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457 



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



That the prevailing changes of level in the delta and allu- 



Mississipp 



i 



dence, rather than the upheaval of land, appears to me 

 established by the fact, that there are no protuberances of 

 upraised alluvial soil, projecting above the level surface of the 



o-reat plain. It is 



true that the gradual elevation of that 



plain, by new accessions of matter, would tend to efface 



from 



mor 



plain westward of the Mississippi, had local upthrows of 

 alluvial strata been of repeated occurrence. 



In regard to the strata composing the lower part of the 

 o-reat delta, an observation of Darby deserves attention. In 

 the steep banks of the Atchafalaya, before alluded to, the 

 following section, says he, is observable at low water : — first 



common 



Mississippi 



ochreous earth, peculiar to Red River, under which the blue 

 clay of the Mississippi again appears ; and 

 is constant, proving, as that geographer 



angement 



rem 



some former 



periods, considerable tracts below their present point of 



union.* Such alternations 



common m 



sub- 



marine spaces situated between two converging deltas ; for, 



must 



intermed 



occupied and abandoned by the waters of each stream ; since 

 it can rarely happen that the season of highest flood will pre- 

 cisely correspond in each. In the case of the Red River and 

 Mississippi, which carry off the waters from countries placed 

 under widely distant latitudes, an exact coincidence in the 

 time of greatest inundation is very improbable. 



Antiquity of the delta and alluvial plain. — After I had 

 examined the pilot station called the Balize, near the mouth 



Mississippi 



I endeavoured to estimate the 



quantity of sedimentary matter contained in the delta and in 

 the alluvial plain, and to calculate the 



minimum 



must 



* Darby's Louisiana, p. 103. 



