444 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 7, 



sidered it in this part of Europe. Admitting the infra-position of the 

 London Clay to the Bracklesham Sands and the Calcaire grassier, we 

 are thereby carried back to another and equally important Tertiary 

 period, one stage older than the group formerly placed at the base 

 of the Tertiaries ; whilst in the Thanet Sands we have a further reces- 

 sion, for only nine species out of the thirty-one found in this deposit 

 range up into the London Clay, and, as before mentioned, the general 

 aspect of the fauna is less distinctly tertiary*. Not only so, but Sir 

 Charles Lyell mentions the occurrence in the Lower Landenian of 

 Orp-le-Grand of a Cardiaster, a genus hitherto only met with in cre- 

 taceous strata, and further states that in the same beds at Tournay 

 the Ostrea lateralis occurs. In supporting the view of these beds 

 being intermediate between the Cretaceous and Tertiary, Sir Charles 

 observes that no Baculites, Belemnites, and Ammonites are found 

 in them, but justly suggests that the same may be said of the true 

 cretaceous strata in many other regions. The Pholadomya Koninckii 

 of the Thanet Sands can hardly be distinguished from a cretaceous 

 species, and Mr. Flower has recently shown me a specimen of the 

 Ostrea lateralis from those beds at the Reculvers, apparently iden- 

 tical with the Exogyra co7iica of the Gault and Chalk. 



At the same time, the physical changes which commenced during 

 the latter part of the Cretaceous period would seem to have been con- 

 tinuous with those which introduced the Tertiary epoch. Arise of apor- 

 tion of the sea-bed, accompanied by the destruction of a portion of the 

 already-formed chalk, and the wear of its flints into pebbles, preceded 

 the commencement of the Maestricht beds ; and a very analogous 

 change, but rather more strongly defined, marks the commencement 

 both of the Thanet Sands and of the Woolwich and Reading series, 

 each successive change being marked, not by a total change in litho- 

 logical structure, but rather by a somewhat gradual diminution in the 

 chalk-like and calcareous character of the strata, and by a successive 

 increase of arenaceous and argillaceous ingredients. The changes in this 

 respect, taking each stage separately, are on the whole of a like order, 

 and are as steps in the same direction. The divisional planes between 

 these stages are, however, abrupt, and indicate more or less consider- 

 able alterations in the sea area of these periods. 



The adaptation of this area at the Thanet Sands period to the ex- 

 istence of the numerous shallow-water burrowing Lamellibranchiates, 

 whatever the duration of the intervening time, woidd necessarily 

 unfit it for the deeper sea Cephalopoda, JJrachiopoda, and other 

 families which prevail in our Cretaceous series. But if we compare 

 the Panopcece, Pholadomyce, and other associated genera of tbe former 

 deposit with the similar groups which flourished under like conditions 

 at an early date of the Cretaceous period, then, although the interval 

 is infinitely greater, a close analogy of forms becomes apparent f. 



* It may be a question hereafter, when the fossils of the Thanet Sands have 

 been more thoroughly investigated, whether these strata may not form a separate 

 division in the Tertiary series, bearing probably the same relation to the Woolwich 

 beds and London Clay as these latter do to the Bracklesham Sands. 



t So marked in some instances is this likeness, that of the two specimens of the 



