Xlii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



brandies of the river being 187 miles. The fall from the first ca- 

 taract to the sea is only two inches in a mile *, and M. Elie de Beaumont 

 states that the bed of the river at Cairo, which is sixteen miles above 

 the head of the delta, is 1 6 feet 4 inches above the Mediterranean, 

 which gives a fall of 1*9 inch in a milef . 



According to Mr. Lyell, the deposit of the Mississippi '' consists 

 partly of sand originally formed upon or near the banks of the river 

 and its tributaries, partly of gravel, swept down the main channel, of 

 which the position has continually shifted, and partly of fine mud 

 slowly accumulated in the swamps. The further we descend the 

 river towards its mouth, the finer becomes the texture of the sedi- 

 ment :{:." A large portion of this alluvial deposit, together with the 

 fluvio-marine strata now in progress, near the mouth, is intermixed 

 with much vegetable matter, derived from the prodigious quantity of 

 drift-wood floated down every summer during the freshets. " In ex- 

 cavating at New Orleans, even at the depth of several yards below 

 the level of the sea, the soil of the delta contains innumerable trunks 

 of trees, layer above layer, some prostrate, as if drifted, others broken 

 off near the bottom, but remaining still erect, and with their roots 

 spreading out on all sides, as if in their natural position. In such 

 situations, they appeared to indicate a sinking of the ground, as the 

 trees must formerly have grown in marshes above the sea-level §." 



The east and west boundaries of the alluvial region, for about five 

 degrees of latitude above the head of the delta, consist of bluffs or 

 cliffs, from 50 to 250 feet in perpendicular height, which continue as 

 far north as the borders of Kentucky, not far below the head of the 

 plain. " They consist in great part of loam, containing land, flu- 

 viatile, and lacustrine shells, of species still inhabiting the same 

 country. These fossil shells occurring in a deposit resembling the 

 Loess of the Rhine, are associated with the bones of the mastodon, 

 elephant, tapir, mylodon, and other megatherioid animals ; also a 

 species of horse, ox, and other mammalia, most of them extinct spe- 

 cies. The loam rests at Vicksburg and other places on Eocene or 

 lower tertiary strata, which in their turn repose on cretaceous rocks." 

 As these bluffs are composed of alluvial and freshwater deposits, we may 

 suppose that they were once overflowed by the river, at a time when 

 the relative level of the Mississippi was very different. During the 

 upheaval of the country, the river may have gradually carried away 

 by denudation large portions of the loam, reducing the alluvial plain 

 to its present level, and leaving bluffs bounding the region from 

 which a large quantity of matter has been removed. Mr. Lyell ap- 

 pears to be of opinion that, in modern times, the levels of the great 

 plain of the Mississippi have been chiefly altered by movements of 

 subsidence, such as those which in 1811-12 gave rise to new lakes and 

 what is called *' the sunk country" near New Madrid in Missouri. 

 That it was subsidence rather than upheaval is, he thinks, " esta- 

 blished by the fact, that there are no protuberances of upraised 

 allmdal soil projecting above the level surface of the great plain. It 



* Newbold, Geol. Proceedings, vol. iii. p. 783. 



t Le9ons de Geologic Pratique, tome i. p. 476. 



J Principles of Geology, 7th edit, page 216. § Ibid, page 214. 



