I. EOCENE OF BRITAIN. (1.) LOWER EOCENE.—The base of the Eocene series in Britain is constituted by about 90 feet of light-coloured, sometimes argillaceous sands (Thanet Sands), which are of marine origin. Above these, or forming the base of the formation where these are wanting, come mottled clays and sands with lignite (Woolwich and Reading series), which are estuarine or fluvio-marine in origin. The highest member of the Lower Eocene of Britain is the "London Clay," consisting of a great mass of dark-brown or blue clay, sometimes with sandy beds, or with layers of "septaria," the whole attaining a thickness of from 200 to as much as 500 feet. The London Clay is a purely marine deposit, containing many marine fossils, with the remains of terrestrial animals and plants; all of which indicate a high temperature of the sea and tropical or sub-tropical conditions of the land.
(2.) MIDDLE EOCENE.—The inferior portion of the Middle Eocene of Britain consists of marine beds, chiefly consisting of sand, clays, and gravels, and attaining a very considerable thickness (Bag-shot and Bracklesham beds). The superior portion of the Middle Eocene of Britain, on the other hand, consists of deposits which are almost exclusively fresh-water or brackish-water in origin (Headon and Osborne series).
The chief Continental formations of Middle Eocene age are the "Calcaire grossier" of the Paris basin, and the "Nummulitic Limestone" of the Alps.
(3.) UPPER EOCENE.—If the Headon and Osborne beds of the Isle of Wight be placed in the Middle Eocene, the only British representatives of the Upper Eocene are the Bembridge beds. These strata consist of limestones, clays, and marls, which have for the most part been deposited in fresh or brackish water.
II. EOCENE BEDS OF THE PARIS BASIN.—The Eocene strata are very well developed in the neighbourhood of Paris, where they occupy a large area or basin scooped out of the Chalk. The beds of this area are partly marine, partly freshwater in origin; and the following table (after Sir Charles Lyell) shows their subdivisions and their parallelism with the English series:—
GENERAL TABLE OF FRENCH EOCENE STRATA.
| UPPER EOCENE. | ||||
| French Subdivisions. | English Equivalents. | |||
| A. | 1. | Gypseous series of Mont Montmartre. | 1. | Bembridge series. |
| A. | 2. | Calcaire silicieux, or Travertin Inférieur. | 2. | Osborne and Headon series. |
| A. | 3. | Grès de Beauchamp, or Sables Moyens. | 3. | White sand and clay of Barton Cliff, Hants. |
| MIDDLE EOCENE. | ||||
| B. | 1. | Calcaire Grossier. | 1. | Bagshot and Bracklesham beds. |
| B. | 2. | Soissonnais Sands, or Lits Coquilliers. | 2. | Wanting. |
| LOWER EOCENE. | ||||
| C. | 1. | Argile de Londres at base of Hill of Cassel, near Dunkirk. | 1. | London clay. |
| C. | 2. | Argile plastique and lignite. | 2. | Plastic clay and sand with lignite (Woolwich and Reading series). |
| C. | 3. | Stables de Bracheux. | 3. | Thanet sands. |
III. EOCENE STRATA OF THE UNITED STATES.—The lowest member of the Eocene deposits of North America is the so-called "Lignitic Formation," which is largely developed in Mississippi, Tennessee, Arkansas, Wyoming, Utah, Colorado, and California, and sometimes attains a thickness of several thousand feet. Stratigraphically, this formation exhibits the interesting point that it graduates downwards insensibly and conformably into the Cretaceous, whilst it is succeeded uncomformably by strata of Middle Eocene age. Lithologically, the series consists principally of sands and clays, with beds of lignite and coal, and its organic remains show that it is principally of fresh-water origin with a partial intermixture of marine beds. These marine strata of the "Lignitic formation" are of special interest, as showing such a commingling of Cretaceous and Tertiary types of life, that it is impossible to draw any rigid line in this region between the Mesozoic and Kainozoic systems. Thus the marine beds of the Lignitic series contain such characteristic Cretaceous forms as Inoceramus and Ammonites, along with a great number of Univalves of a distinctly Tertiary type (Cones, Cowries, &c.) Upon the whole, therefore, we must regard this series of deposits as affording a kind of transition between the Cretaceous and the Eocene, holding in some respects a position which may be compared with that held by the Purbeck beds in Britain as regards the Jurassic and Cretaceous.
The Middle Eocene of the United States is represented by the Claiborne and Jackson beds. The Claiborne series is extensively developed at Claiborne, Alabama, and consists of sands, clays, lignites, marls, and impure limestones, containing marine fossils along with numerous plant-remains. The Jackson series is represented by lignitic clays and marls which occur at Jackson, Mississippi. Amongst the more remarkable fossils of this series are the teeth and bones of Cetaceans of the genus Zeuglodon.
Strata of Upper Eocene age occur in North America at Vicksburg, Mississippi, and are known as the Vicksburg series. They consist of lignites, clays, marls, and limestones. Freshwater deposits of Eocene age are also largely developed in parts of the Rocky Mountain region. The most remarkable fossils of these beds are Mammals, of which a large number of species have been already determined.