1852. J PRESTWICH ON THE THANET SANDS. 253 



sidered very material*. It seems to me, however, that the oysters 

 are not part of the flint bed, but were deposited upon it — sometimes, 

 where Httle sand intervened, in contact with, or even partly amongst, 

 the flints — at other times, and which is more commonly the case, as 

 a separate and distinct layer overlying the flints. I believe that this 

 oyster bed belongs to the lowest part of the central division of the 

 " Lower Tertiaries." It has already been observed that this part of 

 that group often consists of coarse argillaceous green sands mixed with 

 more or fewer round flint-pebbles ; therefore when this stratum comes 

 into contact vdth the one containing the green-coated flints, the simi- 

 larity of che matrix in either case is such as to render it diflicult to 

 draw any line of demarcation between them. Now with the oysters 

 are almost always associated small rounded flints, which do not occur 

 with the green flints beneath the " Thanet Sands," and the two forms 

 of the flints exhibit the results of physical causes so entirely distinct 

 that they cannot possibly be the result of the same agency : the enor- 

 mous wear necessary to produce so perfect a form as the ordinary 

 round flint-pebble could never have left the great, massive, angular, 

 green-coated flints as it were unscathed ; nor could the powerful but 

 transient action necessary to uproot these flints from the chalk have 

 sufliced to wear down and give finish to the more perfectly rounded 

 pebbles. These latter, after having been worn down elsewhere, must 

 have been spread over the former at a subsequent period, and mingled 

 with the Ostrea Bellovacina which was then living on the spot. 



Still it is evident, from the peculiar and distinctive character of 

 these large angular flints lying on the chalk throughout the tertiary 

 area, that their accumulation is attributable to one and the same 

 cause — that their uprooting and dispersion must have been contem- 

 poraneous over the whole district, xlfter this they were, in the in- 

 stance of the Kentish area, covered by a thick deposit of sands, which 

 did not extend into the Berkshire areaf (fig. 1 & 3, supra), or else 

 they have been subsequently denuded throughout the latter district 

 and this basement-bed alone left, and re-covered immediately by the 

 lower beds of the upper series, into the composition of which coarse 

 green sands so often largely enter, forming therefore with the green- 

 coated flint-bed a consecutive mineral series, not distinguishable from 

 one another when viewed as a local phseuomenon J. This thinning 



* At the same time the Ostrea Bellovacina was known to occur at Woolwich, 

 in the upper and middle beds ; at Bromley in some intermediate beds ; and at 

 Northaw and Hertford in beds immediately upon the chalk and under a con- 

 siderable thickness of sands — facts supposed to show the irregular dispersion of 

 this shell throughout the whole series, as well as the irregular grouping and distri- 

 bution of the strata. 



f The green-coated flints extend into the Hampshire tertiary district. There- 

 fore the sea of the earliest tertiary time may have extended over all this area, 

 although the accumulation of strata was afterwards interrupted, or the Thanet 

 Sands have been denuded. 



% Since writing the above, I am however informed by Mr. Lunn that he has 

 found, in the green-flint bed immediately over the chalk at Charlton, remains of 

 shells, apparently oysters, but in too fragmentary a condition for exact determina- 

 tion ; also that the workmen showed him a large and perfect specimen of an 

 oyster which they said came from this bed. This, however, is an exceptional case, 



VOL. VIII. — PART I. S 



