ANTIQUITY OF THE EARTH. 51 



position during the last half-century, and the small portion 

 of the great ravine, which has been eroded within the memory 

 of man is so precisely identical in physical and geological 

 character with the whole gorge of seven miles below, that 

 the river supplies an adequate cause for executing the 

 assigned task, provided we grant sufficient time for its com- 

 pletion. 



The water, after cutting through strata of limestone, fifty 

 feet thick, in the rapids, descends perpendicularly at the Palls 

 over another mass of limestone ninety feet thick, beneath 

 which lie soft shales of equal thickness. The disintegration 

 of the shales causes masses of the incumbent limestone rock 

 to fall down, and thus the cataract recedes southwards. 



By the erosion of the shales and limestone by the water, 

 the conditions being uniform, we are enabled to infer, from 

 what has taken place within a known time, the probable 

 period which this seven miles required for its accomplishment. 



Sir C. Lyell has found proofs that the river extended four 

 miles north of the Palls, having discovered beds of gravel 

 reposing on the cliffs overlying the ravine, containing flu- 

 viatile shells resting on the limestone, of the same species as 

 those now living in the adjoining water. 



Mr. Bakewell estimated the rate of erosion for forty years 

 before 1830 to be about a yard annually. Sir C. Lyell 

 states that one foot would be a much more probable conjec- 

 ture ; in which case, thirty-five thousand years would have 

 been required from the retreat of the Palls from the escarp- 

 ment of Queenston to their present site, if we could assume 

 that the retrograde movement had been uniform' throughout. 

 " This, however, could not have been the case ; as at every 

 step in the process of excavation, the height of the precipice, 

 the hardness of the materials at its base, and the quantity of 

 fallen matter to be removed, must have varied. At some 

 points it may have receded much faster than at present, at 

 others much slower; and it would be scarcely possible to 

 decide whether its average progress has been more or less 

 rapid than now." * 



In pursuing the natural and legitimate mode of interpreting 

 the past by the present, and observing the effect of similar 



* Travels in North America, vol. i. cliap. 2. 



E 2 



