Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/225

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Mr. Webster on the Strata lying over the Chalk.

in the gypsum being thicker, and in the occurrence of a bed of very indurated clay containing fossil fish. The third or uppermost mass is the most important, the beds of gypsum being very thick; it is here alone that the bones of unknown birds and quadrupeds are met with. Freshwater shells are also found in it, though rarely.

Over the gypsum are very thick beds of calcareous and argillaceous marls; one of the latter, which is white and friable, contain silicified trunks of palm trees, and in the former are found many freshwater shells. In this freshwater formation are found neither the meulieres, nor any other flints, except the silex menilites, and the hornstone of the upper gypsum beds.

The total absence of beds of gypsum and of the remains of quadrupeds, in this formation in the Isle of Wight, (at least as far as my observations extend) exhibits a striking proof, that although probably of contemporaneous origin, yet the circumstances accompanying its formation were very different from those under which that of the basin of Paris was produced. The gypsums of Montmartre are well known; and they are considerably different in their appearance from those of England, which belong only to strata far below the chalk. It has been already observed, that selenite is very abundant in our blue clay; but this arises from the decomposition of the pyrites contained in it.

§ 4. Upper Marine Formation.

Over the lower freshwater formation in the Isle of Wight, a stratum occurs, consisting of clay and marl, which contains a vast number of fossil shells wholly marine. Few of these shells agree with the species that have been found in the London clay, and they are also considerably different from them in their state of {hws|preserva|preservation}}