Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/203

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193
Mr. Webster on the Strata lying over the Chalk.

have suffered considerable diminution. The southern half of the island is entirely alluvial, being only a few feet above the level of the sea; and owes its origin to the gradual filling up of the channel which separates the Isle of Sheppey from the rest of Kent. It now consists of flat marsh land, which has been gained from the sea by embankments.

A little to the north of the eastern point, called Shellness, from the great number of recent shells that lie on the shore, a low cliff exhibits the section of the alluvial soil, which consists of clay and gravel. At Warden the high cliff begins on the east, and extends towards Sheerness on the west above six miles in length.

The clay of which these cliffs are composed is in all respects similar to that which has been cut through in the neighbourhood of London at Highgate, and at the Regent's Park; and this place is particularly known on account of its furnishing abundance of the septaria, from which that excellent material for building under water and for stucco is made, known by the name of Parker's cement. These nodular concretions of stone-marl are separated from the clay by the action of the sea, and are collected upon the beach, and exported to various places, where they are calcined and ground.

At Sheerness a well was sunk 380 feet through the blue clay, an account of which is in the Philosophical Transactions: and from this we may obtain an idea of the thickness of the stratum: for to this must be added 200 feet, the height of the cliffs, making in all 550 feet.

The cliffs of Sheppey have long been celebrated for the numerous organic remains found in them, a list of which, added by Mr. Jacob to his Plantæ Favershamienses, is well known. But a much more extensive collection has since been formed by Mr. Francis Crow, of Feversham, who has enriched it by the addition of above 700 different species of fossil fruits, berries, and ligneous seed vessels.


Vol. II.2b