Great Lakes, and the Valley of the Mississippi. 221 



passing near Bloomington, 111., and by some route yet unknown 

 reaching the trough of the Mississippi, which was then much 

 deeper than at present. 



3d. — At this period the continent must have been several 

 hundred feet higher than now, as is proved by the deeply ex- 

 cavated channels of the Columbia, Golden Gate, Mississippi, 

 Hudson, &c, which could never have been cut by the streams 

 that now occupy them, unless flowing with greater rapidity 

 and at a lower level than thev now do. 



The depth of the trough of the Hudson is not known, but it 

 is plainly a channel of erosion, now submerged and become an 

 arm of the sea. As has been before stated, this channel is 

 marked on the sea-bottom for a long distance from the coast 

 and far beyond a point where the present river could exert any 

 erosive action, and hence it is a record of a period when the 

 Atlantic coast was several hundred feet higher than now. 



The lower Mississippi bears unmistakable evidence of being 

 — if one may be permitted the paradox — a half-drowned river ; 

 that is, its old channel is deeply submerged and silted up, so 

 that the " father of waters," lifted above the walls that former* 

 ly restrained him, now wanders, lawless and ungovernable, 

 whither he will in the broad valley. 



The thickness of the delta deposits at New Orleans is vari- 

 ously reported from 1500 feet upwards, the discrepancies being 

 due to the difficulty of distinguishing the alluvial clays from 

 those of the underlying Cretaceous and Tertiary formations. 

 It is certain, however, that the bottom of the ancient channel 

 of the Mississippi has never been reached between New Or- 

 leans and Cairo; the instances cited by Humphreys and Abbot 

 in their splendid study of this river being but repetitions oi 

 the phenomena exhibited at the falls of the Ohio — the river 

 running over one side of its ancient bed. 



The trough of the Mississippi is not dae to synclinal struc- 

 ture in the underlying rocks, but is a valley of erosion simply. 

 Ever since the elevation of the Alleghanies — i.e., the close ot 



JUNE, 1S69 - 15 Ay.t. Lyc. Nat. Hist., Vol. IX. 



