WATERFALLS. 
381 
country, with monuments placed in situations where observations may be made, are the only 
means by which this knowledge can be gained; and when the inhabitants feel sufficient inte¬ 
rest in the operations of nature to establish and preserve such monuments, we shall be able 
to arrive at important conclusions regarding the periods which have elapsed since these 
streams began to wear their channels. Even with our present knowledge, these speculations 
are not always unprofitable ; they lead us to take into consideration numerous influences 
which ordinarily escape observation, and we often find that hasty conclusions are essentially 
modified, or often changed, by the consideration of new facts. 
The different rates of recession in waterfalls is shown when the successive rocks are of 
different degrees of hardness, producing a series of falls. This happens where the highest 
are more destructible than the lower, and by this means the upper fall outruns the others. 
The Genesee river at Rochester presents an example of this kind, where the Medina sand¬ 
stone, the rocks of the Clinton group, and the Niagara group, have each produced a distinct 
fall. This, at one period, was doubtless a single cascade ; but the upper shale wearing 
away faster than the rocks below, allowed the fall to travel rapidly southward till it came 
to the limestone surmounting the shale, where its progress was somewhat arrested. At the 
present time it seems probable that the lower fall is receding faster than the upper, which 
is thus protected. 
The following diagram illustrates the position of these falls, and the rocks over which the 
water passes: 
184. 
1, 2, 3. Upper, middle and lower falls. 
a, b. Medina sandstone. g. Shale of Niagara group. 
c, d, e, f. Shales and limestones of the Clinton group. h. Limestone of Niagara group. 
The upper fall is now upon the northern edge of the limestone, which increases in thickness 
for two miles south, being a medium of constantly augmenting resistance ; while the Medina 
sandstone and the limestone of the Clinton group, are no thicker and no more difficult to wear 
away than they have been for centuries past. Thus it is plain, that under otherwise equal 
circumstances, the lower falls will advance upon the upper, until the whole will become one. 
It will not then, however, be of the height of all these ; for the long rapid between the upper 
fall, and the present place of the lower one, will be nearly as much descent as the fall at 
present. 
