_Older Pliocene and Miocene formations in the United States._--Between the Alleghany mountains, formed of older rocks, and the Atlantic, there intervenes, in the United States, a low region occupied principally by beds of marl, clay, and sand, consisting of the cretaceous and tertiary formations, and chiefly of the latter. The general elevation of this plain bordering the Atlantic does not exceed 100 feet, although it is sometimes several hundred feet high. Its width in the middle and southern states is very commonly from 100 to 150 miles. It consists, in the South, as in Georgia, Alabama, and South Carolina, almost exclusively of Eocene deposits; but in North Carolina, Maryland, Virginia, and Delaware, more modern strata predominate, which I have assimilated in age to the English crag and Faluns of Touraine.[172-A] If, chronologically speaking, they can be truly said to be the representatives of these two European formations, they may range in age from the Older Pliocene to the Miocene epoch, according to the classification of European strata adopted in this chapter.
The proportion of fossil shells agreeing with recent, out of 147 species collected by me, amounted to about 17 per cent., or one-sixth of the whole; but as the fossils so assimilated were almost always the same as species now living in the neighbouring Atlantic, the number may hereafter be augmented, when the recent fauna of that ocean is better known. In different localities, also, the proportion of recent species varied considerably.
[Illustration: Fig. 151. _Fulgur canaliculatus._ Maryland.]
[Illustration: Fig. 152. _Fusus quadricostatus_, Say. Maryland.]
On the banks of the James River, in Virginia, about 20 miles below Richmond, in a cliff about 30 feet high, I observed yellow and white sands overlying an Eocene marl, just as the yellow sands of the crag lie on the blue London clay in Suffolk and Essex in England. In the Virginian sands, we find a profusion of an Astarte (_A. undulata_, Conrad), which resembles closely, and may possibly be a variety of, one of the commonest fossils of the Suffolk crag (_A. bipartita_); the other shells also, of the genera _Natica_, _Fissurella_, _Artemis_, _Lucina_, _Chama_, _Pectunculus_, and _Pecten_, are analogous to shells both of the English crag and French faluns, although the species are almost all distinct. Out of 147 of these American fossils I could only find 13 species common to Europe, and these occur partly in the Suffolk crag, and partly in the faluns of Touraine; but it is an important characteristic of the American group, that it not only contains many peculiar extinct forms, such as _Fusus quadricostatus_, Say (see fig. 152.), and _Venus tridacnoides_, abundant in these same formations, but also some shells which, like _Fulgur carica_ of Say, and _F. canaliculatus_ (see fig. 151.), _Calyptræa costata_, _Venus mercenaria_, Lam., _Modiola glandula_, Totten, and _Pecten magellanicus_, Lam., are recent species, yet of forms now confined to the western side of the Atlantic, a fact implying that the beginning of the present geographical distribution of mollusca dates back to a period as remote as that of the Miocene strata.