It is evident, observe these geologists, from the great range of the hippurite and nummulite limestone, that the South of Europe was occupied at the cretaceous period by an immense sea, which extended from the Atlantic Ocean into Asia, and comprehended the southernmost part of France, together with Spain, Sicily, part of Italy, and the Austrian Alps, Dalmatia, Albania, a portion of Syria, the isles of the Ægean, coasts of Thrace, and the Troad.

In proportion, therefore, as we enlarge the sphere of our researches, we may find in the strata of one era, the mineralogical counterparts of the rocks, which, in a single country like England, may characterise successive periods. Thus, the grits, sandstone, and shale with coal, of the Pyrenees have actually been mistaken by skilful miners for the ancient carboniferous group of England and France. In like manner the cretaceous red marl and salt of northern Spain have been regarded as the same as our new red and saliferous sandstone; and the lithographic limestone of the Morea might be confounded with the oolite of Solenhofen in Germany.

The beginner, perhaps, on hearing these facts, may object to the term cretaceous, as applied to the rocks of the southern region in which there is no chalk. But the term green-sand would have been equally inappropriate as a general name for

this group; and that of hippurite and nummulite limestone, however well suited to the Mediterranean region, would be inapplicable to the chalk of the north. Scarcely any designation would remain unexceptionable as we enlarge the bounds of our knowledge, and we must therefore be content to retain many ancient names, as simply expressing the mineral, or paleontological characters of rocks in the country where they were first studied.

CHAPTER XVI.

WEALDEN GROUP.

The Wealden, including the Weald clay, Hastings sand, and Purbeck beds - Intercalated between two marine formations-Fossil shells freshwater, with a few marine- 2 Cypris Fish

Section Reptiles Birds - Plants showing passage of Wealden beneath chalk-Junction of Wealden and Oolite - Dirt-bed-Theory of gradual subsidence-Proofs that the Wealden strata, notwithstanding their thickness, may have been formed in shallow waterGeographical extent of Wealden — Bray near BeauvaisRelation of the Wealden to the Lower Green-sand and Oolite.

BENEATH the cretaceous rocks in the S. E. of England, a freshwater formation is found called the Wealden, which, although it occupies a small area in Europe, as compared to the chalk, is nevertheless of great interest, as being intercalated between two marine formations. It is composed of three minor groups, of which the aggregate thickness in some places cannot be less than 800 feet.* These subdivisions are,

1st. Weald clay, sometimes including thin beds of sand and shelly limestone

2d. Hastings sand, in which occurs some clays and calcareous grits ;-between

3d. Purbeck beds, consisting of various kinds of limestones and marls

Thickness.

140 to 280 ft.

400 and 500 ft.

about 250 ft.

* Dr. Fitton, Geol. Trans. vol. iv. p. 320. Second Series.

To all these subdivisions, the common name of the Wealden has been given, because they may be best studied in part of Kent, Surrey, and Sussex, called the Weald.

We have seen that the fossils of the chalk and green-sands which repose upon the Wealden are all marine, and the species numerous; and the same remark applies to the Portland stone and other members of the Oolitic series which lie immediately beneath (see Fig. 181.). But in the

Position of the Wealden between two marine formations.

Wealden itself, although the fossils are abundant as to quantity, the number of different species is comparatively small, and by far the greater part of them show that they were deposited in a freshwater lake, or estuary communicating with the sea.*

Fossils of the Wealden. -The shells of this formation are almost exclusively of fluviatile or lacustrine genera, such as Melanopsis, Paludina, Neritina, Cyclas, Unio, and others. The individuals are sometimes in such profusion, that the

* Fitton, Geol. Trans. vol. iv. p. 104. Second Series.

surface of each thin layer of marl or clay is covered with the valves of Cyclas, and whole beds of limestone are almost entirely composed of Paludinæ. Intermixed with these freshwater shells, there are a few which seem to mark the occasional presence of salt water, as for example, a species of Bulla, together with an Oyster, and the Exogyra, a genus of unimuscular bivalves allied to the oyster (see Fig. 182.). The conclusion to be drawn from the presence of a Corbula (see Fig. 183.) and Fig. 182. Fig. 183.

Mytilus is more doubtful; for although these genera are for, the most part marine, still there is a Mytilus living in the Danube, and one species of Corbula inhabits the river La Plata, in South America, as well as the adjoining sea, while another is common to the Caspian, and the rivers Don and Wolga. But admitting all these to have been marine, they by no means outweigh the evidence, both of a positive and negative kind, derived from shells in favour of the freshwater origin of the Wealden. In no part of this deposit do we meet with ammonites, belemnites, terebratulæ, corals, sea-urchins, or other testacea and