278 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 23, 



northern. It was at tliis period that the great break appears to 

 have taken place between the Enghsh and French tertiaries ; for up to 

 this point, the sands, mottled clays, and fluviatile beds are common 

 to both countries, and present much similarity of structure. But at 

 this level the resemblance ceases ; the London clay sets in with its 

 great argillaceous character, whilst in France this period is succeeded 

 by a series of Hght-coloured, calcareous, and generally very fossili- 

 ferous sands and earthy limestones. I do not think this separation 

 of the two districts to have been caused by the elevation of the 

 Wealden : in the first place, because the London clay is found in 

 nearly equal force on both sides of the Wealden elevation ; and 

 in the second place, because there is evidence (see Sections 4, 5, 

 and 6) that that elevation affected these Eocene strata equally with 

 the secondary ones. It would appear therefore, on physical grounds, 

 that the denudation acted from the southward, while it has been be- 

 fore shown, on palseontological evidence, that in this same direction 

 there was probably a considerable rise of the sea-bottom, accompanied 

 by a slight subsidence to the northward. To the disturbing action 

 of the waters flowing off from the sea-bottom thus raised to the 

 southward, I attribute the first spread of the basement bed of the 

 London clay and the partial denudation of the underlying strata. 

 The elevation of the bed of the sea was not sufiicient to convert it 

 into dry land, nor was the change of that violence to destroy, over 

 the whole area acted upon, the animal life of the period. In distant 

 or in more sheltered parts of the sea, as before mentioned, some of 

 the testacea which inhabited it were preserved and transmitted into 

 the deposits formed subsequently to these changes. 



The basement bed of the London clay contains altogether thirty- 

 one known species of testacea, and apparently eight to ten undescribed 

 species. In Hampshire and in the western division of the London 

 district there are in the imderlying strata no remaining traces of any 

 older stock whence this new fauna could have been derived. From 

 London, however, through Woolwich to Upnor, this bed reposes upon 

 fossihferous fluviatile beds, and here apparently there seems to be a 

 transmission upwards, from one period to another, of some of the 

 species, as the Cyrena obovata, C. cuneiformis, C. tellinella, Ceri- 

 thium variahile, Melania inquinata, and Pectiinculus Plumsteadi- 

 ensis, which abound in the estuary and fluviatile beds of Woolwich*. 

 Some of the species also from the lower marine deposits of Heme 

 Bay and Sandwich range upwards into the "basement bed," as the 

 Corbula revoliita, C. longirostris, Natica glaucmoides, and Pectun- 

 culus Plumsteadiensis, and probably the Cyprina Morrisii. There 

 is this difference, however, between the species introduced from the 

 underlying beds, and those which constitute the typical and universal 

 group which will shortly be alluded to, viz. that the fonner are, with 

 the exception possibly of the Cyprina Morrisii^ confined in their 

 range to a limited region, surmounting or not extending much be- 

 yond {i. e. at this period) that previously occupied by them, whilst 

 the latter have a general and unlimited range. 



* Out of these six species four even lived on to the period of the freshwater 

 series of the Isle of Wight, 



