Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/195

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

The last mentioned sand is the lowest stratum there visible. It is above 30 feet in thickness, beautifully white, and in it several pits are annually dug, from which the manufactories are supplied with 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 Colwell 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 immediately 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 introduced when future observations shall shew them to be sufficiently important.

1. Sand and Plastic Clay. From the constant and abundant supply 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 extent

Vol. II.2 a