CHAP. XXXIV.] OF DELTA OF MISSISSIPPI. 253 



manner in which the Mississippi forms long bars of 

 sand, which frequently unite with some part of the 

 coast, so as to dam out the sea and form lagoons, the 

 deposition of sediment in the delta would be much 

 less considerable. A lagoon, like Lake Pontchar- 

 train, once formed, becomes a receptacle of the finest 

 mud, poured into it by an arm of the great river 

 during the flood season, and the space thus parted off 

 from the Gulf by bars of sand, is protected from the 

 action of the breakers and marine currents. 



When I inquired what might be the depth of 

 fluviatile mud in the suburbs of New Orleans, I was 

 told that, in making a railroad near Lake Pontchar- 

 train, piles were driven down sixty feet into the soft 

 mud or slush, and when a boring was made there, 600 

 feet deep, beds of gnathodon were found, but no 

 marine shells. 



The depth of the alluvium may vary in different 

 parts of the great sloping plain; for certain areas, such 

 as the &quot; sunk country,&quot; for example, west of New 

 Madrid, may have been repeatedly depressed, and 

 have been always brought up again to the same 

 superficial level, by the deposition of river mud, or 

 the growth of vegetable matter. 



The age of stumps and erect trunks of the decidu 

 ous cypress, whether living or buried, retaining their 

 natural position, at points near the present termina 

 tion of the delta, ought to be carefully examined, as 

 they might afford evidence of the minimum of time 

 which can be allowed for the gain of land on the sea. 

 Some single trunks in Louisiana are said to contain 

 from 800 to 2000 rings of annual growth, and Dr. 



