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Show 276 EOCENE PERIOD. [Ch. XX. Basin of the Cotentin, or Valognes.-The strata in the environs of y alognes, in the department of La Manche, consist chiefly of a coarse limestone resembling the calcaire grossier of Paris of which M. Desnoyers has given an elaborate description. ' It is occasionally covered with a compact fresh-watet· limestone alternating with fresh-water marls. In these Eocene strata more than 300 species of fossil shells have. been discovered almost all identical with species of the Paris basin. (See Table' s, Appendix I.) Superimposed upon the Eocene strata of this basin is a newer marine deposit, extending over a limited area, the fossils of which agree with those of the Faluns of the Loire*. Here, therefore, the geologist has an opportunity of observing the superposition of the Miocene deposits upon those of the age of the Paris basin. Rennes.-Several small patches, also, of marine strata, have been found by M. Desnoyers, in the neighbourhood of Rennes, which are characterised by Eocene fossils and repose on ancient rocks, as will be seen in tlJe map. Basin of Belgium, or the Netherlands.-The greater part of the tertiary formations of the 'Low Countries consist of clay and sand, much resembling those of the basin of London, afterwards to be described. The fossil shells, also, are of the same species, 49 of which will be found referred to by M. Deshayes, in the tables, Appendix I. Aix, in Provence.-'l'he tertiary strata of Aix and Fuveau in Provence are of great thickness and extent, the lower mem· bers being remarkable for containing coal grit and beds of compact limestone, such as we only find in England in ancient secondary gwups. Yet these strata are for the most part of fresh-water origin, and contain several species of Eocene shells, together with many which are peculiar to this basin. It would require a fuller comparison than has yet been made of the fossil remains of Aix and Fuveau, before we can determine with accuracy the relative age of that formation. Some of the plants seem to agree with those of the Paris basin, while many fl< Desnoyers~ Mem. de la Soc. u' Hi st. Nat. de Paris, 1825. Ch. XX.] ENGLISH EOCENE FOR~ATIONS. 277 Qf the insects have been supposed identical with species now living*. These insects have been fl,lmost exclusively procured from a thin bed of grey calcareous marl, which passes into an argillaceous limestone found in the quarries of gypsum near Aix. The rock in which they are imbedded is so thinly laminated that there are sometimes more than 70 layers in the thickness of an inch. The insects are fot· the most part in an extraordinary state of preservation, and an impression of their form is seen both on the upper and under laminoo, as in the case of the Monte Bolca fishes. 1\f. Marcel de Serres enumerates 62 genera belonging chiefly to the orders Diptera, Hemiptera and Coleoptera. On reviewing a collection brought from Aix, Mr. Curtis observes that they are all of European forms and most of them referrible to existing genera t. With the single exception of an Hydrobius, none of the species are aquatic. The antennoo, tarsi, and trophi are generally very obscure, or distorted, yet in a few the claws are visible, and the sculpture, and even some degree of local colouring, are preserved. The nerves of the wings, in almost all the Diptera, are perfectly distinct, and even the pubescence on the head of one of them, Several of t~e beetles have the wings extended beyond the elytra, as if they had made an effort to escape by flying, or had fallen into the water while on the wing t. BASINS OF LONDON AND HAMPSHIRE. The reader will see in the small map above given (No. 62, p. ~75,) the position of the two districts usually called the basms of London and Hampshire, to which the Eocene formations of England are confined. These tracts are bounded ~y rising grounds composed of chalk, except where the sea Intervenes. That the chalk passes beneath ·the tertiary strata, we can not o~ly. infer f~·om geological data, but can prove by numerous artificial sectwns at points where wells have been sunk, or borings made through the overlying beds. The * 1\I. Marcel d~ Serres, Geog. des Ter. Tertiaires du Midi de la France. 1' Mu~ch~s?n and Lyell. Ed. New Phil. Joum., Oct. 1829. ~ Curbs, 1b1d., where figures of some of the insects are given. |