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Show 248 EOCENE PERIOD. [Ch. XVIII. always produce very complex results; but in proportion as it is more difficult in these cases to discover any fixed order of superposition in the associated mineral masses, so also is it more easy to explain the manner of their origin and to reconcile their 1·clations to the agency of known causes. Instead of the successive irruptions and retreats of the sea, and changes in the chemical nature of the fluid and other speculations of the earlier geologists, we arc now simply called upon to imagine a gulf, into one extremity of which the sea entered, and at the other a large river, while other streams may have flowed in at different points, whereby an indefinite number of alternations of marine and fresh-water beds were occasioned. Second or Uppe·r ma·rine group.-The next group, called the second or Upper marine formation (No. 4 of the tables), consists in its lower division of green marls which alternate with the fresh-water beds of gypsum and marl last described. Above this division the products of the sea exclusively predominate, the beds being chiefly formed of micaceous sand, 80 feet or more in thickness, surmounted by beds of sandstone with scarcely any limestone. The summits of a great many platforms and hills in the Paris basin consist of this upper marine series, but the group is much more limited in extent than the calcaire grossier. Although we fully agree with M. C. Prevost that the alternation of the various marine and fresh-water formations before described admit of a satisfactot·y explanation without supposing different retreats and subsequent returns of the sea, yet we think a subsidence of the soil may best account for the position of the upper marine sands. Oscillations of level may have occurred whereby for a time the sea and a river prevailed each in their turn, until at length a more considerable sinking down of part of the basin took place, whereby a tract previously occupied by fresh water was converted into a sea of moderate depth. In one part of the Paris basin there are decisive proofs that during the Eocene period, and before the upper marine sand was formed, parts of the calcaire grossier were exposed to the Ch. XVIII.] P ARJS BASIN. 249 action of denuding causes. At Valmondois, for example, a deposit of the upper marine sandstone is found*, in which rolled blocks of the calcaire grossier with its peculiar fossils, and fragments of a limestone resembling the calcaire siliceux, occur. These calcareous blocks arc rolled and pierced by perforating shells belonging to no less than fifteen distinct species, and they are imbedded, as well as worn shells washed out from the calcaire grossier, with the ordinary fossils of the upper marine sand. W c have seen that the same earthquake in Cutch could raise one part of the delta of the Indus and depress another, and cause the river to cut a passage through the upraised strata and carry down the materials removed from the new channel into the sea. All these changes, therefore, might happen within a short interval of time between the deposition of two sets of strata in the same delta t. It is not improb'able, then, that the same convulsions which caused one part of the Paris basin to sink down so as to let in the sea upon the area previously covered by gypsum and freshwater marlJ may have lifted up the calcaire grossier and the siliceous limestone, so that they might be acted upon by the waves, and fragments of them swept down into the contiguous sea, there to be drilled by boring testacea. It is observed that the older marine formation at Laon is now raised 300 metres above the sea, whereas the upper marine sands never attain half that elevation. Such may possibly have been the relative altitude of the two groups when the newest of them was deposited. Third fresh-water formation.-We have still to consider another formation, the third fresh-water group (No. 5 of the preceding tables). It consists of marls interstratified with beds ?f flin~ and layers of flinty nodules. One set of siliceous layers Is destitute of organic remains) the other replete with them. * M. Desha yes, Memoires de la Soc. d' Hist. Nut. de Paris, tom. i. p. 243.~~ c sandstone is called, by mistake, gres marin inferieur, instead of supet·icur, to w uch last the author has since asccl'taincu it to belong. t Vol. i, 2u Edit. chap. xxiii.; vol, ii, lst Etlit. chaiJ· xvi. |
