Science, Geology and Physics 



crest of the fall at C, the intervening space being occupied by a 1907 

 gently sloping talus of large limestone blocks, among which the Gllbert 

 water descends in a labyrinth of cascading torrents (PI. XI). 

 At the initial stage, when the American Fall was first separated, 

 the position of its crest was probably at some point (I) between 

 its present position and the outer edge of the visible talus. As 

 sketched, I is 160 feet from C, and if the total retreat of the 

 American Fall in five hundred years was 160 feet the average 

 rate of recession was 0.32 foot per annum. Allowance should 

 be made for difference in rate dependent on the gradual encroach- 

 ment of the protective talus upon the exposed cliff of shale, so 

 that during the earlier part of the period the retreat was more 

 rapid than during the later part. The indication, therefore, is 

 that the present rate of recession is considerably less than 0.32 

 foot per annum, a result in harmony with that based on the maps 

 and sketches. 



The assumptions underlying each of the estimates are factors 

 of such importance that neither result can claim a high measure 

 of precision. It appears to be safe to say that the present aver- 

 age rate of recession of the American Fall can not be so great 

 as 0.5 foot per annum, and is probably as small as 0.2 foot per 

 annum, or about one twenty-fifth of the rate of recession of the 

 Horseshoe Fall. 



The Map of 1842. 

 The detection of an important error in the outline of the 

 American Fall as mapped in 1842 tends naturally to bring into 

 question all other results of the survey of that year. Inasmuch 

 as the outline of the Horseshoe Fall as determined in 1842 is 

 one of the most important data used in the computation of the 

 rate of recession, it has been subjected to critical examination 

 and all practicable checks have been applied. 



The factors bearing on the estimate of the rate of recession 

 are not so related that rigid mathematical methods can be applied 

 to their discussion. The conflict of data and the mutual support 



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