(218 A. Tylor on Changes of the Sea-Level effected by 
It will be seen that the level of the water in the Mississippi, 
near its junction with the Ohio, nearly 600 miles from the Gulf 
of Mexico, is 275 feet above that of the sea. The slope of the 
alluvial plains through which the river winds will therefore be 
less than 1 foot in 10,000. 
The hills bordering the valley of the Mississippi are cut through 
in several places by the river, thereby exposing good sections of 
their component strata, consisting of alluvial deposits thought to 
be much more ancient than those we are about to consider. 
An area of 16,000 square miles is occupied by the more modern 
alluvial formation between the head of the delta and the junction 
of the Ohio.* It is supposed to be, in the average, 264 feet deep, 
and is from 30 to 80 miles wide. The true delta extends over 
14,000 square miles, occupying a frontage of 24 degrees on the 
coast-line of the Gulf of Mexico, and extends 180 miles inland. 
At its southern extremity its surface is hardly above the level © 
high tides, but it rises gradually as it passes inland, and at New 
Orleans is nearly 10 feet above the sea-leve 
A boring near Lake Pontchartain, of 600 feet, failed to penetrate 
the modern alluvium; and wherever excavations are made, the 
remains of trees are frequently found, apparently in the places 
where they grew, but now far below the sea-level. Sir Charles 
Lyell computes its average depth at 528 feet, and consequently 
nearly the whole of this modern deposit is below the sea-level, 
yet is supposed not to contain marine remains. The fall of the 
Mississippi during a course of 600 miles is shown by fig. 8; the 
depth of the channel varies from 81) to 200 feet until it approaches 
the Balize, where it shallows to 16 feet. The rise of the tide at 
this point is only 2 feet. The depth of the alluvial deposit be- 
low the river-channel is also indicated, together with the surface 
of the more ancient formation upon which the Mississipp! M4 
formed this great alluvial deposit, the bottom of which is D0 
more than 500 feet below the present sea-level. ee’ 
Mr. Charles Ellet, Jun., ina Report to the American Secretaly 
of War, January 29, 1851, communicates the information from 
which the diagrams figs. 1 and 2 are constructed. See p- 4% 
The theory of Mr. 'C. Ellet is, that the velocity of the stratum 
of fresh water (fig. 1) is communicated entirely to the underly 
stratum, composed of salt water, partially to the next stratum © 
but not at all to stratum 4, which is stationary : stratum 5 1s also 
marine, but it flows in an opposite direction to the rest, 40 id 
stores the salt water which is carried away by the friction of te 
upper stratum, No. 1, against the suface of No. 2. . 1 
It is supposed that the rapid increase of deposit at the bar, fig. é 
arises from stratum No. 5 carrying mud to that point, where * 
* Lyell’s Second Visit to the United States, 1849, vol. ii, pp. 146-152, 155, !” 
194,195, 208, 243, &e. eS ae a 
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