Page:Popular Science Monthly Volume 31.djvu/491

Rh 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 explanation 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 remnants 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 Potsdam sandstone, a formation of great thickness composed of shales and friable sandstones.

From the Wisconsin River to the Falls of St. Anthony the formations 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 measurements 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 similar 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 hundred to five hundred and fifty feet above the water-level of the river, and have been laterally furrowed and eroded by streams flowing from