Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/227

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217
Mr. Webster on the Strata lying over the Chalk.

the north side of the Isle of Wight, and may be seen at Cowes, Ride, and Bembridge.

In the enumeration of the various strata in the basin of Paris, our attention is particularly called to a thin bed of bivalve shells covering the upper beds of gypsum and gypseous marls of the lower freshwater formation, the shells of which are placed close, and as it were locked into each other. This bed, though apparently of little importance, is remarkable by its great extent; having been observed over a space of more than 10 leagues in length, and more than 4 in breadth, and always retaining the same situation and thickness. It is still farther distinguished as marking out the beginning of the new series of strata deposited by the sea.

Over this thin bed of bivalves there is one of greenish marl without fossils; and then several alternating beds of argillaceous and other marls and sands, containing marine fossils, shells, and bones of fish. Two beds of oysters are particularly noticed; and it is observed that these must have lived upon the spot where they now are, because they are locked into each other as in their natural beds, with many of their hinges entire. The uppermost beds of marls are however not constant; sometimes, as at Chellis, there is only a thin bed of sand between the green marl and the upper freshwater formation. It is in these marls that the silex menilite is found.

The changes which the surface of the earth has experienced have no doubt destroyed so much of the last depositions in the London as well as in the Isle of Wight basins, that it is impossible to say what the complete series was originally.

Of the existence of this upper marine formation therefore in the London basin, I must speak with diffidence, more particularly since an extensive freshwater formation might never have existed

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