252 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 21, 



deposit, and the irregularity in this respect of the other group, we are, 

 I think, justified in maintaining over the whole area the divisions 

 which we have found more evident and marked further eastward, and 

 in assigning to the lower one a distinct and separate place in the 

 Eocene series. 



Bearing upon this question is another point of physical structure, 

 the details of which will he given at length in my next paper, but 

 which I may briefly allude to here. It has been mentioned that the 

 upper part of the Thanet Sands is usually light-coloured and very 

 uniform in texture, whilst in many places the lowest bed of the over- 

 lying division consists of rounded black flint-pebbles imbedded in a 

 coarse green sand. When their mineral characters are thus perfectly 

 distinct, the worn and indented line of junction of the two groups is 

 often very marked. Sometimes it seems as if the mud and pebbles 

 of the upper bed had been driven and splashed into the soft upper 

 surface of the underlying " Thanet Sands." All the phsenomena 

 point in the same direction, and, taken generally, go to prove a change 

 of conditions between the two periods. In the one case we have a 

 deposit, littoral probably, but distinctly marine, and in the other a 

 variable accumulation of coeval marine, sestuarine, and fluviatile 

 strata. 



At the same time it is to be observed, that when, as frequently 

 happens, neither division contains any fossils, if some of the other 

 distinctive features are wanting, and the upper group becomes more 

 arenaceous, lighter in colour, and the conglomerates more diffused, 

 the separation of the two groups is sometimes extremely difficult, for 

 tthey seem to pass one into the other and to form a series, the upper 

 part of which is hardly distinguishable in general appearance from 

 the lower *. It is only when the characters of one or the other be- 

 come, as it were, more concentrated, that the separation is clearly 

 marked. 



There is an objection to these subdivisions which may at once 

 strike those who have been accustomed to view the '^ Plastic Clay 

 Formation^' as a whole and as a series of recurring strata, arising 

 from the occurrence at the base of the "Thanet Sands" of Wool- 

 wich and Upnor, as well as of the " Mottled Clays" of Reading and 

 Newbury, of the layer of argillaceous greensand with the rough green- 

 coated flints, reposing throughout the range of both divisions imme- 

 diately upon the chalk, and presenting throughout very similar mineral 

 characters. This common character has always been considered an 

 argument in favour of the synchronism of the two groups. At the 

 same time it did not escape observation that the Ostrea Bellovacina 

 was never found in this bed in Kent, whereas it was common at 

 Beading and Newbury ; but as even elsewhere in the latter district 

 the occurrence of this fossil was by no means a constant character, 

 its absence altogether in the former district was naturally not con- 



* Even at the Reculver Cliff, notwithstanding the occurrence of organic remains, 

 this is particularly the case, owing apparently to the materials of the upper bed of 

 the Thanet Sands, having become mixed up with the lower part of the overlying 

 division. 



