222 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 co- 

 vered 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. 



