Eocene System

Eocene System, in Geology. This is the lowest or oldest series of strata included in the Tertiary division. The name Eocene (eos, 'dawn,' kainos, 'recent') was introduced in 1830 by Lyell, whose classification of the Tertiary formations was based on the proportions of living species of molluscs which the strata contain. The oldest members of the series, containing as they do only an extremely small proportion of such recent forms (according to Lyell, 3½ per cent.), were looked upon as indicating the dawn of the existing state of the molluscan fauna. With necessary modifications, Lyell's classification has maintained its place, but the names of the various subdivisions of the Tertiary strata, first suggested by an examination of the European deposits, are now used without reference to the precise percentage of living molluscan species which the beds may chance to contain. The Eocene beds rest unconformably upon the Chalk or Cretaceous strata—hence there is a break in the succession. A movement of elevation followed upon the close of the Cretaceous period, and the sea disappeared from middle Europe. Probably the British area at this time was connected with what are now the opposite coasts of the Low Countries and

France, and considerable denudation of the later Cretaceous deposits supervened. Ere long, however, partial subsidence set in, and at the beginning of Eocene times the sea commenced to invade those regions in the south-east of England, the north of France, and the low grounds of Belgium, in which we meet with the oldest accumulations of Tertiary times.

The Eocene system, as developed in England, occurs chiefly in two districts, called respectively the London and Hampshire basins. In the north of France it forms another large basin, in the centre of which is Paris, and it likewise spreads over a considerable area in Belgium. The strata are partly of fresh-water and partly of marine and brackish-water origin, and consist principally of more or less unconsolidated sands, clays, loams, marls, &c., with which are here and there interstratified layers of limestone and lignite. Grouping the Anglo-French and Belgian strata together, we have the following succession:

In the south of Europe the Eocene system is developed on a much larger scale. In place of the limited basins of the Anglo-French and Belgian areas we encounter a thick and widespread series of limestones and calcareous sandstones. These accumulations appear on both sides of the Mediterranean, in Spain, and in Morocco. They enter largely into the composition of the Apennines, the Alps, the Carpathians, and the Balkans; they extend through Greece, Egypt, and Asia Minor, and thence through Persia and the Himalayas to the coasts of China and Japan. The most characteristic feature of these thick calcareous beds is the occurrence in prodigious numbers of nummulites—a foraminifer. Another noteworthy feature of the Eocene system of southern Europe is the occurrence in the Alpine region of Bavaria of certain sandstones, &c. (locally called Flysch), in which appear many gigantic erratics of various crystalline rocks—the precise source of which is not known. They may possibly have been derived from the archæan masses of Bohemia. In North America the more important Eocene strata are the great lacustrine deposits of the Rocky Mountain region and the plateaus lying to the west. But marine deposits of the same age occur also along the borders of the Atlantic, the Mexican Gulf, and the Pacific.

The flora of the Eocene is represented chiefly by dicotyledonous and monocotyledonous plants, and had a prevalent Indo-Australian character. Palms, screw-pines, cypresses, and various proteaceous plants grew in England in early Eocene times, and with these were associated species of custard apple, gourd, melon, oak, walnut, magnolia, &c. Amongst the forms of later Eocene times were species of gum-tree, nettle-tree, and banksia, fan-palms and screw-pines, aroids and cacti. Evergreens were represented by laurels, cypresses, and yews, while deciduous trees were represented by species of maple, plane, willow, poplar, elm, beech, chestnut, hornbeam, walnut, fig, &c. The fauna, like the flora, is also indicative of somewhat tropical con- ditions. Among the characteristic molluscs were species of Conus, Voluta, Cyprea, Oliva, Nautilus, &c. Sharks were numerous, such as Otodus, Lamna, and Carcharodon; and reptiles were represented by turtles, terrapins, snakes, lizards, crocodiles, and alligators. None of the great Mesozoic Saurians, however, survived into Tertiary times. Among the more remarkable birds were Odontopteryx, the jaws of which had tooth-like denticulations, and Dasornis, akin apparently to the extinct gigantic Dinornis of New Zealand. Early forms of modern types were also present, such as species of heron, gull, vulture, buzzard, woodcock, quail, pelican, flamingo, ibis, &c. Many of the mammals of Eocene times were remarkable for the union of characters intermediate between marsupials and carnivores. Tapir-like animals, such as Palæotherium, seem to have abounded. Another common form was Anchitherium, which had affinities to the Palæotheres and true horses. There were also many hog-like animals with intermediate or generalised characters, as well as transitional hornless forms of deer and antelopes. From the Eocene also come the earliest of the Prosimie—the lemuroid Cenopithecus, as well as ancestral forms of bats, hedgehogs, and squirrels. From the American Eocene lacustrine beds have been obtained remains of some very remarkable types. Among these are four-toed and five-toed ancestors of the horse, and Dinoceraps, an animal as large as the elephant, furnished with three pairs of horns and two long tusk-like canines (see DINOCERATA). Another strange order of animals (Tillodonts) combined characters which are now met with in such distinct groups as the ungulates, rodents, and carnivores.

The Eocene deposits of the Anglo-French and Belgian area appear to have accumulated in an inland sea opposite the mouths of one or more great rivers—the rivers evidently draining a continental land surface. In south Europe large areas which are now land were under water in Eocene times. The Mediterranean at that period extended considerably farther north and south than it now does, while eastwards it stretched into Asia, and seems to have communicated with the Indian Ocean. At this time the Pyrenees, the Alps, the Carpathians, and even the Himalayas were inconspicuous heights. Since the Nummulitic sea washed their base, they have been upheaved for thousands of feet—the old floor of that sea is now met with in the Alps at heights of over 10,000 feet, and in the Himalayas at an elevation of not less than 16,500 feet.