OCR Text |
Show 250 EOCENE PERIOD. [Ch. XVIII. Gyrogonitcs, or fossil seed-vessels. of chara~, are found ab~n .. dantly in these strata, and all the ammal and vegetable remru.ns arrree well with the hypothesis, that after the gulf or estuary h~d been silted up with the sand of the upper marine forma· tion, a great number of marshes and shallow lakes existed, like those which frequently overspread the newest parts of a delta. These Jakes were fed by rivers or springs which contained, in chemical solution or mechanical suspension, such kinds of sediment as we have already seen to have been deposited in the lakes of Central France during the Eocene period. The Pa·risian groups all Eocene.-Having now given a rapid sketch of the different groups of the Paris basin, we may observe generally that they all belong to the Eocene epoch, although the entire series must doubtless have required an immense lapse of ages for its accumulation. The shells of the different fresh-water groups, constituting at once some of the lowest and uppermost members of the series, arc nearly all referrible to the same species, and the discordance between the marine testacea of the calcaire grossier and the upper marine sands is very inconsiderable. A curious observation has been made by M. Deshayes, in reference to the changes which one species, the Cardittm porulosum, has undergone during the long period of its existence in the Paris basin. Different varieties of this cardium are chamcteristic of different strata. In the oldest sand of the Soissonnais (a marine formation underlying the regular beds of the calcaire grossier), this shell acquires but a small volume, and has many peculiarities which disappear in the lowest beds of the calcaire grossier. In these the shell attains its full size, and many peculiarities of form, which are again modified in the uppermost beds of the calcaire grossier, and these last cha· racters are preserved throughout the whole of the ' upper marine ' series*. Microscopic shells.-In some parts of the cal~a~re gross~er microscopic shells are very abundant, and of d1stmct species * Coquilles characterist. des Terrains, 1831. Ch.XVIII.J MAMMIFEROUS REMAINS iN GYPSUM. 251 from those be:ore mentioned of the older Pliocene beds of Italy. "\Ve may remmd those readers who are not familiar with these minute fossil bodies, that they belong to the order Cephalopoda, the animals of which are most free in their movements, and most advanced in their organization, of all the mollusca. The multilocular cephalopods have been separated, by d'Orbigny, into two subdivisions: first, those having a syphon or internal tube connecting the different chambers, such as the nautilus and ammonite; and, secondly, those without a syphon, to which the microscopic species now under consideration belong. They are often in an excellent state of preservation, and their forms are singularly different from those of the larger testacea. W c have given a plate of some of these, from unpublished drawings by M. Deshayes, who has carefully selected the most remarkable types of form. The natural size of each species :figured in plate 4, is indicated by minute points, to which we call the reader's attention as they might be easi.ly overlooked. ' Bones of quadrupeds in gypsum.-We have already con ... sidered the position of the gypsum which occurs in the form of a saccharoid rock in the hill of Montmartre at Paris, and other central parts of the basin. At the base of that hill it is seen distinctly to alternate with soft marly beds of the calcaire grossier, in which cerithia and other marine shells occur. But the great mass of gypsum may be considered as a purely fresh-water deposit, containing land and fluviatile shells, toO'cther with fragments of palm-wood, and great numbers 0 0f skeletons of quadrupeds and birds, an assemblage of orrranic remains which has given great celebrity to the Paris basin. 0 The bones o~ fr:sh- wa.ter fish, also, and of crocodiles, and many land and.fluviatlle reptiles occur in this rock. The skeletons of mamm~ l.Ia ar: usually isolated, often entire, the most delicate extremities bemg preserved as if the carcasses clothed with their flesh and ski~ had been floated down soon after death, and while they '~e.re still swoln by the gases generated by their first decom poSitiOn. The few accompanying shells are of those light kinds |