Science, Geology and Physics 



warJs. The narrowness of the gorge near Queenston, where *8 41 

 it is just large enough to contain the rapid current of water, 

 accords well with the same hypothesis, and there is no ground 

 for suspecting that the excavation was assisted by an original 

 rent in the rocks, because there is no fissure at present in the 

 limestone at the Falls, where the moving waters alone have 

 power to cut their way backwards. 



I have already remarked that there will always be insuperable 

 difficulties in the way of estimating with precision the rate of the 

 retrogression of the Falls in former ages, because at every step 

 new strata have been successively exposed at the base of the 

 precipice. According to their softer or harder nature, the under- 

 mining process must have been accelerated or retarded. This 

 will be understood by reference to the annexed section (fig. 4), 

 where the line b, c, d, represents the present surface of the river 

 along which the Falls have receded. The strata (1,3, and 7) 

 are of soft materials; the others, (2, 4, and 8), which slightly 

 project at their termination in the escarpment, are of a more 

 compact and refractory kind. It has been necessary to exagger- 

 ate the southward dip of the strata in this diagram, which is in 

 reality so slight as to be insensible to the eye, being only, as 

 before mentioned, about twenty-five feet in a mile, the river chan- 

 nel sloping in an opposite direction at the rate of fifteen feet in 

 a mile. These two inclinations, taken together, have caused, as 

 Mr. Hall has pointed out in his Survey, a diminution of forty 

 feet in the perpendicular height of the Falls for every mile that 

 they receded southward. By reference to the section, the 

 reader will perceive that when they were situated at the Whirl- 

 pool (c), the quartzose sandstone (2), which is extremely hard, 

 was at the base of the precipice, and here the Great Cataract may 

 have remained nearly stationary for ages. 



In regard to the future retrocession of the Falls, it will be 

 perceived by the same section (fig. 4), that when they have 

 travelled back two miles, or to i, £, the massive limestone (8), 



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