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Show 328 SECONDARY FORMATIONS. [Ch. XXIII. genera distinct from those established for the classification of tertiary and recent fish. Chasm between the Eocene and Maest·richt formations.There appears, then, to be a greater chasm between the organic 1·emains of the Eocene and Maestricht beds, than between the Eocene and Recent strata; for there are some living shells in the Eocene formations, while there are no Eocene fossils in the newest secondary group. It is not improbable that a greater interval of time may be indicated by this greater dissimilarity in fossil remains. In the 3rd and 4th chapte~s we endeavoured to point out that we have no right to expect, even when we have investigated a greater extent of the earth's surface, that we shall be able to bring to light an unbroken .chronological series of monuments from the remotest eras to the present; but as we have already discovered a long succession of deposits of different ages, between the tertiary groups first known and the recent formations, so we may, perhaps, hereafter detect an equal, or even greater series, int_ermediate between the Maes. tricht beds and the Eocene strata. Duration of secondary periods.-Tbe different subdivisions of the secondary group No. l, extending from the chalk of Maestricht to the lower green-sand inclusive, may, perhaps, relate to a lapse of ages as immense as the united tertiary periods, of which we have sketched the eventful history in this volume. Such a conjecture, at least, seems warranted, if we can form any estimate of the quantity of time, by comparing the amount of vicissitude in animal life which has occurred during its lapse. Position of former continents.-The existence of sea as well as land, at every geological period, is attested by the re· mains of terrestrial plants imbedded in the deposits of all ages, even the most remote. We find fluviatile shells not unfrequently in the secondary strata, and here and there some fresh-water formations ; but the latter are less common than in the tertiary series. For this fact we have prepared the reader's mind, by the views advanced in the third chapter Ch. XXIII.] FORMER CONTINENTS WIIERE NOW SEA, 329 respecting the different circumstances under which we conceive the secondary and tertiary strata to have originated. 'Ve have there hinted, that. the former may have been accumulated in an ocean like the Pacific, where coralline and shelly limestone are forming, or in a basin like the bed of the western Atlantic, which may have received for ages the turbid waters of great rivers, such as the Amazon, and Orinoco, each draining a considerable extent of continent. The tertiary deposits, on the other hand, may have been accumulated during the growth of a continent, by the successive emergence of new lands, and the uniting together of islands. During such changes, inland seas and lakes would be caused, and afterwards filled up with sediment, and then raised above the level of the waters. That the greater part of the space now occupied by the European continent was sea when some of the secondary rocks were produced, must be inferred from the wide areas over which several of the marine groups are diffused ; but we do not suppose that the quantity of land was less in those remote ages, but merely that its position was very different. In the above tabular view of the secondary rocks, we have shown that immediately below the division No. I, or ' the chalk and green-sand,' is placed a fresh-water formation called, in the south-east of England, the Wealden. This group has been ascertained to extend from west to east (from Lulworth Cove to the boundary of the Lower Boulonnois) about 200 English miles, and from north ... west to south-east (from Whitchurch to Beauvais), about 220 miles, the depth or total thickness of the beds, where greatest, being about 2000 feet*"· Now these phenomena most clearly indicate, that there was a constant supply in this region, for a long period, of a considerable body of fresh water, such as might be supposed to have drained a continent, or a large island, containing within it a lofty chain of mountains. Dr, Fitton, in speaking of these appearances, recalls to our recollection that the delta of the new 1 y-discovered Quorra, or Niger, in Africa, stretches into the interior • Fitton's Geology of Hastings, p. 58, |