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



have siifFereci 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, knov/n 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 330 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 Plantse 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 dif- 

 ferent species of fossil fruits, berries, and ligneous seed vessels. 



Vol. II. 2 b 



