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^HOCEEMNGS OP THE 



[Jan., 1857. 



dean Fall did at that time communicate with the main stream by 

 the outlet A D, it will follow that while the centre of the American 

 Fall receded from B or C to E, the centre of the Canada Fall re- 

 ceded in the same interval from B or C to F. From C to F is at 

 least three times the distance fromC to E,and from Bto F is about 

 four times the distance from B to E, so that the recession of the 

 Canada Falls is, on this supposition, at least from three to four times 

 *as rapid as that of the American Fall. This point is overlooked, or at 

 least omitted by Desor, so that his result drawn from the American 

 cannot be applied without modification to the Canada Fall. 



I have carefully examined the accounts of the topography and 

 phenomena- of the Falls, and of the geological structure of the pre* 

 cipice and surrounding region, given by H. D. Rogers (Amer-. 

 Jour. ScL, March, 1885, voL xxvii,* p. 326), G. E. Hayes, (ibid, 

 1839, vol. xxxv, p. 86,) James Hall (Report of Fourth Oeol. Disk, 

 of New York, p. 383), and C. Lyell (Travels in N. Amer., 1845, 

 vol. i., p. 22), and do not find that any of these writers have dis- 

 cussed this inequality of recess, or alluded to the causes which 

 have produced it.f Nor do I find that they use any specifically 

 appropriate term to express the mode in which the rocky strata 

 forming the precipice are removed, and the Falls recede. It is not 

 by the wear of the rock by attrition or erosion, by the friction of 

 the water merely, or by its friction and momentum combined. The 

 process is described by Rogers (op, cit. p. 329), by Hall (op. cit, 

 p. 385), and by Lyell (op. cit., p. 26); the softer shaly calcareous 

 strata which form the lower part of the precipice, are more easily 

 acted on by the agencies brought to bear upon it, moisture, frost, 

 impact of water in motion, &c, and give way sooner than the 

 harder limestone strata above; these latter are thus undermined, 

 and fall in masses from the action of the weight and momentum of 

 the waters above. To avoid misconception, I shall express the sum 

 or result of these actions by the term abruption. 



Having shown that the Canada Fall recedes not less than three 

 or four times as fast as the American Fall, we must next look to 



* Hall refers erroneously to vol. xxviii. 



■f The only allusion I can find, on a second examination made since writing this 



paper, is the following remark of Rogers (op. cit., p. 328). " it is manifest 



that the American Fall is no part of the receding cataract. It enters the gorge 

 laterally, having been left by the other fall at least a quarter of a mile in the rear. 

 The true width of the valley at the Falls is, therefore, no greater than its average 

 width below; as neither the American Cataract nor Goat Island contribute to 

 its breadth." The italics are Rogers'. 



