32 E.. W. Hilgard on the Tertiary formations 
The Red Bluff group.—tin a late paper, above referred to, Con- 
rad proposed to distinguish the lowest fossiliferous stratum ordi- ~ 
narily visible at Vicksburg, and subsequently studied by him 
(No. 4 of my Vicksburg section, ué supra), as a separate group, 
which he considers. as cliaracterized by the occurrence of Ostrea 
Georgiana, and for which he proposes the name of Shell Bluff 
group. I have elsewhere (this Jour., July, 1866) explained my 
reasons for dissenting from Conrad as to the position between 
the Claiborne and Jackson grou , which he assigns to this new 
division. To the propriety of distinguishing it, however, as a 
sub-group of the Wickahiane age, I fully agree, though doubting 
that of giving it the name of a locality from which, as Conrad 
remarks, but one coincident fossil is known—0O. Geor orgrana— 
while another also. ee ig there—O. selleformis—in Missis- 
sippi and Alabama is confined to the Claiborne group. Ina pro- 
file of 80 feet, as occurring at "Shell Bluff, ge data like those 
extant regarding this locality, cannot fairly made a ground 
of conclusions contrary to the order Pe elaborately ob- 
sery For aught that is on record, the whole Jackson group 
may be represented between the beds in which 0. Georgiana 
and 0. selleformis respectively occur at that place, if (as seems 
probable from its non-occurrence in the Jackson group of Missis- 
sippi and Alabama) the former shell should beso restricted in 
its range as Conrad supposes. 
I believe the white limestone (No. 1 of my Vicksburg section) 
which underlies the lignite at Vicksburg, but is visible only at 
extraordinarily low stages of water, to be of the Jackson age, both 
from its stratigraphical position and the pean character of 
the specimens I have seen. But whether it is or not} there can 
be no Semnatle doubt that the usual Jackson Gan which are 
largely developed on the Yazoo above Vicksburg, underlie at 
Maire ta as well as on Pearl river and Chickasawhay, the Vicks- 
_" urg gro! 
al The Comba bed at Vicksburg is preéminently the habitat 
of a shell common to the Jackson and Vicksburg stages, but mos 
abundant in the former, viz., Meretrix Sobrina Con.; of the two 
Madrepores described by Conrad, and of Fulgoraria rong: 
all occurring, more or less, in the Vicksburg stage proper. Of 
the fossil first mentioned, I have after a freshet oak hundreds 
washed out, mingled with numerous masses of Madrepores, 
sometimes of several pounds weight, with Fulgoraria, Natica? 
Vicksburgensis, Ostrea Georgiana, etc. The bed has therefore af- 
finities both above and below, and moreover occupies precisely 
oe stratigraphical position of the bed at Red Bluff (Miss. Rept., 
135). Here the fossils are much more numerous and the affini- 
ties in both directions are therefore better expressed. Character- 
istic and abundant above all, however, is a Hees which I 
=) 
