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PROCEEDINGS OF THE 



[Jan., 1857, 



tion than in another, in the direction of dip,* for example, than in 

 one ohlique or transverse to it. We are driven to this view by the 

 apparent inadequacy of other causes to the explanation of the phe- 

 nomena, and the phrase "unequal facility of abruption" maybe 

 regarded as framed to express this result, and our ignorance of the 

 real cause of the difference, and not intended to indicate a differ- 

 ence of hardness or of texture, possessed by the rocks in different 

 directions. The mechanical actions resulting in the abruption of 

 the rock, are too complex to be referred to any such sole point of 

 difference. 



The preceding views and discussions were communicated to 

 Prof. Hall, the eminent Geologist of the New York Surveys, in 

 August last. In his reply, he regards the chief points noticed 

 above as well taken, but considers the hypothesis of unequal facili- 

 ty of abruption, (or as I then expressed it, of abrasion,) as unten- 

 able, and refers the more rapid recess of the main Fall to the 

 fourth, of the causes given above, namely, that this Fall gains 

 or increases in depth of water, by receding in the line of dip, since 

 the uppermost stratum, or bed of the Falls, sinks deeper and deeper,, 

 in the very direction in which the Falls recede; the American Fall 

 losing, at the same time, in depth, as the other gains. He also 

 says, "that the depth of water at Canadian Fall must be less than 

 fifty feet, and is probably much less than that, while on the Ameri- 

 can Fall the water is very shallow; this is possibly more than five 

 feet, but if the water were evenly distributed over the entire 

 width of the Fall, it would be less than five feet," If, then, we 

 take 40 feet as a near approximation to the depth of water at the 

 Canadian Fall, also as above 1 to 15 as the minimum ratio of recess, 

 it would then require a depth of only 2| feet a the American Fall, 

 to make the volumes proportional to the rates of recess, and surely 

 the depth cannot be less than that. 



I may not clearly apprehend the views of Prof. Hall, but it does 

 not appear to me that the dip of the strata can produce the 

 whole effect that he has assigned to it. It is probable that, when* 

 there was but a single Fall at A K, and the two streams mingled 

 their waters at the line A D, the level of the surface of the water 

 was nearly the same in both streams. If it be true that the floor 

 over which the main Fall poured its waters was, as the Fall receded 



*The direction of the dip of the strata coincides very nearly with the general 

 direction of the gorge or chasm. 



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