Eocene (Gr. ^wf, dawn, and mivog, recent), the lowest division or earliest epoch of the tertiary formation or period. It was named by Sir Charles Lyell, who divided the tertiary period into three epochs, called respectively, according to time, eocene, miocene, and pliocene, under the generally existing belief that the eocene contained among its fossils from 5 to 10 per cent, belonging to existing species. It was known that the next older formation, the cretaceous, belonging to the reptilian age, contains no existing species, and therefore this lowest tertiary epoch was named for the purpose of expressing the idea that it was the dawn of the existing state of the testaceous fauna. Since the division was made and brought into general use, it has been found that all the species of the eocene epoch are extinct. The miocene, however, contains from 10 to 40 per cent, and the pliocene from 50 to 90 per cent, of species now existing. The eocene rocks are well developed in the London and Paris basins, the cities of London, Paris, and Brussels being built upon the formation; a circumstance which has tended to attract much attention to its study, as well as the numerous fossils which it contains, and by which it was subdivided into lower, middle, and upper eocene, called also in the United States the Clai-. borne, Jackson, and Vicksburg epochs.

It extends along a great portion of the S. E. coast of the United States, principally in Maryland, Virginia, the Carolinas, and the gulf states. The beds reach their most complete and" distinct development in Mississippi and Alabama. Those of the Claiborne epoch or lower eocene, near Claiborne, Ala., consist of about 25 ft. of clay overlaid by a bed of lignite 4 ft. thick, and succeeding this marl with oysters, then marly arenaceous limestone, again succeeded by marl with oysters, upon which is sand with shells showing beach origin, the whole being about 125 ft. thick. In Mississippi the beds reach a thickness of 425 ft. The epoch is represented on the Pamunkey river in Virginia, and at Marlborough in Maryland, by dark greensands. In the upper Missouri there is a great lignite group 2,000 ft. thick, containing much lignite, numerous leaves of plants, and shells of mollusks; and a lignite formation also occurs in Texas. The beds of the Jackson epoch, or middle eocene, are represented at Jackson, Miss., and consist of lignitic clay and beds of white and blue marls, containing numerous marine shells, and remains of the zeu-gloclon, and extinct species of whale supposed to have been about 70 ft. long, whose large vertebrae, 18 in. long by 1 ft. in diameter, are found in such quantities in some places as to be used in making walls.

The Jackson beds cross the state as a narrow band about 80 ft. thick, E. N. E. through" Jackson and Scott counties. The beds of the Vicksburg epoch, or upper eocene, are represented at Vicksburg, Miss., and consist of: 1, lignitic clay, 20 ft. thick; 2, ferruginous rock of Red Bluff, containing numerous marine fossils, 12 ft. thick; 3, compact limestones and blue marls, with marine fossils, 80 ft. thick; making a total thickness of 112 ft. The Vicksburg group occurs in Munroe, Clarke, and "Washington counties, Ala.; as limestone at Tampa Bay, Fla.; and as gray marl on Ashley and Cooper rivers, S. C. In the upper Missouri region there are freshwater beds 1,000 ft. thick, consisting of white and drab clays, layers of sandstone, and local beds of limestone; but they are considered by Leidy as miocene. In them are found the remains of the titanotherhtm, an herbivorous animal, somewhat resembling the modern tapir, and about twice the size of a horse.