On the opposite page we present, in [Plate XXIII.], an imaginary landscape of the Eocene period. We remark amongst its vegetation a mixture of fossil species with others belonging to the present time. The Alders, the Wych-elms, and the Cypresses, mingle with Flabellaria; the Palms of extinct species. A great Bird—a wader, the Tantalus—occupies the projecting point of a rock on the right; the Turtle (Trionyx), floats on the river, in the midst of Nymphæas, Nenuphars, and other aquatic plants; whilst a herd of Palæotheria, Anoplotheria, and Xiphodon peacefully browse the grass of the natural meadows of this peaceful oasis.
With a general resemblance in their fossils, nothing can be more dissimilar, on the whole, than the lithological or mineral characters of the Eocene deposits of France and England; “those of our own island,” says Lyell,[84] “being almost exclusively of mechanical origin—accumulations of mud, sand, and pebbles; while in the neighbourhood of Paris we find a great succession of strata composed of limestones, some of them siliceous, and of crystalline gypsum and siliceous sandstone, and sometimes of pure flint used for millstones. Hence it is by no means an easy task to institute an exact comparison between the various members of the English and French series. It is clear that, on the sites both of Paris and London, a continual change was going on in the fauna and flora by the coming in of new species and the dying out of others; and contemporaneous changes of geographical conditions were also in progress in consequence of the rising and sinking of the land and bottom of the sea. A particular subdivision, therefore, of time was occasionally represented in one area by land, in another by an estuary, in a third by sea; and even where the conditions were in both areas of a marine character, there was often shallow water in one, and deep sea in another, producing a want of agreement in the state of animal life.” The Eocene rocks, as developed in France and England, may be tabulated as follows, in descending order:—
| English. | French. | ||||||||
| Upper Eocene. | – | Hempstead beds. | – | Fluvio- marine series. | – | Calcaire de la Beauce. Grès de Fontainebleau. | |||
| Bembridge beds. | – | Calcaire silicieux or Calcaire Lacustre Moyen. Gypseous series of Montmartre. | |||||||
| Middle Eocene. | – | Osborne beds. Headon beds. | – | Grès de Beauchamp and Calcaire Marin. | |||||
| Upper Bagshot sand. | – | – | Upper Sables Moyens. | ||||||
| Barton clay. Bracklesham beds. | – | Middle Bagshot. | – | Lower Sables Moyens, Lower Calcaire Grossier, and Glauconie Grossière. | |||||
| Lower Bagshot beds. | – | – | Lits coquillières. Glauconie Moyenne. | ||||||
| Lower Eocene. | – | London clay. | Wanting. | ||||||
| Woolwich and Reading beds, or Plastic clay. | – | – | Argile Plastique. Glauconie Inférieure. | ||||||
| Oldhaven beds. | |||||||||
| Thanet sands. | Sables Inférieurs. | ||||||||
The Woolwich and Reading Beds, or the Plastic Clay of older writers, consists of extensive beds of sand with occasional beds of potter’s clay, which lie at the base of the Tertiary formation in both England and France. Generally variegated, sometimes grey or white, it is employed as a potter’s earth in the manufacture of delf-ware.
In England the red-mottled clay of the Woolwich and Reading Beds in Hampshire and the Isle of Wight is often seen in contact with the chalk; but in the south-eastern part of the London basin, Mr. Prestwich shows that the Thanet Sand (consisting of a base of fine, light-coloured sand, mixed with more or less argillaceous matter) intervenes between the Chalk and the Oldhaven Beds, or in their absence the Woolwich and Reading beds, which lie below the London Clay. The Thanet Sands derive their name from their occurrence in the Isle of Thanet, in Kent, in the eastern part of which county they attain their greatest development. Under London and its southern suburbs the Thanet sand is from thirteen to forty-four feet thick, but it becomes thinner in a westerly direction, and does not occur beyond Ealing.[85]
The Woolwich and Reading beds in the Hampshire basin rest immediately on the Chalk, and separate it from the overlying London Clay, as may be seen in the fine exposure of the Tertiary strata in Alum Bay, at the western extremity of the Isle of Wight, and in Studland Bay, on the western side of the Isle of Purbeck, in Dorsetshire.
In the London basin the Woolwich and Reading beds also rest on the Chalk, where the Thanet Sands are absent, as is the case, for the most part, over the area west of Ealing and Leatherhead.
The beds in question are very variable in character, but may be generally described as irregular alternations of clays and sands—the former mostly red, mottled with white, and from their plastic nature suitable for the purposes of the potter; the latter also of various colours, but sometimes pure white, and sometimes containing pebbles of flint.
The Woolwich and Reading beds are called after the localities of the same names; they are fifty feet thick at Woolwich, and from sixty to seventy feet at Reading.