bearing on theoretical Speculations. 267 



Thames over young Niagara. Meanwhile considering them as 

 coaeval, I will ask how 1 am to account for the mighty effects 

 ascribed to the Thames, within a period during which the ut- 

 most effects that can be ascribed to Niagara are the gradual 

 wearing away of the bar over which it rushes, for a distance not 

 exceeding 7 miles ; for the general range of the mountains which 

 occasions those falls extends only that distance eastwards from 

 their present site, when the hills entirely subside into the flats 

 bordering Lake Ontario, and of course the original site of the 

 falls cannot have been beyond the extreme escarpment of 

 those hills. Mr. Lyell, taking for his datum that the falls have 

 receded near 50 yards within the last 40 years, calculates that 

 they must have occupied 10,000 years in retrograding from the 

 original to the actual position, and that it will require 30,000 

 more for them to reach Lake Erie. But if both the actions 

 commenced together, must it not have completed this effect 

 ages before the Thames could have excavated even a third 

 of its present valley? Taking Mr. Lyell's own determinations, 

 I do not know a more striking instance of the comparatively 

 inconsiderable power of fluvial erosion acting under circum- 

 stances that must every way give it its maximum of intensity : 

 but I must confess my doubts whether the falls actually do 

 recede, as far as their general line is concerned, at the rate of 

 50 yards in 40 years. I suspect that some partial degrada- 

 tion of the strata has here been mistaken for the general re- 

 trogradation. My grounds of suspicion are these : The falls 

 are, as is well known, divided in the centre by an extremely 

 small islet; but from the periods of our earliest accounts it 

 should appear that this islet has occupied exactly the same 

 relative position, with regard to the falls, that it holds at the 

 present moment. The celebrated narrative of the Indian whose 

 canoe drifted against this islet, whence he was subsequently 

 so wonderfully rescued, more than a century ago, involves a 

 full description of all the particulars of this locality, and proves 

 it to have been then very nearly the same as at present. 



Cataracts indeed appear generally to have undergone sur- 

 prisingly little change from the earliest periods to which hi- 

 story extends. The cataracts, or rather rapids, of the Nile 

 above Syene, when examined by the scavans of Buonaparte's 

 expedition, agreed pretty closely in locality, features, and 

 extent, with the description given by the Grecian Father of 

 History. I have always inclined to consider the cascades of 

 Tivoli as another evidence of the slight changes effected in 

 this way during a long series of centuries. But Mr. Lyell's 

 remarks on this locality (although I cannot say they have 

 changed my opinion) in every way claim an attentive exami- 



2 M 2 nation, 



