Cn. X.] PLAIN OF THE MISSISSIPPI. 121 



same rate at which its foundations sink, so that these may go down hun- 

 dreds or thousands of feet perpendicularly, and yet the sea bordering the 

 delta may always be excluded, the whole deposit continuing to be terres- 

 trial or freshwater in character. This appears to have happened in the 

 deltas both of the Po and Ganges, for recent artesian borings, penetrating 

 to the depth of 400 feet, have there show^n that fluviatile strata, with 

 shells of recent species, together with ancient surfaces of land supporting 

 turf and forests, are depressed hundreds of feet below the sea level.* 

 Should these countries be once more slowly upraised, the rivers would 

 caiTe out valleys through the horizontal and unconsolidated strata as they 

 rose, sweeping away the greater portion of them, and leaving mere frag- 

 ments in the shape of terraces skirting newly-formed alluvial plains, as 

 monuments of the former levels at which the rivers ran. Of this nature 

 are " the bluffs," or river cliffs, now bounding the valley of the Mississippi 

 throughout a large portion of its " course." The upper portions of these 

 bluffs which at Natchez and elsewhere often rise to the height of 200 feet 

 above the alluvial plain, consist of loam containing land and freshwater 

 shells of the genera Helix, Pupa,, Succinea, and Ziymnea, of the same 

 species as those now inhabiting the neighboring forests and swamps. In 

 the same loam also are found the bones of the Mastodon, Elephant, Mega- 

 lonyx, and other extinct quadrupeds.f 



I have endeavored to show that the deposits forming the delta and 

 alluvial plain of the Mississippi consist of sedimentary matter, extend- 

 ing over an area of 30,000 square miles, and known in some parts to be 

 several hundred feet deep. Although we cannot estimate correctly how 

 many years it may have required for the river to bring down from the 

 upper country so large a quantity of earthy matter — the data for such a 

 computation being as yet incomplete — we may still approximate to a 

 minimum of the time which such an operation must have taken, by as- 

 certaining experimentally the annual discharge of water by the Mississippi, 

 and the mean annual amount of solid matter contained in its waters. The 

 lowest estimate of the time required would lead us to assign a high an- 

 tiquity, amounting to many tens of thousands of years to the existing 

 delta, the origin of which is nevertheless an event of yesterday when con- 

 trasted with the terraces formed of the loam above mentioned. The ma- 

 terials of the bluffs were produced duiing the first part of a great oscilla- 

 tion of level which depressed to a depth of 200 feet a larger area than the 

 modei'n delta and plain of the Mississippi, and then restored the w^hole 

 region to its former position.^ 



Loess of the Valley of the Rhine, — A similar succession of geograph- 

 ical changes attended by the production of a fluviatile formation, singu- 

 larly resembling that which bounds the great plain of the Mississippi, 

 seems to have occurred in the hydrographical basin of the Ehine, since 



* See Principles, 8th ed. pp. 260-268, 9th ed. 257-280. 



f See Principles of Geol. 9th ed., and LyeU's Second Visit to the United States, 

 vol. ii. p. 257. 



X Lyell's Second Visit to the United States, vol. ii. chap. 34. 



