458 



ANTIQUITY OF DELTA AND ALLUVIAL PLAIN. [Ch. XIX. 



of matter. 



Mr 



v 



to be about 13,600 square British miles in extent. In the 



Messrs. Humphr 



somewhat 



The 



average depth of the fluviatile formation in this area I sup- 



more 



calculation I assumed it to be 528 feet, or T * ff of a mile. My 

 conjectures on this head were founded partly on the depth of 

 the Gulf of Mexico, between the southern point of Florida and 

 the Balize, and partly on borings 600 feet deep, in the delta 



New 



near Lake Pontchartrain, i 



bottom of the alluvial matter was said not to have been reached 



at that depth — a resul 



12 45 



) confirmed, as we shall presently see, 

 iment. For the quantity of sediment 

 ned in the water I adopted Mr. Kiddell's estimate of 

 in weight, and this does not differ materially from the 



isrs. Humphreys and Abbot after a 



Me 



measur 



contents as T ^Vi 



From the data above stated as to the thickness of the delta- 

 deposits, and the quantity of solid matter brought down 

 annually by the river, (which would amount to 3,702,758,400 

 cubic feet,) I inferred that the accumulation of the whole 



have taken 67.000 years. But in the course of 



must 



Humpl 



elusion that the quantity of water annually discharged by the 

 Mississippi into the gulf had been greatly underrated. They 

 also remarked that the river pushes along the bottom of its 

 channel even to its mouth a certain quantity of sand and gravel, 

 equal, according to them, to about T \j of the mud held in sus- 

 pension by the river, of which. I had taken no account. Allow- 

 ing, therefore, for this addition and for the larger discharge 

 of muddy water, they make the whole mass of transported 

 matter nearly double that which I had assumed ; conse- 

 quently the number of years required for the growth of the 

 whole delta would be reduced to about one half, or to about 

 33,500 years, if my former assumed data as to the probable 

 thickness of the deposit be adopted. 



But in 1854* another Artesian well was bored 



* Eeport of Survey of Mississippi Eiver, p. 101. 



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