Mr. Webster on the Strata lying over the Chalk. 185 



The last mentioned sand is the lowest stratum there visible. It 

 is above 30 feet in thickness, beautifully vvrhite, and in it several 

 pits are annually dug, from which the manufactories are supplied 

 v/ith their materials for the best flint glass. This sand may be 

 traced round the foot of the hill on the north side, and forms the 

 bottom of Totland and Col well bays, dipping gradually to the north. 



>Over it lies a horizontal bed of black clay E which contains 

 fossil shells, and sometimes selenite. 



Upon reviewing the whole of this lower marine series of strata 

 in Alum bay, and comparing it with other sections of the strata im- 

 mediately over the chalk, we shall find it useful, for the present at 

 least, to separate it into two great divisions: 1. Sand and plastic 

 clay, 2d. London clay. From the irregularities in the beds in the 

 few places where there are good sections, these divisions however 

 can as yet scarcely be considered as distinctly determined. Thus 

 much is certain, that the plastic clay and sand is always below and 

 never above the London clay. Other sub-divisions may be intro- 

 duced when future observations shall shew them to be sufficiently 

 important. 



\. Sand and Plastic Clay. From the constant and abundant sup- 

 ply of water which is found on boring through the London clay, 

 and from the accounts of the proprietors of the numerous pits of 

 plastic clay in Dorsetshire, the sand must be considered as the 

 most extensive and continuous formation, and the clay as filling up 

 basins or hollows in it. Hence, as may naturally be expected, 

 we find each of these substances in different places in immediate 

 contact with the chalk. In the Isle of Wight clay is next to it, but 

 in the numerous sections on the banks of the Thames, sand is the 

 lowest, or the clay is wanting. 



The beds of plastic clay in the Isle of Wight are of unusual ex- 

 Vol. II. 2 a 



