1854.] 



ON CHANGES OF THE SEA LEVEL. 



It will be seeu that the level of the water in the Mississippi, 

 near its junction with the Ohio, nearly 600 miles from the 

 Grulf of Mexico, is 275 feet above that of the sea. The slope 

 of the alluvial plains through which the river winds will there- 

 fore be loss than 1 foot in 10,000. 



The hills bordering the valley of the Mississippi are cut 

 through in several places by the rivei', thereby exposing good 

 sections of their component strata, consisting of alluvial de- 

 posits 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 aver- 

 age, 264 feet deep, and is from 30 to 80 miles wide. The 

 true delta extends over 14,000 square miles, occupying a fron- 

 tage of 2;t degrees on the coast-line of the Gulf of Mexico, and 

 extends 180 miles inland. At its southern extremity its sur- 

 face is hardly above the level of high tides, but it rises gradu- 

 ally as it passes inland, and at New Orleans is nearly 10 feet 

 above the sea-level. 



A boring near Lake Pontchartain, of 600 feet, failed to pen- 

 etrate 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 ia 

 shown by fig. 1 ; the depth of the channel varies from SO 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 below the river-channel is also 

 indicated, together with the surface of the more ancient for- 

 mation upon which the Mississippi has formed this great allu- 

 vial deposit, the bottom of which is now more than 500 feet 

 below the present sea-level. 



Mr. Charles Ellet, Jun., in a Eeport to the American Sec- 

 retary of War, January 29, 1851, communicates the informa- 

 tion from which diagrams are constructed. (See page 57.) 



The theory of Mr. C. Ellet is, that the velocity of the stra- 

 tum of fresh water is communicated entirely to the un- 

 derlying stratum, composed of salt water, partially to the next 

 stratum 3, but not at all to stratum 4, which is stationary : 

 stratum 5 is also marine, but it flows in an opposite direction 

 to the rest, and restores the salt water which is carried away 

 by the friction of the upper stratum. No. 1, against the sur- 

 face of No. 2. 



It is supposed that the rapid increase of deposit at the bar, 

 arises from stratum No. 5 carrying mud to that point, 

 where its velocity is partially neutralized by impinging against 

 stratum No. 1. 



From the following particulars of the deltas of the Ganges 

 and 1*0, it would appear that they are similarly situated 

 to the Mississippi. " An Artesian well at Fort William 

 near Calcutta, in the year 1835, displayed at a depth of 50 feet 

 a deposit of peat with a red-coloured wood similar to that now 

 living. At 120 feet clay and .sand with pebbles were met with. 

 At the depth of 350 feet a frgshwater tortoise and part of the 

 humerus of a ruminant were found. At 380 feet, clay with 



* Lycll s Second Visit to tlio United Stutcs, 1849, vol. ii, pp. 146- 

 l-')2, l"')5, 100, 104, lO.-j, 203, 243, &c. 



lacustrine shells was incumbent upon what appeared to be 

 another dirt^bedor stratum of decayed wood. At 400 feet they 

 reached sand and shingle."* 



In the delta of tuc Po, a well bored 400 feet failed to pene- 

 trate the modern alluvial deposits ; very near the bottom it 

 pierced beds of peat, similar to those now forming. The 

 coarser particles of mud which have alreadj' passed the mouths 

 of rivers may contribute to the marine or fluvio-marine deposits 

 forming outside deltas ; but this can only be to a limited ex- 

 tent, as the great bulk of the mud is far too fine to settle near 

 the coast. Little material could be obtained from cliffs along 

 the sea coasts, but we have information of marine currents 

 specially bringing sand and mud from other parts of the sea- 

 bottom to the neighborhood of deltas. (See Mr. Ellet's ob- 

 servations.) 



For these reasons, if the further examination of the deltas of 

 the Mississippi and other rivers should lead to the discovery of 

 some recent marine or fluvio-marine strata, it may turn out 

 that such deposits have been more rapidly accumulated than 

 the purely fluviatile beds with which they may be associated. 

 In estimating the age of deltas, allowance, however, ought to 

 be made for such contingences, and also for their organic 

 contents. 



Let us now turn to fig. 2, which exhibits Sir Charles Lyell's 

 transverse section of the channel and plains of the Mississippi, 

 and at all points throughout a course of several hundred miles. 

 The dotted lines are introduced to show the variation of the 

 water-level in the wet and dry seasons : b, b represents the ar- 

 tificial levee; d d, the banks and plains ; m, m the swamps of 

 the Mississippi. " The banksf are higher than the bottom of 

 the swamps, because when the river ovei-flows, the coarser part 

 of the sediment is deposited on the banks, where the speed of 

 the current is first checked" (Lyell). The channel, however, 

 is so wide and deep, that even if there were no artificial banks 

 to prevent floods, the river would carry into the Gulf of 3Iex- 

 ico the principal mass of the mud it had received with the 

 water of its tributaries ; for it is only for a short time in the 

 year that the level of water in the river is above that of the 

 adjoining plains. The swamps and the numerous lakes formed 

 by deserted river-bends communicate at all times of the year 

 with the main stream. In these places mud could be con- 

 stantly deposited mingled with the remains of the vegetation 

 which grows luxuriantly in the swamps. The only supply of 

 inorganic matter for rising the level of the vast plains throush 

 which the river winds for hundreds of miles, must be the mud 

 deposited upon them during the periodical floods. These are 

 very much prevented by the artificial levee; but when they 

 do occur, their force is augmented by the water being arti- 

 ficially dammed up. 



" I have seen, .says an eye-witness, when the banks of the 

 Mississippi burst, the water rush through at the rate of ten miles 

 an hour, sucking in fiat boats and carrying them over a wat- 

 ery waste into a dense swamp forest " (Lyell). It would ap- 

 pear that the Mississippi diSers in size and proportion more 

 than in other respects from our rivers. For instance, when 

 floods occur upon our own alluvial plains, they arc most con- 

 spicious at a distance from the stream which caused them, in- 

 dicating that the parts of the plains nearest the banks are 



* Lyell, he. cit. p. 248 ; and Principles, p. 207-270. 



\ There is a similnr section of the Nile and its banks published in 

 the fonrtli volume of the Quarterly .Tournal of the Geological Society, 

 p. 344, but comnuuiicatcd by Lieut. Kcwbold jn 1842. 



