244 PROCEEDINGS 01? THE GEOLOGICAL SOCIETY. [March 20, 



land rising from beneath the wide-spread ocean of the Oxford Clay 

 and remaining but partially submerged during the accumulation of 

 the Portland strata, gradually enclosed as it were within itself one 

 or more centres of depression. These, cut off, it may be, at first but 

 partially from the retreating ocean, and receiving as catchment-basins 

 the accumulated drainage of the older and newer lands, passed gra- 

 dually and with many changes of condition from inland seas or 

 branches of the sea to wide but shallow lakes — the Wealden lakes, 

 as I would venture to suggest, of England and of Hanover. 



It would be difficult, perhaps impossible, to determine the precise 

 period at which such land had attained its greatest ascendancy, 

 whether directly at the close of the Portlandian era or at some later 

 time ; and it matters little to my present purpose when such period 

 may be fixed. Certain it is that with the close (or, more correctly, 

 towards the close) of the Wealden epoch, the gain was once more 

 on the side of the ocean. 



Allowing, then (as all will, I think, allow), that the gain of the 

 ocean on the land at the close of the Wealden period involved sooner 

 or later the whole Wealden area of the south-east of England, the 

 second object of this paper is to show that the intrusion of such 

 ocean into the Wealden basin did not at once, or for a long period, 

 cut off the flow of rivers into what had now become a part of the 

 Neocomian sea. 



There are thus two separate suggestions which I propose to put 

 forward in this paper in relation to the Wealden and its rivers : — 



1st. That the Wealden strata are a fluvio-lacustrine rather than 

 a purely fluviatile or fluvio-estuarine deposit ; 



2nd. That the Wealden rivers continued in existence, although 

 probablj- in much diminished volume, during the accumulation of 

 most of the succeeding Neocomian strata. 



1. In attributing to the Wealden formation a fluvio-lacustrine 

 origin, it is necessary to refer to a nearly similar origin the greater 

 portion of the Purbeck strata ; and this I have no hesitation in 

 doing. The adjacent land-surfaces were probably the same during 

 both these periods ; the drainage-area was, there is little doubt, also 

 virtually the same. And, excepting that the species altered with 

 the flow of time, the fauna of the one formation is, with few excep- 

 tions, the counterpart of the other. 



The Purbeck strata, as a rule, contain little evidence of direct 

 fluviatile action. Drift wood, or the remains of vegetation, is rarely 

 to be found in them, except in direct connexion with a terrestrial 

 surface. The insects and insectivorous mammalia whose remains 

 are occasionally present were probably inhabitants of the imme- 

 diately adjacent shores, of sand-dunes, or of some small islands. 

 Few geologists can regard the Purbeck marble as other than a quiet- 

 water, if not a lacustrine, deposit; and the resemblance of the " lime- 

 stone with flint nodules " of the Purbeck to the freshwater lacustrine 

 strata of Cantal, in Central Erance *, is certainly remarkable. 



It was shown by Professor Eorbes, in his description of the Dorset- 

 * Ljellj El. of GeoL, " Lacustrine strata of Auvergne." 



