142 ANCIENT SUBSIDENCE OF DELTA. [CHAP. XXX. 



the exact height above the level of the sea, of the fossil cypress 

 swamp at Port Hudson, I presume it is less than thirty feet ; and 

 in order to explain the superposition of 150 feet of fresh-water 

 sediment, we must imagine the gradual subsidence of fluviatile 

 strata to a depth far below the level of the sea, followed by an 

 upward movement to as great an amount. The depression must 

 have taken place so slowly as to allow the river to raise the 

 surface by sedimentary deposition continually, and never permit 

 the sea to encroach and cover the area. Jt is quite conceivable, 

 for example, that the present delta and alluvial plain should sink 

 150 feet without the salt water coming up even to New Orleans, 

 provided the land went down only a few feet or inches in a cen 

 tury, and provided the ground was raised vertically to the same 

 amount by fluviatiie mud, sand, or vegetable matter. But if the 

 land should go down even ten or twelve feet at once, the whole 

 delta would be submerged beneath the sea. Were the downward 

 movement here supposed to be followed by an upheaval to the 

 extent of about 150 feet, and should the river then cut a channel 

 through the upraised mass, we might expect to see the modern 

 formation exhibit appearances similar to those of high antiquity 

 above described at Port Hudson. 



I shall endeavor, in the sequel, to show that oscillations of 

 level, like those here assumed to account for the phenomena at 

 Port Hudson, will explain other appearances, observable, not 

 only in cliffs bounding the valley of the Mississippi, but in ancient 

 alluvial terraces bordering the Ohio, and other tributaries of the 

 great river. 



