184?6.] PRESTWICH ON THE ISLE OF WIGHT TERTIARIES. 245 



testacea, wliich we have therefore traced throughout the entire 

 series of these fluvio-raarine strata, both at Headon Hill and White 

 Cliff Bay. It will be observed, however, that, on the whole, marine 

 conditions are more predominant at the latter than at the former 

 place. 



In these remarks, having merely in view the unity of the series, I 

 have only given the characteristic and more abundant fossils, worked 

 out and marked on the spot from the different beds. The sequence 

 appears complete and unbroken, the whole series consisting of de- 

 posits more or less marine and estuary, with interpolated freshwater 

 beds, all characterized respectively by the same organic remains, 

 varying only according to the variable conditions of the waters; at 

 one time the abode of the Potamides, Melania, Cytherea, Neritina, 

 Cyrena, and Fusus ; and at another, more particularly, of the Pla- 

 norbis and LimncEa. All these fossils range at intervals throughout 

 the series, associated, when under predominating marine conditions, 

 with other marine and estuary genera, and exhibiting no break 

 in the sequence beyond that a\ hich would result from a contem- 

 poraneous fauna, subject to variations of marine and freshwater 

 agencies. 



With respect to the age and synchronism of these fluvio-marine 

 deposits, I feel considerable hesitation in hazarding an opinion. 

 We still want further evidence, and we especially require a more 

 extended and careful comparison of the marine testacea with those 

 of the Paris basin. At an early period in the history of tertiary 

 geology they were considered to represent the lower freshwater, 

 upper marine and upper freshwater formations of Cuvier and Bron- 

 gniart; and this was assumed, first, from a supposed unconform- 

 able superposition on the London clay ; secondly, from their having 

 been accumulated, like the Paris series, under predominating fresh- 

 water conditions, and exhibiting a very similar sequence of strata ; 

 and, thirdly, from analogy of organic remains. 



With respect to the first reason, there can, I think, be little doubt, 

 as I shall endeavour to prove further on, that all the beds of the 

 series, from the chalk to the top of the fluvio-marine beds, are in 

 conformable stratification. 



With regard to the second reason, it is now known that analo- 

 gous freshwater conditions prevailed to some extent in the Paris 

 basin prior to this period ; for the upper strata of the calcaire gros- 

 sier are frequently argillaceous, and contain subordinate beds of 

 white and green freshwater marls, sands, and fine earthy limestones, 

 closely resembling some of the overlying freshwater deposits, and, 

 like tiiem, containing also several species of Paludina^ Limncca^ 

 Planorbis^ and Melania^ showing evidence therefore of the com- 

 mencement of freshwater conditions in the upper (ralcaire grossier 

 — conditions traceable also occasionally in the gres de Beauchamp, 

 and subsequently increasing in force, and ranging, with t.light inter- 

 ruptions, to the completion of the Paris beds*. 



* Constant Prevost has ably argued in support of his theory of aflluents, that 



