height was 137 feet^ (French measure), which equals 145 

 English feet. The reduction in the height of the falls was due to the 

 falling of the gorge walls into the channel at Whirlpool Eapids. Kalm's 

 statement being accepted as correct, it would seem that the Whirl- 

 pool Eapids have reduced their height by 22 feet since the year 1750. 

 The lately reduced height of the falls, as mentioned, was finally estab- 

 lished from the soundings under the Canadian Falls. 



On page 22, in his bulletin on the recession of the falls,"^ Doctor Gilbert 

 suggests that the rate of recession of the American Falls may be 0.32 foot 

 a year, which is near that deduced by me (0.27 foot) as the mean since 

 the separation of the two cataracts. However, these figures are not of 

 value in investigating the age of the falls, as they are not derived from 

 direct measurement, but they are of importance in showing that there is 

 no apparent discrepancy between the relative work of the two falls, and 

 also that any omission of this question did not derogate from the general 

 conclusions as to the age of the Falls of Magara based on work of the 

 main cataract. 



Eelative Efficiency of the two Falls of !N"iagara 



This is the other problem oijiitted from my monograph. In the two 

 cases the rock formations are nearly identical, bnt the immediate results 

 are very different. The main cataract, falling 158 feet to the cauldron 



8 Op. clt, p. 3B7. 

 « Op. clt., p. 435. 

 ' Bulletin of the U. S. Geological Survey, no. 306, 1907. 



