GROWTH OF THE MISSISSIPPI DELTA. 



[Ch. XIX. 



expect to pass occasionally through brackish-water strata 



formed in lagoons, and through others 



containing 



a 



purely 

 marine shells ; and if there have been oscillations in the level 



of the ground, as may well be supposed in the case of tens 

 of thousands of years, there will be alternations of fluviatile 

 beds and freshwater cypress-swamps, with clays and sands 



containing marine shells. On the whole, I am not disposed 



made 



T 



accumulation 



delta, as extravagant. 



The rate at which the river accomplishes a given amount 

 of work is no doubt nearly double what I supposed, as shown 



Humphreys 



mud 



which has 



been carried down into the gulf, is far greater than that 



my 



We 



the thickness of the alluvial deposit in the plain above the 

 delta, the superficial area of which is about equal to that of 

 the delta itself. I assumed it to average half that of the 

 delta, or 264 feet. I grounded this conclusion partly on the 

 idea that the valley had been subsiding, as a part of it sank 

 during the earthquake of New Madrid in 1811-12, and partly 

 on the fact that the Mississippi is continually shifting its 

 course in the great alluvial plain, cutting its channel to the 



depth of 100 feet, and sometimes even to 250 feet, 



6111110* up 



much 



these changes alone must, I think, have given a considerable 

 depth to the alluvial deposit, independently of the filling up 

 of the original basin of the great river, the capacity of which 

 was probably increased by repeated subsidences. 



If we ascend 



Mississipp 

 >rt Hudson 



for 



miles above New 



the river, a cliff continually undermined by the stream. This 

 cliff I examined in 1846, and the state of it had been well 



am 



At the base of it, about forty feet above the level of the gulf, 

 is a buried forest, with the stools and roots in their natural 

 position, and composed of such trees as now live in the swamps 

 of the delta and alluvial plain, the deciduous cypress being the 





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