C. 1. Argile de Londres at base 1. London clay. of Hill of Cassel, near Dunkirk. C. 2. Argile plastique and 2. Plastic clay and sand with lignite. 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. LIFE OF THE EOCENE PERIOD.