Ra Nr ht Migs EY Se eee me ee Eh see Se Ee ae 
pert i ne 2) 
E. W. eigend on Conrad's division of the Hocene. 69 
In the numerous localities where I have studied the beds of 
the Jackson group, I have never found a single Orbitoides asso- 
ciated with them. The constant concomitant of the latter fossil, 
the Pecten Poulsoni, ~~ is ee from the Jackson strata, being 
replaced by P. nuper 
But if the Grisiteides canadian is no member of the Jackson, 
but on the contrary, a characteristic one of the Vicksburg group, 
then it is clear that the strata of the “Shell Bluff group’ 
Vicksburg lie above, and not below the Jackson strata. For ws 
cannot be supposed. that the latter, which occupy so extensive’ 
an area above Vicksburg (see the map accompanying my Report,) 
should suddenly come to an end, and leave no trace of a repre- 
sentative between the Shell Bluff pee the Vicksburg groups did 
it belong there. 
There is only one other locality in the state, as far as known, 
where O. Georgiana (i.e. the large air occurring at Vicks- 
burg) is found, viz: in Jasper county, Miss., where “it was col- 
lected by Prof. W. D. Moore, late of the. University of Miss. 
It there occurs again in the same outcrop with Pecten Poulson, 
Orbitoides, and a Schizaster, which is also a leading Vicksburg 
fossil ; this locality being Tikewise considerably south of the 
shell prairies of the Jackson 
As there is nothing to justify the assumption of a sudden 
termination of the strata of the latter group, which, on the con- 
trary, may be seen disappearing under those forming the transi 
tion to the Vicksburg strata, with remarkable regularity, along 
the course of both Pearl and Chickasawhay rivers, (see p. 135 
of my Report), the conclusion is inevitable that the Jaakson group 
as older than the Shell Bluff group as defined by Conrad. 
hat there may be a considerable difference in the geological 
horizons of the Jackson and Vicksburg groups proper, sufficient 
“to admit of the existence of a fauna deserving to 
into a distinet group, is proved, not only by the paucity of coin- 
cident species, (see list, ibid, p. 182), but no less by the consid- 
erable thickness of the intervening strata in eastern Mississippi, 
on the Chickasawhay river, which near Red Bluff Station 
(ibid, p. 185,) amounts to over one hundred feet. 
Here, as at Vicksburg, we have, underlying the Orbitoides, 
marls and limestones, a stratum of inconsiderable t ickness, but 
literally teeming with shells, which are a strange mixture of the 
faunas of Jackson and Vieksbur rg, with numerous peculiar 
species (see list, ibid, p. 136). Here also, we have a Madrepora, 
distinct from, but closely allied to, the eS ae in a 
“ Georgiana bed” at Vicksbur rg; where in its t e find an 
extraordinary eee of valves of Meretrix si i : rara avis 
in the Vicksburg strata proper, but abundant in the 
group. Busycon undulatum, also, is a Jackson form, it not 
