. Elements of geology. Geology. PART II. CHAPTER XVI. 201 Wealden Group. Fossils. Fig. 181. freshwater' marine mariiiej. Chalk. Oolite. Weald Clay, ) Hastings sand, > Wealden, Purbeck beds, ) Position of the Wealden between two marine formations. Fossils of the Wealden.—The shells of this formation are almost exclusively of fluviatile or lacustrine genera, such as Me- lanopsis, Paludina, Neritina, Cyclas, Unio, and others. The indi- viduals are sometimes in such profusion, that the surface of each thin layer of marl or clay is covered with the valves of Cyclas, and whole beds of limestone a
. Elements of geology. Geology. PART II. CHAPTER XVI. 201 Wealden Group. Fossils. Fig. 181. freshwater' marine mariiiej. Chalk. Oolite. Weald Clay, ) Hastings sand, > Wealden, Purbeck beds, ) Position of the Wealden between two marine formations. Fossils of the Wealden.—The shells of this formation are almost exclusively of fluviatile or lacustrine genera, such as Me- lanopsis, Paludina, Neritina, Cyclas, Unio, and others. The indi- viduals are sometimes in such profusion, that the 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 Palu- dinse. 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 Mytilus is more doubtful,- for al- though these genera are for the most part marine, still there is a Mytilus living in the Danube, and one species of Corbula in- habits the river La Plata, in South America, as well as the ad- joining 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 posi- tive and negative kind, derived from shells in favour of the fresh- water origin of the Wealden. In no part of this deposit do we meet with ammonites, belemnites, terebratulas, corals, sea-urchins, or other testacea and zoophytes so characteristic of the chalk above, or the oolite below the Wealden. Fig. 182. Fig. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Lyell, Charles, Sir, 1797-1875; Gibbes, Lewis Reeve, 1810-1
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