CHAP. XI. SHELLS OF THE NATCHEZ DEPOSIT. 201 



its eastern side by a table-land, d e, about two hundred feet 

 higher than the river, and extending twelve miles eastward 

 with a gentle upward slope. This elevated platform ends 

 abruptly at d, in a line of perpendicular cliffs or bluffs, the 

 base of which is continually undermined by the great river. 



The table-land, d e, consists at Vicksburg, through which 

 the annexed section, fig. 26, passes, of loam, overlying the 

 tertiary strata, //. Between the loam and the tertiary form- 

 ation there is usually a deposit of stratified sand and 

 gravel, containing large fragments of silicified corals and 

 the wreck of older paleozoic rocks. The age of this inter- 

 vening drift, which is one hundred and forty feet thick at 

 Natchez, has not yet been determined; but it may possibly 

 belong to the glacial period. Natchez is about eighty miles 

 in a straight line south of Vicksburg, on the same left bank 

 of the Mississippi. Here there is a bluff, the upper sixty 

 feet of which consists of a continuous portion of the same 

 calcareous loam as at Vicksburg, equally resembling the 

 Rhenish loess in mineral character and in being sometimes 

 barren of fossils, sometimes so full of them that bleached 

 land-shells stand out conspicuously in relief in the vertical 

 and Aveathered face of cliffs which form the banks of streams, 

 everywhere intersecting the loam. 



So numerous are the shells that I was able to collect at 

 Natchez, in a few hours, in 1846, no less than twent}'- species 

 of the genera Helix, Helicina, Pupa, Cyclostoma, Achatina, 

 and Succinea, all identical with shells now living in the same 

 country ; and in one place I observed (as happens also occa- 

 sionally in the valley of the Rhine) a passage of the loam 

 with land-shells into an underlying marly deposit of sub- 

 aqueous origin, in which shells of the genera Limnea, 

 Planorbis, Paludina, Physa, and Cyclas, were imbedded, 

 also consisting of recent American species. Such deposits, 

 more distinctly stratified than the loam containing land- 



