Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/232

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Mr. Webster on the Strata lying over the Chalk.

the London clay, nor having been actually identified with any strata which have been found in sinking through it with certainty, leave the question still undetermined; and the agreement of the fossils of the Woolwich and Plumstead beds with those of the upper marine formation in the Isle of Wight, not only in their species but in their state of preservation, is sufficiently striking to suggest the idea of a similarity in the circumstances of their production.

The considerably greater elevation of Highgate-hill, and of other places known to consist of the London clay, above the Woolwich beds, is scarcely of itself a satisfactory reason for supposing that the latter exist under the former, since the changes to which this part from the chalk upwards has been subjected at various periods, are sufficiently pointed out by the extensive banks of pebbles of very ancient date, and the other proofs of the agency of water. The great irregularity in the surface of the chalk stratum is obvious from its disappearing so suddenly on the north bank of the Thames; and it is not difficult to imagine that from currents and other local causes, the deposition of the London clay might not have taken place in certain spots, which might yet have been covered by the sea that gave rise to the upper marine stratum on the Isle of Wight. It must be confessed however, that for want of proper sections, we have not yet sufficient data for determining this point with accuracy, and I shall content myself at present with having described such facts as I have myself noticed, leaving this subject to future investigation.