120 HalVs Remarks on Niagara Falls 



commenced the work of excavation, cutting down the higher 

 strata, and rapidly imdermining and removing those below. 

 It is only necessary to refer to the accompanying section, in 

 order to discover what materials the river had to work upon 

 at this period. 



It is impossible that there could ever have been a perpen- 

 dicular fall of the whole height of the cliff at Lewiston, for 

 the limestone at the top, being so much thinner than at the 

 present falls, would soon be broken down by the pressure of 

 the immense body of water precipitated over its edge upon 

 the shale below. It may even be doubted whether the shale 

 would be excavated fast enough to form a perpendicular fall, 

 and it is probable that the water would be projected over a 

 declivity of the upper shale (No. 7,) to the limestone below, 

 which together with the higher layers of the sandstone would 

 form the crest of a second fall. From this again the water 

 would be precipitated as far as the sandstone, (No. 2,) where 

 a third fall would be formed. Thus, instead of a single fall 

 of three hundred and fifty feet, we should have the whole 

 height divided into three falls, at some distance from each 

 other. In consequence of the thinness of the upper limestone, 

 that fall would recede faster than either of those below it ; 

 and the middle faster than the lower one. Even under these 

 circumstances, the wearing action would go on much faster 

 than at present. Finally, however, the recession would be- 

 come less and less rapid, from the thickening of the limestone 

 above ; and from this cause, the two lower falls having the 

 same resistance to overcome as at first, would gradually ap- 

 proach the upper, till the whole became one. 



At the same time there are other circumstances to be taken 

 into consideration, and among the most important of these 

 will be the dip of the strata and the ascent of the bed of the 

 stream ; both together tending to bring the strata down to 

 the level of the water as Ave progress southward. This fact 

 has also an important bearing upon the rate of recession ; for 

 while a hard mass remains at a considerable height above 

 water, with a soft one below, the excavation of the softer one 

 and undermining of the upper hard one, tends to the recession 



