Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/223

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213
Mr. Webster on the Strata lying over the Chalk.

The whole of the north shore of the Isle of Wight has been for ages in a state of constant ruin by the action of the sea and the sliding down of the soil. It is difficult therefore to find any past of the strata in their original situation; on this account freshwater and marine shells are frequently found together in confusion. I have however observed some places where they occur in alternate layers.[1]

The cerithia, cyclades, cytheræa, oysters, and other fossil shells, which are so numerous on the shore near Cowes, are derived from the blue clay of the upper marine formation, which is situated above that which we are now considering, and of which the sloping banks chiefly consist.

That occasional alterations and mixture of marine and freshwater shells should occur, might[errata 1], a priori, be expected. They would denote either the gradual nature of the change that has taken place in an arm of the sea before it became completely a lake of freshwater, or the occasional irruptions of the ocean at a subsequent period.

These beds may be traced a considerable way eastward of Ride, and I believe as far as Nettlestone, but I have not had an opportunity of ascertaining their precise boundaries. Neither have I been able to learn with certainty that any part of this formation is to be found on the coasts of Hampshire on the opposite side of the Solent; and I do not find it eastward on the Sussex side.

Woodward, however, in his valuable catalogue of fossils, frequently makes mention of freshwater shells in a marly stone from

  1. The only shells which I noticed thus alternating with those of freshwater, appeared to belong to the genus Cerithium, but not having specimens of them it has since occurred to me, and it will deserve future observation, whether these were not the Potamides Lamarkii. They are abundant at Gurnet Point.

Errata

  1. Original: may was amended to might: detail