THE FALLS OF THE MISSISSIPPI. 475 



These few reflections as to the falls and gorge of Niagara, fully- 

 demonstrated by forces now in active operation, we shall apply to the 

 Mississippi. Here also a mighty water-way has been cut out by 

 erosion, a fact which is universally conceded, but no definite expla- 

 nation of the process has heretofore, so far as we have been able to 

 learn, been advanced. It remained for a geology-reading inventor by 

 the name of Robert Bates to suggest a theory which, illuminated with 

 what little investigation we have been able to give it, promises to offer 

 a solution of the question, or to assist in its solution. The theory 

 briefly is, that the erosion was accomplished by means of a mighty 

 cataract which began far down the river near its original mouth, and 

 by gradual retrocession dug out the valley-like gorge which is so 

 marked a feature in the upper part of its course, and left the high 

 bluff walls on either hand, at the same time depositing heavy beds of 

 sand at the bottom of the canon, the product of the erosion above, 

 and that St. Anthony Falls are the ever decreasing and receding rem- 

 nants of the once most stupendous cataract the world ever saw, having 

 a perpendicular descent of perhaps six hundred feet. 



Stretching over almost the entire Mississippi Valley immediately 

 overlying the Azoic rocks lie the old and extensive beds of the Pots- 

 dam sandstone, a formation of great thickness composed of shales and 

 friable sandstones. 



From the Wisconsin River to the Falls of St. Anthony the forma- 

 tions through which the Mississippi has cut its way are first, the St. 

 Lawrence, or, as Owen has termed it, lower magnesian limestone, 

 very analogous to the Niagara formation in density and durability. 



This stratum is from one hundred and fifty to two hundred and 

 twenty-five feet in thickness, and lies in a nearly horizontal position, 

 dipping somewhat to the west, but not to any great extent between 

 the Wisconsin River and St. Anthony Falls. Second, underlying this 

 and forming a part of the Potsdam group is the St. Croix sandstone, 

 with perhaps other sand-rock and shales local in extent. A section of 

 the bluff in Houston County, Minnesota, gives the St. Lawrence 

 crowning it a thickness of nearly two hundred feet, with three hundred 

 and twenty feet of sand-rock and shale beneath. 



Another measurement of a bluff in the town of Richmond, Winona 

 County, indicates a little over one hundred and ten feet of limestone 

 overlying four hundred feet of St. Croix sandstone. Other measure- 

 ments have been made in different localities, but without doubt these 

 already given indicate the general positions and relative thickness of 

 the different strata. The conditions, it will here be observed, are sim- 

 ilar to those existing at Niagara, viz., a hard limestone superimposing 

 a soft sandstone and shale deposit. 



These bluff walls rise on either hand to a height of from three hun- 

 dred to five hundred and fifty feet above the water-level of the river, 

 and have been laterally furrowed and eroded by streams flowing from 



