270 Mr. A. Tylor on Changes of the Sea-Level. 



govern the formation of alluvial plains along the lower parts of 

 all river-courses. 



The diagram (fig. 8) represents a section of 600 miles of North 

 America, through the alluvial plains and delta of the Mississippi*, 

 together with a section of the Gulf of Mexico, from a point 100 

 miles east of the Balize to the continent of South America. The 

 sea-bottom is marked from the soundings on the Admiralty 

 Chart, and the depth of the Mississippi and its fluviatile deposit 

 are inserted from statistics collected by Sir C. Lyellf. 



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 modem 

 alluvial formation between the head of the delta and the junction 

 of the Ohio J. 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 2\ 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 of high tides, but it rises gradually as it passes inland, 

 and at New Orleans is nearly 10 feet above the sea-level. 



A boring near LakePontchartain, 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 80 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 



* For a most valuable detailed description of the physical geography, 

 &c. of the Mississippi and Ohio valley, see Mr. C. EUet's paper, Smith- 

 sonian Contributions, vol. ii. 1851. 



t See note, p. 262. 



X Lyell*8 Second Visit to the United States, 1849, vol.ii. pp. 146-162, 

 155, 169, 194, 195, 203, 243, &c. 



