and the Geology of the Surrounding Country. 123 



strata, would cause the disappearance of the whole beneath 

 the water, before reaching that point. 



In support of the hypothesis that streams do cut back their 

 channels, we may adduce sufficient evidence. An observation 

 of the last four or five years upon the waterfalls of Western 

 New York, has furnished positive evidence of their recession. 

 Among these may be noticed the falls on Jacock's run, near 

 Genesee, and Fall brook, a few miles further south, both of 

 which have evidently receded, from the undermining of the 

 platform over which the water is precipitated. The amount 

 during this time is very small, but sufficient to be appreciated. 

 The lower falls of Portage furnish another example of reces- 

 sion, where large masses have been removed, the channel 

 deepened, and cut backwards many feet in the space of four 

 years. 



Lateral streams flowing into ravines or river courses, furnish 

 the most palpable evidence of the excavating power of water. 

 The channel of Wolf creek, which comes into the Genesee 

 through a perpendicular wall of rock, is a good example of 

 this kind, where the evidence is conclusive that the excavat- 

 ing power is alone due to the stream. The junction of a 

 small stream with the Genesee, on the west side, below Roch- 

 ester, furnishes another example of this power. This stream 

 has cut its channel through soft shale for fifty feet or more, a 

 bed of limestone eighteen feet thick, a bed of shale of equal 

 thickness, and below this another bed of limestone nearly 

 equal to the upper one. Many more examples of a similar 

 kind might be named, on the Seneca and Cayuga lakes, and 

 their valleys continued to the south. 



A remarkable fact connected with these lateral water courses 

 is that their dimensions bear some proportion to their present 

 quantity of water. And although the actual quantity flowing 

 in any stream may seem insufficient to excavate its bed, yet I 

 believe, generally, the largest streams will be found in the 

 largest ravines, and the small lateral streams have always a 

 proportionately smaller channel. If in any case we find the 

 stream debouching into the lake, river, or valley, through a 

 chasm with nearly perpendicular sides, and little or no wider 



