_Mottled or Plastic Clays_, _&c._ (3. _b_, Table, p. 197.).--No formations can be more dissimilar on the whole in mineral character than the Eocene deposits of England and Paris; those of our own island 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 a coarse white limestone, and compact siliceous limestone with beds of crystalline gypsum and siliceous sandstone, and sometimes 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, and to settle their respective ages. It is clear that 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 the 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. At the commencement, however, of the Eocene formations in France and England, we find an exception to this rule, for a marked similarity of mineral character reigns in the lowest division, whether in the basins of Paris, Hampshire, or London. This uniformity of aspect must be seen in order to be fully appreciated, since the beds consist simply of sand, mottled clays, and well-rolled flint pebbles, derived from the chalk, and varying in size from that of a pea to an egg. These strata may be seen at Reading, at Blackheath, near London, and at Woolwich. In some of the lowest of them, banks of oysters are observed, consisting of _Ostrea bellovicina_, so common in France in the same relative position, and _Ostrea edulina_, scarcely distinguishable from the living eatable species. In this formation at Bromley, Dr. Buckland found one large pebble to which five full-grown oysters were affixed, in such a manner as to show that they had commenced their first growth upon it, and remained attached to it through life. In several places, as at Woolwich on the Thames, at Newhaven in Sussex, and elsewhere, a mixture of marine and freshwater testacea distinguishes this member of the series. Among the latter, _Melania inquinata_ (see fig. 188.) and _Cyrena cuneiformis_ are very common. They probably indicate points where rivers entered the Eocene sea. [Illustration: Fig. 187. _Cyrena cuneiformis_, Min. Con. Natural size.] [Illustration: Fig. 188. _Melania inquinata_, Des. Nat. size. Syn. _Cerithium melanoides_, Min. Con.]