476 THE POPULAR SCIENCE MONTHLY. 



the adjacent country, and in a much greater degree by glacial action, 

 as it is a well-established fact that the river had cut its way (except for 

 a short distance below St. Anthony) prior to the glacial epoch. On 

 account of this lateral furrowing the bluff walls present a broken and 

 serrated appearance, but this, when rightly considered, does not in the 

 least militate against the correctness of the cataract theory. The 

 valley gorge, which is from two to six miles, is at present somewhat 

 wider at the top where the cliffs appear, and where wind and frost 

 have been free to act, than the water originally cut it, and the debris 

 falling below has formed a talus which, increased and modified by 

 glacial action, has to a considerable extent effaced the wall-like ap- 

 pearance which is such a marked feature of the comparatively freshly- 

 cut canon of Niagara. 



Another fact which has hitherto received no satisfactory explana- 

 tion is the deep accumulation of sand in the valley-bottom. There 

 are no data sufficient to determine the depth of this deposit, but as the 

 great river nowhere flows upon a rock-bed, but everywhere, except in 

 its extreme northern section, has a sand bottom ; and, as cities and 

 villages are built within the bluffs on the compact accumulations at 

 the sides of the present channel, we conclude that it must have a 

 depth of several hundred feet. 



If the gorge had been chiseled out by a process of gradual wear 

 (which would have been the case if the strata had been of uniform re- 

 sisting power), then the river should flow upon a rock-bed, and not 

 upon 6and, for the latter would, as it now does, protect the underlying 

 strata from all wear. Upon the cataract hypothesis this peculiar con- 

 dition can be met with an easy and satisfactory explanation. The 

 descent of the Mississippi is very gradual. Directly at the base of 

 the falls, wherever they may have originated, the sand-rock would 

 be cut down to a depth determined by the comparative hardness of 

 the rocks and the volume of water. For a short distance below the 

 descent the rock would be swept clean of sand and debris, except, per- 

 haps, the large limestone chunks fallen from above, but as the stream 

 came to flow more evenly below the falls, sand from the erosion above 

 would be deposited at the bottom. Two other existing conditions 

 would assist in the deposit : first, the very slight fall of the river ; and, 

 second, the detached masses of limestone broken from the crest of the 

 falls would help to collect and retain the sediment, and thus the accu- 

 mulation would begin. All these facts, taken in connection with the 

 further fact that no bluffs appear above St. Anthony Falls, while they 

 are continuous below, except where broken by the lateral erosion, 

 have a tendency to establish the theory advanced. Just how far to 

 the south the requisite conditions of stratification exist, we have not 

 as yet ascertained, but they probably exist wherever the limestone- 

 capped bluffs bound the river. If the limestone formation is want- 

 ing at any place, there rapids would have taken the place of falls, 



