Science, Geology and Physics 



in the outline of the fall which was mentioned nearly a century 1907 

 ago as diminishing its resemblance to a horseshoe consisted in ' ert 

 the development of an angle near the head of the curve and on 

 the side toward Goat Island. . . . Within the last thirty 

 years the recession has been especially rapid in that angle, and 

 there has developed a deep recess or notch. This appears to 

 have been occasioned by a local weakness of the limestone, pre- 

 sumably its subdivision by a belt of vertical joints. Within the 

 notch the mode of recession has been so far modified that the 

 upper layers of limestone have been removed before the lower, 

 so that at certain stages of the process the water after falling 

 from the crest has been caught by a shelf. The configuration 

 can be better understood by an examination of PL I (p. 41) 

 which is based on a photograph made in or near the year 1 886. 

 Whatever the method of erosion in the notch, it appears to be 

 superadded to the general erosion by undermining, and an 

 acceleration of the rate may plausibly be ascribed to it. 



If we regard the general method of recession by the process 

 of sapping or undermining as normal, and the influence of joint 

 systems as exceptional and temporary, the rate of recession com- 

 puted for the period from 1842 to 1875 should be accepted as 

 normal and the best available for use in geologic computations; 

 but this involves the assumption that the limestone ledge was not 

 affected in other parts of the gorge by belts of weakness similar 

 to the one which has been exposed during the last few decades. 

 It seems to me better, on the whole, to assume that the limestone 

 eroded between 1842 and 1905 is fairly representative, so far 

 as strength is concerned, of all that portion of the limestone ledge 

 in which the cataract has done its work. 



The maps of 1842 and 1905 represent the earliest and latest 

 surveys, but do not include quite all the data worthy of consid- 

 eration in this connection. A sketch by Basil Hall, made with 

 the aid of a camera lucida, in 1827, has a claim for accuracy 

 by no means to be disregarded. In the use of the camera lucida 



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