Niagara Falls 



1907 of data can be weighed only by nonmathematical methods, and 

 Gilbert ^ resu it f their study is an opinion rather than a decision. The 



general tenor of the evidence, including the five surveys and the 

 Basil Hall sketch, leaves no question that the annual rate of 

 recession has been about four or five feet. If full authority be 

 ascribed to the map of 1 842, the estimated annual rate of reces- 

 sion is 5.3 feet. If full authority be ascribed to the tangent line 

 based on the sketch of 1 827, the estimated rate is about one foot 

 less. It is my opinion that the map affords the better record. 

 Giving to it the greater weight and to the tangent a smaller 

 weight, I think the best practicable estimate of the rate is 

 between 5.3 and 4.2 feet, but nearer to the former; and I select 

 five feet partly because a statement in even feet avoids the impli- 

 cation of high precision which might be suggested by a decimal. 

 As an estimate of the average rate of recession during the period 

 of definite observation, I think this can not be in error more 

 than one foot. 



Summary and Conclusion. 



The data for computing the rate of recession of Niagara Falls 

 include surveys of the crest line made in 1842, 1875, 1886, 

 1890 and 1905, and camera lucida sketches made in 1827. 

 During the period covered by these data the local conditions 

 affecting the rate of recession have not differed to an important 

 extent from the natural conditions. The present and prospective 

 diversions of water for economic uses interfere with the course 

 of nature and may be expected to modify the rate of recession. 

 The natural rate of recession of the Horseshoe Fall is desired by 

 geologists in connection with estimates of the age of the river. 

 The geologic bearing of a rate modified by human agency is less 

 direct. The rate of recession of the American Fall is of interest 

 to geologists because somewhat representative of the river's 

 activity in gorge making when the volume of water was much less. 



The rate of recession of the Horseshoe Fall, or the rate of 

 lengthening of the Niagara gorge, during the sixty-three years 



668 



