Niagara Falls 



1907 that portion of the Horseshoe where the sheet of falling water is 

 Gilbert heavy enough to clear away the debris and maintain a deep pool. 



The retreat of the cliff on either side of this portion is secondary, 

 and appears to have little or no bearing on the question of the 

 rate at which the gorge is growing longer. I have therefore 

 restricted attention to the central part of the Horseshoe curve. 



As the two crest lines compared are irregular in outline, a 

 certain confusion arises if the recession of different parts is con- 

 sidered separately. At one place the recession seems to have one 

 direction, at another place to have another direction, and various 

 complications ensue when attempt is made to combine measure- 

 ments made in different directions. In view of this difficulty it 

 has appeared to me both convenient and legitimate to assume 

 some one direction as the general direction of recession and at 

 all points measure the amount of recession on lines parallel to 

 that direction. From an inspection of the crest lines as wholes 

 and in their relation to each other I have inferred such a general 

 direction of recession, and assuming it to apply to the entire 

 central tract of the Horseshoe, have drawn the system of parallel 

 lines seen in fig. 4. There are six of these lines, each extending 

 from the crest line of 1842 to that of 1905. Their interspaces, 

 according to the scale of the map, are 100 feet wide. The 

 average length of these lines represents approximately the average 

 recession of the cataract in the part where the sheet of falling 

 water is heaviest. Their lengths are, severally, 430, 292, 260, 

 276, 317 and 412 feet, giving an average length of 331 feet. 

 Their distance divided by the number of years, 63, gives as the 

 average annual recession 5.3 feet. 



The indication is that during the thirty years following 1875 

 the lengthening of the gorge went on at a somewhat faster rate 

 than during a similar period preceding that date. While it is 

 quite possible that the apparent variation in the rate is sufficiently 

 accounted for by the irregularity of the breaking away of the 

 limestone sill, it is also possible that the rate has been influenced 

 by a special condition affecting the mode of recession. A change 



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