CHAP. XXX.] ANCIENT SUBSIDENCE OF DELTA. 183 



I have dwelt at some length on the geological 

 phenomena disclosed in the interesting sections of 

 these bluffs, because I agree with Bartram and Car 

 penter, that they display a series of deposits similar 

 to the modern formations of the alluvial plain and 

 delta of the Mississippi. They lead us, therefore, to 

 the important conclusion, that there have been changes 

 in the relative level of land and sea since the esta 

 blishment, in this part of the continent, of a geogra 

 phical state of things approximating to that now 

 prevailing. Then, as now, there were swamps in 

 which the deciduous cypress and other trees grew, 

 and became buried in mud, without any intermixture 

 of sand or pebbles. At that remote period, also, 

 drift wood was brought down from the upper country, 

 and enclosed in sandy strata. Although I could not 

 ascertain the exact height above the level of the sea, 

 of the fossil cypress swamp at Port Hudson, I pre 

 sume it is less than thirty feet ; and in order to 

 explain the superposition of 150 feet of freshwater 

 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. It 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 century, and provided the 



