OCR Text |
Show 244 EOCENE PERIOD. [Ch. XVIII. several geologists, especially of M. Constant Prevost*, have led to O'reat modifications in the theoretical views entertained b respecting the order in which the several groups were formed; and it now appears that the formations Nos. 1, 2, and a, of the table of MM. Cuvier and Brongniart, instead of havinO' 0 originated one after the other, arc divisible into four nearly contemporaneous groups. Superposition of dijfe·rent formations in the Pa,ris basin.A comparison of the two accompanying diagrams will enable the reader to comprehend at a glance the different relations which the several sets of strata bear to each other, according to the original, as well as the more modern classification. We shall now proceed to lay before the reader a brief sketch of the several sets of strata referred to in the above systems. Immediately upon the chalk a layer of broken chalk flints, often cemented into a breccia by siliceous sand, is very commonly found. 'These flints probably indicate the action of the sea upon reefs of chalk when a portion of that rock had emerged and before the regular tertiary beds were superimposed. To this partial layer no reference is made in the annexed sections. Plastic clay and sand.-Upon this flinty stratum, or, if it be wanting, upon the chalk itself, rests frequently a deposit of clay and lignite (No. l of the above tables). It is composed of fresh-water shells and drift-wood, and was, at first, regarded as a proof that the Paris basin had originally been filled with fresh water. But it has since been shown that this group is not only of very partial extent, but is by no means restricted to a fixed place in the series; for it alternates with the marine calcaire grossier (No. 2 of the tables), and is repeated in the very middle of that limestone at Veaugirard, Bagneux, and other places, where the same Planorbes, Paludinre, and Lim· nei occur t. M. Desnoyers pointed out to me a section in the suburbs of Paris, laid open in 1829, where a similar intercalation was seen in a still higher part of the calcaire grassier. * Bulletin des Sci. de Ia Soc. Philom., May, 1825, p. 74. t Prevost, Sur les Submersions It~ratives, &c. Mcm. de Ia. Soc. d'Ilist. Nat. de Paris, tome iv. p. 7 4. Ch. XVIII.] PARIS BASIN, 245 These observations relieve us from the difficulty of seeking a cause why vegetable matter, and certain species of fresh-water shells and a particular kind of clay, was first introduced into the basin, and why the same space was subsequently usurped by the sea. A minute examination of the phenomena leads us simply to infer, that a river charged with argillaceous sediment entered a bay of the sea and drifted down, from time to time, fresh-water shells and wood. C'alcaire gros.Yier.-The calcaire grossier above alluded to is . ' composed of a coarse limestone, often passing into sand, such as may perhaps have been derived from the aqueous degradation of a chalk country. It contains by far the greater number of the fossil shells which characterize the Paris basin. No less than 400 distinct species have been derived from a single locality near Grignon. They are imbedded in a calcareous sand chiefly formed of comminuted shells, in which, nevertheless: individuals in a perfect state of preservation, both of marine, terrestrial, and fresh-water species, are mingled together" and were evidently transported from a distance. Some of the marine shells may have lived on the spot, but the Cyclostoma and Limnea must have been brought there by rivers and currents, and the quantity of triturated shells implies considerable movement in the waters. Nothing is more remarkable in this assemblage of fossil testacea than the astonishing proportion of species refenible to the g~nus Cer~thium *. 'l'here occur no less than 137 species of this genus m the Paris basin, and almost all of them in the calcaire grossier. Now the living testacea of this genus inhabit the sea near the mouths of rivers, where the waters are brackish ~o that their abundance in the marine strata of the Paris basin i~ 1~ perfect harmony with the hypothesis before advanced, that a r:ve~· flowed into the gulf, and gave rise to the beds of clay and h.gmte before mentioned. But there are ample data for inferrmg ~hat the gulf was supplied with fresh water by more than one nver, for while the calcaire grassier occupies the northern * See the tables of :M. Desha.yes, Appendix 1., Jl. 26. |