ON THE POTOMAC AND THE HUDSON. 105 



mergence of the continent, is another question. To be able to trace back with 

 clearness erosions accomplished on even the last continent, is more than I ever 

 expected to be able to do. The above facts come nearer to extending our vision 

 across another mighty chasm, and witnessing events in surface geology upon a still 

 earlier continent, than any I have ever met with. But- whether this be a problem 

 resolvable by the geologist I am in doubt. 



It is only recently that this subject of distinguishing between postdiluvian and 

 antediluvian river beds has arrested my attention. And from the number of cases 

 that have already fallen under my observation, I cannot doubt that they are quite 

 frequent. I have some other examples to which I shall, refer on a subsequent page. 



13. Gorge at Great Falls on the Potomac, tioelve miles west of Washington city. — 

 The top of these falls is 112 feet above tide water; and the water at the cataract 



descends 82 feet. The rock is a hard mica slate, whose strike is N. a little "W. by 

 the needle, and whose dip is about 70° easterly. Consequently the water has 

 acted upon the edges of the strata, and in circumstances poorly adapted to erosion. 

 Yet as you stand upon the high bank near the falls, and look to the south, you see 

 a gulf, from 60 to 65 feet high, with almost perpendicular walls of naked rock, 

 extending nearly four miles. One cannot stand there and not be satisfied that the 

 river must have worn out that gulf. Indeed, in going towards Georgetown, he will 

 see that in many other places the work of erosion has been going on. And when 

 we see the unfavorable position of the rock for being acted upon at this place, and 

 the great amount of erosion, we can hardly avoid the conviction that a greater 

 work has been done here than at Niagara; as indeed we might expect, when we 

 remember that the rock over which the Potomac flows is probably much the 

 oldest. 1 



14. Passage of the Hudson through the Highlands. — This celebrated gorge is 

 nearly twenty miles long, and is remarkable for being worn out so that its bottom 

 is below mean tide water. The hills on its sides rise in some instances as much as 

 2600 feet, and in many places the walls are very precipitous. The rock is gneiss, 

 of a kind not easily disintegrated or eroded. Nor is there any evidence of any 

 convulsive movement in the strata. 



While, therefore, this is clearly a case of erosion, it seems almost equally 

 obvious that the waters of the present river could not have done it : for they are 

 too quiet, and have so little descent that tide water extends nearly 100 miles 

 up the river beyond the Highlands : and, moreover, the low level of the bed of 

 the gorge precludes the idea of a former cataract, whose recess might have accom- 

 plished the erosion. This, therefore, was probably a work mainly performed in 

 «ome past period, when the continent was at a higher level. It was doubtless the 

 joint result of oceanic and fluviatile action : for it is too crooked to allow us to 

 impute it all to the ocean. Very probably the whole process was gone through at 

 different periods, with long intervals, it may be, of rest. There is no evidence 



1 I visited this spot in 1849, in company with Professors Henry and Guyot, Count Pourtales, and 

 Mr. Saxton ; and from these gentlemen I obtained several of the facts mentioned in the text. This 

 same gorge is given as an example of the erosion of a river in Hutton's Theory of the Earth, by Playfair. 

 14 



