1847.] LYELL ON THE NUMMULITE LIMESTONE OF ALABAMA. 15 
softer varieties of which it much resembles, is I believe due to the 
_ decomposition of corals, and like our chalk downs, the surface of the 
country where it prevails is sometimes marked by the absence of 
wood, by which all the other deposits in this part of Alabama are con- 
tinuously covered. The spots where few or no trees appear are called 
“bald Prairies,” but in some places, and at Bettis’ Hill among others, 
the orbitolite rock produces what is termed a “ cedar knowl,”’ the red 
cedar, Juniperus virginiana, having exclusive possession of the ground. 
I was much struck with the resemblance of such calcareous tracts, 
covered with the tree above-mentioned, to certain chalk regions in the 
south of England, where the only wood which grows on the white 
rock consists of yew-trees accompanied here and there by shrubs of 
juniper. 
At St. Stephen’s, on the left bank of the Tombecbee river in Ala- 
bama, a similar limestone with orbitoides forms a perpendicular bluff. 
The water of the river at the time of my visit was too high to enable 
me to collect fossils from the beds at the base of the cliff, but I was 
afterwards furnished with them through the kindness of Prof. Brumby 
of Tuscaloosa. They are imbedded im a ferruginous ochreous-coloured 
sand, and consist in part of shells common to Claiborne Bluff, such as 
Terebra costata, Conrad, Cardita parva, Dentalium thalloides, Fla- 
bellum cuneiforme, Lonsd., Scutella Lyelli, Con., and several more. 
I shall now conclude by adverting briefly to the result of a compari- 
son which I made of the fossils contamed im the eocene strata of 
Vicksburg on the left bank of the Mississippi river, the position of 
which is indicated at 4 a in the annexed woodcut, with those of other 
eocene beds forty-five miles farther inland or eastward, at Jackson in the 
same State (36). {nthe former of these at 4 a, the Orbitoides Man- 
telli abounds, together with Pecten Poulson, Dentalium thalloides, 
Sigaretus arctatus, Con., Terebra costata, Con., and a few others com- 
mon to Claiborne, but the great bulk of the associated fossils do not 
agree specifically with those of Claiborne Bluff. I found these di- 
stinet species of Vicksburg obtained by me to be referable to the ge- 
nera Voluta, Conus, Terebra, Fusus, Murex, Cassis, Pleurotoma, 
Oliva, Solarium, Natica, Turritella, Corbula, Panopea, Crassatella, 
Iucina, Venus, Cardium, Arca, Pinna, Pecten and Ostrea, with 
several corals, the whole having a decidedly tertiary and eocene aspect. 
The genus Pleurotoma for example, which is represented by several 
species, is one of the forms most characteristic of tertiary as distin- 
guished from secondary formations. 
At Jackson, which as before stated is more than forty miles to 
the eastward, older eocene beds crop out near to the area occupied 
by cretaceous deposits, as at 6, woodcut No. 2. Here, on the Pearl 
river, | found no specimens of Orbitoides Mantelli, although some are 
said to have been met with in the vicinity; but I observed that a 
larger proportion among the fossils were of species common to Clai- 
borne Bluff than at Vicksburg. Among these may be mentioned 
Cardita planicosta, Cardita rotunda, Cytherea equorea, Natica like 
one which I collected at Claiborne, Flabellum cuneiforme, Lonsd., 
and Endopachys alatum, Lonsd. (Turbinolia Maclurii of Lea); these I 
