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 be prepared to find that the simultaneous deposition of two or more sets of strata in one basin, some freshwater and others marine, must have produced 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 relations 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 are 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 freshwater beds would be occasioned. LOWER EOCENE, FRANCE. _Lits coquilliers_ (3. _a_, Table, p. 175.).--Below the calcaire grossier are extensive deposits of sand, in the upper parts of which some marine beds, called "lits coquilliers," occur, in which M. d'Archiac has discovered 200 species of shells. Many of these are peculiar, but the larger portion appear to agree with species of the calcaire grossier, so that the line of demarcation usually adopted between the French Lower and Middle Eocene formations, seems not to be very strongly drawn. _Sands and plastic clay_ (3. _b_, Table, p. 175.)--At the base of the tertiary system in France are extensive deposits of sands, with occasional beds of clay used for pottery, and called "argile plastique." Fossil oysters (_Ostrea bellovacina_) abound in some places, and in others there is a mixture of fluviatile shells, such as _Cyrena cuneiformis_ (fig. 187. p. 204.), _Melania inquinata_ (fig. 188.), and others, frequently met with in beds occupying the same position in the valley of the Thames. Layers of lignite also accompany the inferior clays and sands. Immediately upon the chalk at the bottom of all the tertiary strata there is often a conglomerate or breccia of rolled and angular chalk flints, cemented by siliceous sand. These beds appear to be of littoral origin, and imply the previous emergence of some portions of the chalk, and its waste by denudation. [Illustration: Fig. 169. _Cardium porulosum_. Paris and London basins.]