OF SOUTH CAROLINA. 135 
Towards the top the upper stratum becomes less calcareous, and is charged with sulphate of 
iron. ‘These beds may be traced, at intervals, nearly to the preceding locality. 
At Port’s Ferry the same beds occur, and are composed of precisely the same materials. The 
ledge of marl stone is about four feet thick, presenting, on the upper side, an even surface, but below 
it is much water-worn and shelving. In addition to the fossils at Britton’s, I found here, in the 
marl stone, very perfect casts of ammonites placenta, Mor. for the first time in the State. 
The stratum above the marl stone contains soft concretionary nodules of lime, like those I ob- 
served at Petersfield. The beds now show a very decided dip, wherever they are sufficiently stra- 
tified to render it observable. So long as the thick upper beds alone appeared above the surface, it 
was difficult to observe it; but the ledge of marl stone, which is exposed at low-water, makes it 
quite evident. For instance, at Britton’s Ferry it was, when I observed it, on a level with the 
river; at Port’s Ferry it was two or three feet above it; and at Williams’s Bluff, above this, the 
bed below the marl stone comes fairly into view. Of course, in making these observations, the 
direction of the river must not be lost sight of; for where a bend coincides with the strike, the rock 
will appear horizontal, and it is only where the direction of the stream is at right angles with the 
strike, that the dip can be correctly determined. 
The last mentioned bluff presents an exceedingly interesting section. 'The alternating beds of 
marl and marl stone are seen gently emerging from the water. Above the marl stone is a white 
stratum, very rich in calcareous matter, and this is succeeded by irregular masses of calcareous 
rock, filled with the common cretaceous fossils, Bxogyra costata’and Ammonites. 
Similar alternations of hard and soft strata occur at Gibson’s Bluff. The marl, however, which is 
of greater thickness, is capped with a bed of compact limestone, the weathered surface of which is 
paved with fossils, composed of crystallized carbonate of lime. I identified, among these, Plagi- 
ostoma dumosum? well preserved Venilia Conradi, and Crassatella—species undetermined.— 
These, with other fossils, occur here in horizontal layers, and are detached from the matrix with 
difficulty, for this limestone is equally hard and compact with that of the older fossilliferous rocks. 
At Giles’s Bluff, on the land of Col. Davis, the calcareous beds attain their greatest height, which 
is about 20 feet above the river. The section, however, differs little from those just examined, for 
it shows the usual alternation of hard and soft beds, containing nearly the same fossils. The for- 
mation at this point, namely, between Gibson’s and Giles’s Bluff, has its maximum of calcareous 
matter, and for the simple reason that the middle beds are most exposed, that is, those in which the 
indurated beds occur. The upper part of the soft marl is generally poorer in lime than the bottom 
of the same bed, perhaps because much of it has been removed by the percolation of water. The 
bed underlying the marl stone is also poor. 
Now it is the upper part of the marl that is exposed below Britton’s Ferry; and it is the lower 
beds—those below the marl stone—that come to the surface higher up on Lynch’s Creek. The 
following section, (Fig. 26,) will make this plain. 
