AND THE SALT D E T O 8 1 T O jN PETITE ANSE 1 SEAN P. 9 



of the top. Siuli is tlie case at New Texas Landing, for example, where the clay 

 is abundantly traversed by cypress roots, though no large stumps are visible. 

 At Proffit's Island, a few miles below Port Hudson, the Stratified Drift is seen on 

 the river for the last time, in the gravel beds of that locality, near low-water level. 

 At Walker's Landing, near Pass Manchac, several generations of cypress stimips 

 appear, one above the other, for some twelve feet above low-water level, in clay 

 similar to that of the stump stratiun at Port Hudson. They are overlaid by eight to 

 ten feet of more sandy deposits, from which a streamlet had waslu^d an abundance 

 of whitened shells of PUuiorhis, Pahidiud, Helix, Helkina — all species now living 

 in the lakes of the bottom. The cypress wood here is in a better state of preserva- 

 tion than that of the stumps at Port Hudson, thus appearing to indicate a more 

 recent origin. Little value, however, can attach to this circumstance, upon which, 

 within certain limits, differences of the matrix and position seem to exert a greater 

 influence than time. 



It will be difficult, on this account, to distinguish in every case the equivalents 

 of the Port Hudson stump stratum from similar and more recent deposits, now 

 demonstrably in course of formation in cypress swamps, which in their turn may 

 become covered with river or lake deposits. It is mainly where the superincumbent 

 strata have not been removed by denudation, or where continuity can be proven, 

 that we can readily and positively identify these equivalents. 



Even below New Orleans, clays opposing considerable resistance to denudation, 

 and differing in aspect from those of modern cypress swamps, occasionally appear 

 in the river's banks. But the main body is evidently below tide-level, forming the 

 "blue clay bottom" of the Gulf coast. A special discussion of this portion of the 

 region explored, however, is given in a separate paper.^ 



THE FIYE ISLANDS. 



The chain of five " islands" rising, partly from the sea, partly from the sur- 

 rounding marsh, has been described by Mr. Thomassy (I. c.) from ])ersoual obser- 

 vation— ^somewhat fancifully, it is true, since in all of them he thought he found 

 proofs of "powerful volcanic convulsions." But notwithstanding his preconceived 

 idea of comparing these elevations to the extinct " mudlunq)s" of the passes of the 

 Mississippi, his description is sufficiently faithful to show the general resemblance 

 of their geological structure ; so that after examining the three middle ones, 1 

 thought it superfluous to visit the first and fifth of the chain, viz., Belle Isle, and 

 MiUer's Island or Orange Grove. The former appears as a promontory west of the 

 mouth of the Atchafalaya; the latter forms the southeastern shore of Lake 

 Pcigneur; but neither of them is usually mentioned on maps. A line laid from 

 the first-mentioned point to the last touches, in succession, Cjte Blanche, Grande 

 Cote or Weeks' Island, and Petite Anse or Avery's Island, the genera, trend being 

 about northwest. (See Map.) 



' "On the Geology of the Delta, and the Mudlumps of the Passes of the Mississippi;" Amer. Journ. 

 ScL, March, April, and May, 1871. See also " Report ou the Aga of the Mississippi Delta," iu Ecp. 

 U. S. Engineer Dep't. for 1S70. 



2 May, 1872. 



