206 Geology of the Lakes and the Valley of the Mississippi. 



burg, in which the exfoliation of each successive winter is distinctly 

 marked. This exfoliation is greater in the shale below by reason of 

 its softer consistence, than in the limestone above ; consequently, un- 

 til the bottom of the basin shall gain a point above the shale — if in- 

 deed it ever shall gain it — the incumbent limestone must continue to 

 tumble in, as the supporting bed continues to be removed. But ob- 

 servation does not sustain the position that the portion of the shale 

 visible at the cataract is thinner than the portion visible at the em- 

 bouchure, though were the strata exactly horizontal, the inclination 

 of the channel would indicate it. It indicates no more, however, 

 than its own coincidence with their direction and dip ; corroborative 

 of which, is the evidence of the walls whose equality of height in 

 their whole extent, exhibits a parallelism betv/een the bottom of the 

 trough and the surface of the plain. In fact, the plain itself, being 

 without those steps that would otherwise be produced by the lateral 

 edges of the broken layers, shews conclusively that it but conforms 

 to the inclination of the surface rock. It is improbable that the wa- 

 ter is very deep in any part of the trough, or that the excavation has 

 been carried below the floor of the shale. The sea-green torrent at 

 the brink of the cataract, is drawn out in its descent, into attenuated 

 foam like rolls of cotton ; and instead of plunging deep, as a solid 

 column would do, into the sheet below, falls lightly on it in the form 

 of spray. That it does so, is not only apparent to the eye, but pro- 

 ved by the comparative tranquihty of the basin, v^^hich is such as to 

 permit a ferry boat to pass within its verge — indeed to the foot of 

 the rock which divides the chute. The writer of this notice crossed 

 so near the spray as to became wet by it without feeling any unusual 

 motion of the boat. Did a dense column rush against the bottom, or 

 even penetrate to a considerable depth, the agitation of its rebound 

 would be tremendous. The force of a faUing stream is perhaps in a 

 proportion inverse to the square of the distance. Hot water falling 

 from a stopcock through the distance of a foot into the cold water of 

 a bathino- tub, is imperceptible to the finger at half an inch below the 

 surface : falling through a distance of ten feet, it would be separated 

 by the resistance of the air into thin drops. But the force even of a 

 combined column must in any event be broken by the debris which 

 lie at the foot of the fall — at least till they are swept away by it — 

 and that they remain there, is another proof of the comparative fee- 

 bleness of the descending tide. Loose rocks of the same size are 

 unable to maintain themselves against a common flood tide on the 



