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Show 242 EOCENE PERIOD. (Ch. XVIII, SUO'O'ested the very natural idea that there existed formerly a 00 f chain of lakes, reaching from the highest part o the central mountain-O'roup of France, and terminating in the basin of Paris, whi~h he supposes was at that time an arm of the sea. Notwithstanding the great changes which the physical geo~ graphy of that part of France must since have undergone, we may easily conceive that many of the principal features in the confiO'uration of the country may have remained unchanged, 0 or but slightly modified. Hills of volcanic matter have indeed been formed since the Eocene formations were accumulated, and the levels of large tracts have been altered in relation to the sea ; lakes have been drained, and a gulf of the sea turned into dry land, but many of the reciprocal relations of the different parts of the surface may still remain the same. The waters which flowed from the granitic heights into the Eocene lakes may now descend in the same manner into valleys once the basins of those lakes. Let us, for example, suppose the great Canadian lakes, and the gulf into which their waters are discharged, to be elevated and laid dry by subterranean movements. The whole hydrographical basin of the St. Law~ renee might be upraised during these convulsions, yet that river miO'ht continue, after so extraordinary a revolution, to b drain the same elevated regions, and might continue to convey its waters in the same direction from the interior of the continent to the Atlantic. Instead of traversing the lakes, it would hold its course through deposits of lacustrine sand and shelly marl, such as we know to be now forming in Lakes Superior and Erie; and these fresh- water strata would occupy the site and bear testimony to the pristine existence of the lakes. Marine strata might also he brought into view in the space where an inlet of the sea, like the estuary of the St. Lawrence, had once reeeived the continental waters; and in such formations we mjoo ·ht discover shells of lacustrine and fluviatile species inter-mingled with marine testacea and zoophytes. . Subdivisions of strata in the Paris basin.-The area which }las been c.alled the Paris basin is about one hundred and Cll. XVIII.] PARIS BASIN. 243 eighty miles in its greatest length from north-east to southwest, and about ninety miles from east to west. This space may be described as a depression in the chalk (see diagram No. 2, p. 16), which has been :filled up by alternating groups of marine and fresh-water strata. MM. Cuvier and Brongniart attempted in 1811 to distinguish five different formations, and to arrange them in the following order, beginning with the lowest:- . {Plastic clay. 1. First fresh-water formation, , . . Lignite. First sandstone. 2. First marine formation •.• , • , • Calcaire grossier. { Siliceous limestone. 3. Second fresh-water formation , • Gypsum, with bones of animals. Fresh-water marls. { Gypseous marine mads. 4. Second marine formation •••• , • Upper marine sands and sandstones. Upper marine marls and limestones. { Siliceous millstone, without shells. , 5. Third fresh-water formation, Siliceous millstone, with shells. , Upper fresh-water marls. These formations were supposed to have been deposited in succession upon the chalk; and it was imagined that the waters of the ocean had been by turns admitted into and excluded from the same region. But the subsequent investigations of No. 58. No. 59, M. Constant Prevost. |