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Show 246 EOCENE PERIOD. [Ch. XVIII. part of the Paris basin, another contemporaneous deposit, of fresh-water origin, appears at the southern extremity. Calc:ai·re siliceux.-This group (No. 3 of the foregoing tables) is a compact siliceous limestone, which resembles a precipitate from the waters of mineral springs. It is, for the most part, devoid of organic remains, but in some places it contains fresh-water and land species, and never any marine fossils. The siliceous limestone and the calcaire grassier occupy distinct parts of the basin, the one attaining its fullest development in those places where the other is of slight thickness. They also alternate with each other towards the centre of the basin, as at Sergy and Osny, and there are even points where the two rocks are so blended together, that portions of each may be seen in hand specimens. Thus in the same bed, at Triel, we have the compact fresh-water limestone, characterized by its Limnei, mingled with the coarse marine limestone through which the small multilocular shell, called milliolite, is dispersed in countless numbers. These microscopic testacea are also accompanied by Cerithia and other shells of the calcaire grassier. It is very extraordinary that, although in this instance both kinds of sediment must have been thrown down together on the same spot, each still contains its own peculiar organic remains*· These facts lead irresistibly to the conclusion, that while to the north, where the bay was probably open to the sea, a maI ·ine limestone was formed, another deposit of fresh-water origin was introduced to the southward, or at the head of the bay. For it appears that during the Eocene period, as now, the ocean was to the north, and the continent, where the great lakes existed, to the south. F_rom the latter region we may suppose a body of fresh water to have descended charged with carbonate of lime and silica, the water being perhaps in sufficient volume to convert the upper end of the bay into fresh water, like some of the gulfs of the Baltic. Gypsum and marls.-The next group to be considered is "' M. Prevost has pointed out this limestone to me, both in situ at Triel, and in hand specimens in his cabinet. Ch. XVIII.] PARIS BASIN. the gypsum, and the white and green mat·ls, subdivisions of No. 3 of the table of Cuvier and Brongniart. These were once supposed to be entirely subsequent in origin to the two groups already considered; but M. Prevost has pointed out that in some localities they alternate repeatedly with the calcaire siliceux, and in others with some of the upper members of the calcaire grossier. The gypsum, with its associated marls and limestone, is in greatest force towards the centre of the basin, where the two groups before mentioned are less fully developed;· and M. Prevost infers, that while those two principal deposits were gradually in progress, the one towards the north, and the other towards the south, a river descending from the east may have brought down the gypseous and marly sediment. It must be admitted, as highly probable, that a bay or narrow sea, 180 miles in length, would receive, at more points than one, the waters of the adjoining continent; at the same time we must observe, that if the gypsum and associated green and white marls of Montmartre were derived from an hydrographical basin distinct from that of the southern chain of lakes before adverted to, this basin must nevertheless have been placed under circumstances extremely similar; for the identity of the rocks of Velay and Auvergne with the fresh-water group of Montmartre, is such as can scarcely be appreciated by geologists who have not carefully examined the structure of both these countries. Some of our readers may think that the view above given of the arrangement of four different sets of strata in the Paris basin is far more obscure and complicated than that first presented to them in the system of MM. Cuvier and Brongniart. We admit that the relations of the several sets of strata are less simple than the first observers supposed, b~ing much more ana~ logous to those exhibited by the lacustrine groups of Central France before described. The simultaneous deposition of two or more groups of strata in one basin, some of them fresh-water and others marine, must |