1852.] PRESTWICH ON THE THANET SANPS. 23/ 



The sands which in Kent immediately overlie the chalk are here 

 formed into a separate division, as I believe them to be entirely of 

 marine origin, and distinct from the sands incumbent on the chalk at 

 Hertford, Reading, Newbury, and elsewhere westward of London, 

 but with which these Kent sands have been hitherto considered syn- 

 chronous. If such were the case, then certainly the mottled plastic 

 clays and light-coloured sands lying between the chalk and the Lon- 

 don clay at Reading might be regarded as the equivalent of the whole 

 group of strata reposing upon the chalk at Woolwich and Upnor ; 

 but it can, I think, be proved that the Reading and Hertford beds 

 are higher in the series than the Kentish sands, and that it is from the 

 gradual thinning-out of the latter, as they range westward, and not 

 by actual synchronous deposition, that this mottled clay group reposes 

 immediately upon the chalk in Berkshire, and occupies therefore rela- 

 tively to that formation the same position as the thick mass of lower 

 sands in Kent*. 



The grounds for this opinion will probably be better understood 

 from the description of the several groups rather than by a prior enu- 

 meration of abstract reasons. As before mentioned, I purpose com- 

 mencing in this instance, as more convenient for the general argu- 

 ment, with the lowest beds, viz. — 



" The Thanet SANDsf." 



I have used this term in consequence of these sands being best ex- 

 hibited, and marked by organic remains, in part of the Isle of Thanet 



ordinairement discontinus, vers la base et vers le haut de ce groupe, ont designe 

 celui-ci sous le nom de groupe de Vargile plastique ; mais cette expression, assez ■ 

 juste lorsqu'on ne considere que les environs de Paris, devient au contraire tout a 

 fait fausse lorsqu'on embrasse la totalite du bassin. On ne tarde pas a reconnaitre 

 en efFet que I'argUe plastique proprement dite n'est qu'un accident de quelques 

 metres d'epaisseur subordonne a la partie inferieure d'une masse sablonneuse qui 

 en a 80 et meme davantage." These observations will apply to a certain extent 

 to the English series ; but the beds of mottled clay are in this country far more 

 largely developed and hold a more important place in the Lower Eocene strata 

 than M. d'Archiac describes them to occupy in France. 



* Such is the difficulty of obtaining clear sections between the west of Kent and 

 Berkshire, that even now it would not be safe to pronounce as a certainty that the 

 whole of the Lower Tertiaries of the former county were of more recent origin 

 than the Thanet Sands. It is possible that a small portion of the lowest beds 

 may be synchronous with the latter. This, however, would only modify the divi- 

 sions here proposed, and would not affect their general correctness. 



t The only accounts we have of this lowermost bed are short abstracts of papers by 

 Mr. Morris, Proc. Geol. Soc. vol. ii. p. 595, " On the Coast section from Ramsgate 

 to Pegwell Bay ;" and again at p. 450, " On the Strata usually termed Plastic 

 Clay," wherein he describes the Woolwich and Upnor beds, and gives their relation 

 to the Bognor and Heme Bay strata, in the same volume both Dr. Mitchell (p. 7) 

 and Mr. Richardson (pp. 78 and 222) give some account of the cliffs at the Re- 

 culvers and Heme Bay, but these descriptions are very general. Brief notices of 

 these sands, as forming part of the Plastic Clay series, occur also in Phillips and 

 Conybeare's * Geology of England' (pp. 37-51), and in papers by Mr. Webster 

 (Trans. Geol. Soc. vol. ii. p. 196), by Dr. Buckland (Trans. Geol. Soc. vol. iv. 

 p. 284), and by the Rev. H. M. De la Condamine (Quart. Journ. Geol. Soc. vol. vi. 

 p. 440). 



VOL. VIII. PART I. R 



