336 MAN AND THE GLACIAL PERIOD. 



which are, in turn, ground to powder by the constant com- 

 motion in which they are kept, and thus the channel is 

 cleared of debris. 



Below these two main strata there is considerable 

 variation in the hardness of the rock, as shown in the 

 accompanying diagram, where 3 and 5 are hard strata 

 separated by a soft stratum. In view of this fact it seems 

 probable that, for a considerable period in the early part 

 of the recession, instead of there being simply one, there 

 was a succession of cataracts, as the water unequally wore 

 back through the harder strata, numbered 5, 3, and 1 ; 



Fig. 105. — Section of strata along the Niagara gorge from the falls to the lake : 

 1, 3, strata of "hard rock ; 2, 4, of soft rock. 



but, after having receded half the distance, these would 

 cease to be disturbing influences, and the problem is thus 

 really the simple one of the recession through the strata 

 numbered 1 and 2, which are continuous. So uniform in 

 consistency are these throughout the whole distance, that 

 the rate of recession could never have been less than it is 

 now. We come, therefore, to the question of the rapidity 

 with which the falls are now receding. 



In 1841 Sir Charles Lyell and Professor James Hall 

 (the State Geologist of New York) visited the falls together, 

 and estimated that the rate of recession could not be 

 greater than one foot a year, which would make the time 

 required about thirty-five thousand years. But Lyell 

 thought this rate was probably three times too large ; so 



