THE MISSISSIPPI DELTA 239 



called the " blue delta clay " by Humphreys and Abbott ; it will here be 

 called the Port Hudson Clay, because it is entirely independent of the 

 modern delta formation built up by the river. 



In this stratum, when exposed in-shore or in shallow water, there 

 frequently appear stifmps of the deciduous cypress, suggesting that in 

 former geological times a cypress swamp extended out gulfward, perhaps 

 to the deep-water line at the edge of the continental shelf.^ Beneath this 

 Port Hudson clay stratum lie formations materially different, and of 

 such a character, both physical and biological, as clearly proves them to 

 be not river alluvium, but of marine, brackish and paludal origin. But 

 these formations, as well as the Port Hudson Clay, have nothing to do 

 with the present problems of the delta, beyond serving as the floor on 

 which it is built forward. The depth of the sands and silts of the true 

 delta is practically from thirty to forty feet, and rarely reaches above 

 sixty feet. That so great a river should show so small a depth of 

 alluvium, when compared with such rivers as the Nile, Ganges, Hoangho 

 and others, at first appears incredible; but it becomes intelligible when 

 considered in connection with the existence of the underlying Port 

 Hudson clay stratum, and the extraordinarily rapid extension of the 

 mouth of the river towards the Gulf; the advance of the bar at the 

 mouth of the Southwest Pass being, at the time mentioned, about 340 

 feet per annum. That this advance, however, is not made by the usual 

 process of delta formation, is clearly shown on the accompanying map 

 of the mouths of the Mississippi, and the " normal " delta of the Volga. 



Continental Shelf 



As is well known, a continental shelf, covered by a comparatively 

 shallow depth of water, runs out for about thirty miles beyond the 

 present mouths of the Mississippi Elver, then breaks ofE into the deep 

 waters of the Gulf. The original surface stratum of this shelf is the 

 Port Hudson clay (the "blue delta clay" of Humphreys and Abbott) ; 

 but it is now coming to be gradually covered with the delta deposits of 

 river sediment; and it would be natural to connect the shallow-lying 

 shelf with the unusually rapid advance of the river mouths. 



It is not easy to see at first sight why even the existence of the Port 

 Hudson clay stratum should interfere with the ordinary, merely con- 



* In view of the many phenomena indicating that the present course of the 

 Mississippi Eiver is comparatively young, and that in times not far remote its 

 waters flowed toward the Arctic Ocean, as contended by Professor G. H. Tight, 

 such a condition of things would simply indicate a temporary cessation of an 

 oscillation which, taking into consideration the deep submerged channels of 

 western Louisiana and the present elevation of the Loess hills of Mississippi and 

 Louisiana above the level of the river, as discussed by me {Am. Journ. Sc, Vol. 

 48, November, 1869, p. 335) would amount to more than 800 feet. 



