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 for 

 mation there is usually a deposit of stratified sand and 

 gravel, containing large fragments of silicified corals and 

 the wreck of older palaeozoic 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 Ehenish 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 weathered 

 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 twenty species 

 of the genera Helix, Helicina, Pupa, Cydostoma, 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 Ehine) 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 embedded, 

 also consisting of recent American species. Such deposits, 

 more distinctly stratified than the loam . containing land- 



