EOCENE, e'6-sen, a subdivision of geo logical time. Lyell in 1833 first used the term Eocene (dawn of the recent) for the earliest of his three subdivisions of the Tertiary. The term found favor quickly since early Tertiary life differed greatly from late Cretaceous. To ward the close of Cretaceous time, the sea re ceded from a large part of North America and by the end of Eocene time the continent had nearly its present form. The climate continued warm, Greenland and Alaska being temperate. Of Cretaceous animals the reptiles suffered most, the ichthyosaurs, dinosaurs and plesio saurs passing away as well as many peculiar mollusks. Eocene fishes were mostly of mod ern character (teleosts), birds were more numerous and highly developed than in the Cretaceous Age; while mammals developed wonderfully. True carnivorous mammals ap peared, as also the ancestors of the horse, rhinoceros, tapir, pig and the ruminants, be sides bats, primitive camels and monkeys. A study of organic types indicates that Asia, North America and Europe were connected in Eocene time ;. and, by the Antarctic continent, South America and Australia; while South America was separated from North America, and Africa and southern Asia from Europe and northern Asia.
The Eocene rocks of the Altantic border are nearly all loose sands and clays of ma rine origin and contain in New Jersey beds of greensand once of some economic importance as a source of phosphate of lime for agricultural use. The rocks of the Gulf border were partly
laid down in fresh or brackish water and partly in salt water. They comprise shales, sandstones and limestones. In Florida are valuable de posits of phosphate rock, and in Texas are ex tensive beds of lignite, of workable size, but poor quality. The interior province formations, mostly clays and sands, were laid down as alluvial fans and cones (q.v.), in brackish or fresh water lakes which stretched, though not contemporaneously, from Mexico to Alaska. The largest of these lakes covered eastern Utah and western Colorado, and must have been 450 miles long and 250 miles wide. In Utah are lignite deposits of some importance. In the Pacific border province the Lower Eocene stages are wanting and the epoch is represented by a single series, the Tejon shales and sand stones, partly marine and partly terrestrial, with workable deposits of lignite in California and Oregon. See CENOZOIC ERA; GEOLOGY; TERTIARY SYSTEM.