210 T.. A. Conrad on the Bocene Formation 
Arr, XI[X.—Eocene Formation of the a Hills, §¢c., Mis- 
ateaiyips 3 by T. A. Con 
Tue elevated bluffs on the Mississippi above New Orleans, on 
which stand the towns of Natchez, Rodney, Grand Gulf, and 
Vicksburg have a similar geological origin, and these interesting 
hills present sections of two formations or depositions widely dif- 
ferent in age, as well as in the phenomena presented to our notice 
and in the causes which produced them. The lower strata are 
wholly of marine origin, and.as I stated in a former publication, 
they are members of the Hocene, admirably characterized by the 
peculiar forms. which existed in the seas of that period, The 
shells are extremely abundant, and as in all other localities of the | 
Union which have been explored, the line of demarcation be- 
tween this formation and the Miocene is as strongly marked as a 
total diversity of species can make it.. My: investigations during 
a late tour in Mississippi, have been chiefly confined to the vi- 
cinity of Vicksburg. ‘The hills here rise steeply from the Missis- 
sippi river and are some miles in extent., They have been wash- 
ed into frequent and sometimes very profound ravines which cut 
through and expose the Eocene strata, and the sides of the hills 
ravines are whitened ‘in many places by the shells which 
have been washed out of the ferruginous marl ‘or fossiliferous 
mixture of sand and clay. The strata appear to be nearly hori- 
zontal and. the greatest elevation about sixty feet above the ordi- — 
nary high-water level. ‘The lowest stratum exposed is a bluish 
compact limestone, which is quarried for the purpose of paving 
the streets of Vicksburg. It is full of shells and casts of shells 
of such species as are common in the marls above. . One of the , 
most abundant bivalves is Pecten Poulsoni, (Morton,) a species 
Oceurring in the white limestone near Claiborne, Alabama. A 
very thin wafer-shaped Nummulite, described by Dr. Morton, is 
common in the limestone as well as in all the strata above, and 
connects the formation of Vicksburg with the Eocene white lime- 
stone of St. Stephen’s, Alabama. A new species of Pinna is 
one of the most striking fossils of the limestone at Vicksburg, 
and which is rare above it. Over this rock are various strata of 
sandy marl, sometimes indurated and ferruginous, clay, and clay 
nd sant mixed, 4 all of which are very analytes dm: fossil shells: 
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