248 PBocEEDiNas OF THE GEOLOGICAL HociEXY. [March 20, 



bably a new species of Ceritliium, but subsequently figured by Man- 

 tell under the name of Potamides carhonarius (Mant. Geol. Isle of 

 Wigbt, pi. vi. f. 5). It is apparently rare at Atherfield, but occurs 

 abundantly in the same stratum at Sevenoaks and Haslemere. The 

 mode of its occurrence is peculiar : the shells are frequently eroded 

 or waterworn as to their surface ; they are often broken, and exhibit 

 the appearance of having been washed or drifted into their present 

 position. I believe the shell to have been in habit a true Potamides 

 rather than a Ceritliium, as suggested by Professor Forbes*, that it 

 was semiaquatic and was forced into its present position at the time 

 of the irruption of the ocean into the "Wealden area. A small species 

 of Ostrea, a Cardium, and a species of Corhida or Potamomyci occur 

 occasionally in this passage-bed at Sevenoaks and Haslemere, and 

 also at a lower level in the Isle of "Wight, and clearly indicate, 

 where present, the temporary prevalence of brackish water. I shall 

 have to refer again to these species, and to the conditions under 

 which they occur. 



The lowest portion of the succeeding (Neocomian) stratum, which 

 rests upon the passage-bed I have just described, includes, in the words 

 of Fitton, " a large quantity of a kind of gravel containing numerous 

 fragments offish-bones." It is just such an accumulation of sediment 

 as would result from the dispersion of shore-deposits over the floor 

 of a moderately deep lake. The fish-bones mentioned by Fitton are 

 those possibly of inhabitants of the "Wealden waters ; their presence 

 at the junction of the two formations may be due to the suddenness 

 of their destruction by the change from fresh te salt water. 



2. With respect to the second subject of this paper, which relates 

 to the existence of the Wealden rivers after the close of the Wealden 

 era, there is unfortunately much difficulty in obtaining evidence. 

 The existence of such rivers during the accumulation of the Neoco- 

 mian deposits has been indeed inferred by Fitton and other eminent 

 geologists ; but to the present moment proof is wanting. It is, 

 however, just one of those points in geological history on which, 

 proof being wanting, indirect evidence becomes important ; to obtain 

 such evidence, one must look back in the first place from the I^eoco- 

 mian to the AVealden era. 



The close of the Wealden formation, as seen at Atherfield, Seven- 

 oaks, Haslemere, and other points in the south-east of England, 

 resulted, as I have supposed, from the depression of a lacustrine 

 area beneath the sea-level. I have supposed that such depression 

 was effected gradually, but that the final intrusion of the ocean into 

 the Wealden basin was a sudden intrusion, such a one as might have 

 resulted from the destruction of some previously existing barrier 

 between the ocean and a freshwater lake or inland sea. 



Now I have shown, and it is besides a well-known fact to most 

 geologists, that the freshwater Wealden beds of Kent, Surrey, Sussex, 

 and Hampshire are everywhere at once succeeded by the marine 

 "Atherfield beds" of the English Neocomian. It has been shown 

 by Mr. Godwia-Austen and others that these " Atherfield beds " are 

 * Mem. Geol. Survey (18G2), p. 18. 



