Science, Geology and Physics 



cataract; (2) that the gorge before the falls was created by this 1907 

 process of recession, the position of the falls having originally 

 been where the mouth of the gorge now is, and (3) that it is pos- 

 sible, by sufficiently accurate observations, to determine the rate 

 at which the change is taking place. 



Associated with the idea of measuring the rate of recession 

 was that of applying it to the determination of the time con- 

 sumed by the river in the making of the gorge. By some of the 

 earlier writers the age of the gorge was obscurely connected with 

 the age of the world as estimated from Biblical data; by others 

 it was recognized as a small fraction of geologic time. With 

 the progress of knowledge of the local geologic history there was 

 increasing interest in the time estimates for the river, and the 

 various conditions affecting the estimate came to be scrutinized 

 with much care. As developed by careful study, the problem 

 proved to be complex and difficult. It came to be recognized 

 not only that the rate of recession in different parts of the gorge 

 must have varied with the height of the cataract, the temporary 

 width of the stream, and the thickness of the capping limestone, 

 which is different in different places, but also in a very important 

 way with the volume of water carried by the river, which has 

 been subject to extreme fluctuations. The influence of these 

 various conditions assumed prominence in the discussion, and 

 altho the rate of present recession came to be fairly well known, 

 opinions still differed widely as to the total period repre- 

 sented by the gorge. The age of the gorge is outside the scope 

 of the present paper, and the subject is here mentioned only to 

 show the basis of the strong interest which has been felt in the 

 determination of the present rate of recession. 



In 1841 James Hall, then geologist of the fourth district of 

 New York, undertook the preparation of an authoritative map 

 of the crest of the falls, and employed for that purpose E. L. 

 Blackwell, a civil engineer. The work was completed in the 

 autumn of 1842, at which time a series of monuments were 

 established at the principal trigonometric points. The map was 



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