50 



But the problem is extremely complicated and involves 

 many elements that are incapable of exact determination. 

 Many assumptions have to be made and the value of the 

 results depend upon their accuracy. 



Five instrumental surveys were made between 1842 

 and 1905, with the object of determining the precise con- 

 figuration of the crest lines of the two cataracts. The 

 first was begun by E. L. Blackwell under the direction of 

 James Hall, in 1841 and finished in the autumn of 1842. 

 A series of monuments were established by this survey 

 for the use of future investigators. The second survey 

 was made by the U. S. Lake Survey in 1875 under the 

 direction of Major C. B. Comstock, the field work being done 

 by F. M. Towar; the third in 1886 for the U. S. Geological 

 Survey by R. S. Woodward; the fourth in 1890 by A. S. 

 Kibbe under the direction of John Bogart, State Engineer 

 of New York, and the fifth by the U. S. Geological Survey 

 and the State Engineer of New York, the field work 

 being done by W. Carvel Hall in 1905. A survey was also 

 made in 1906 by J. W. Spencer and one or two others have 

 been made more recently. 



Based on the five surveys mentioned, Mr. G. K. 

 Gilbert made a careful study of the rate of recession of 

 the falls in a report published in 1907. He concluded 

 that between 1842 and 1905 the main cataract had receded 

 at an average rate of about 5 feet (1-5 m.) per annum. 

 In reaching this value for the rate of recession Mr. Gilbert's 

 calculations tended to a result near 4-5 feet (1-37 m.) 

 per annum, but as he states, he chose the nearest whole 

 number as being, in all probability, as near the truth as 

 a small fraction under that number, since precise results 

 are not to be obtained with such data as are available 

 for this kind of a calculation. By this rate the time 

 taken by the cataract in making the Upper Great gorge 

 would be 2,400 or 2,500 years. He has not given an 

 estimate of the total duration of the falls, but some years 

 ago expressed himself as favouring a long time rather than 

 a short one. 



From his exhaustive studies on the rate of recession, 

 J. W. Spencer finds the rate of recession of Horsehoe fall 

 to be 4-2 feet (1-28 m.) per annum, and at the Erie stage 

 •42 (-128 m.) of a foot per annum. Dr. Spencer made 

 careful allowances for many modifying factors in making 

 his calculations for the total duration of the falls, but 



