250 THE HISTORY OF THE NIAGARA RIVER. 



Plate VII exhibits the outline of the crest of the falls, together with 

 the brink of the cliff in the vicinity of the falls, as determiued by Mr. 

 Woodward in 1886, and also shows a part of the same outliue as deter- 

 mined by Professor Hall 44 years earlier.* If both were precise, the 

 area inclnded between the two lines would exactly represent the reces- 

 sion of the Horseshoe and American falls in 44 years, and the retreat 

 of the cliff face at Goat Island in the same time. I regret to say that 

 there is internal evidence pointing to some defect in one or both sur- 

 veys, for there are some points at which the Woodward outline projects 

 farther towards the gorge than the Hall outline, and yet we can not 

 believe that any additions have been made to the face of the cliff. 

 Nevertheless, a critical study, not merely of these bare lines on the 

 chart, but also of the fuller data in the surveyors' notes, leads to the 

 belief that the rate of recession in the central part of the Horseshoe 

 Fall is approximately determined, and that it is somewhere between 4 

 and 6 feet per annum. The amount fallen away at the sides of the 

 Horseshoe is not well determined, but this is of less importance, for 

 such falling away affects the width of the gorge rather than its length, 

 and it is the length with which we are concerned. 



The surveys likewise fail to afford any valuable estimate of the rate 

 of retreat of the American Fall, merely telling us that its rate is far less 

 than that ot the Horseshoe — a result that might be reached independ- 

 ently by going back in imagination to the time when the two falls were 

 together at the foot of Goat Island, and considering how much greater 

 is the distance through which the Horseshoe Fall has since retreated. 

 The rate of retreat of the central portion of the Horseshoe is the rate 

 at which the gorge grows longer. 



Now if we were to divide the entire length of the gorge by the space 

 through which the Horseshoe Fall retreats in a year, we might regard 

 the resulting quotient as expressing the number of years that the falls 

 have been occupied with their work. This is precisely the procedure 

 by which the majority of time estimates have been deduced, but in my 

 judgment it is not defensible. It implies that the rate of retrogression 

 has been uniform, or, more precisely, that the present rate of retrogres- 

 sion does not differ from the average rate, and this implication is open 

 to serious question. I conceive that future progress in the discussion 

 of the time problem will consist chiefly in determining in what ways 

 the conditions or circumstances that affect the rate of retrogression 

 have varied in past time. In order to discuss intelligently these condi- 

 tions, it is necessary to understand just what is the process by which 

 the river increases the length of its gorge. 



There can be no question that the cataract is the efficient engine, 

 but what kind of an engine is it? What is the principle on which it 

 works I 



* The south side of this chart is placed uppermost (in violation of the conventional 

 rule) 80 that it may accord with the bird's-eye views. 



