1S46.] PRESTWICH ON THE ISLE OF WIGHT TERTIARIES. 243 



eastward, and ending possibly in a point of comparatively slight 

 change and of nearly uniform deposit, — conditions applicable to the 

 contemporaneous strata of the central and eastern portions of the 

 London basin. 



We now enter upon what may be termed, for the sake of distinc- 

 tion, the Fluvio-marine deposits, forming lithologically a distinct 

 local group, exhibiting divisions of condition, but not of age, and 

 palaeontologically connected with and passing into the group below. 

 These beds have therefore no relation with those similar ones partly 

 of freshwater origin, which are known as the lower and upper fresh- 

 water and intervening marine formations of the Paris basin (groups 

 4, 5 and 6 of diagram 2). 



The argillaceous strata last described are suddenly succeeded by 

 a thick bed of sand without organic remains, but this bed is appa- 

 rently of marine origin, since Professor Sedgwick has shown that the 

 equivalent bed at Hordwell contains marine shells, and also that in 

 the lower part of Hempstead Cliff marine or estuary fossils predomi- 

 nate. Further, the lower marls and clays of the overlying formations 

 described as freshwater contain marine fossils. As this sand bed is 

 well-exhibited and well-known at Headon Hill, where it forms the 

 base of the hill, it may be convenient to term it the Headon Hill 

 sand. It was formed during a period when the sea became shallow, 

 estuaries replacing the open sea, and at a time also when a drift was de- 

 posited different from that previously accumulated ; marls and earthy 

 limestones succeeding the quartzose and green sands and pure clays. 



At White Cliff Bay we find in stratum No. 21, immediately over- 

 lying the Headon Hill sands, an assemblage of fossils, presenting a 

 singular mixture of marine, estuary and freshwater shells. Amongst 

 them are the following species: Cytherea incrassata, Valuta spiiiosa^ 

 Pleurotoma colon, Calyptrcca trochiformis, and other marine shells, 

 associated with the Potamides cinctusy Cyrena obovata, Paludina 

 lenta, Melania fasciata, &c. (and Planorbis according to Prof. E.. 

 Forbes and Captain Ibbetson). 



In stratum No. 24 we have the same estuary and freshwater- 

 shells, with only the Valuta and Cytherea among marine forms, 

 and, in addition to those species already quoted we find Nerithia^ 

 concava, and a Nutica identical with an unnamed species from tha 

 calcaire grossier of Damery. 



In 26 I only observed Cyrena ohovata and Melania fa sciata. 



In 28 the Potamomya plana occurs with the Potamides cinctuSf 

 Cyrena obovata, and Paludina lenta. 



In 29 and 31, Paludina lenta and Serptda tenuis are found ; and 

 in 32 the Limnoia and Planorbis. 



At Alum J3ay, or rather Headon Hill, this part of the series 

 (Nos. 31 to somewhere about S^) is mucii less developed, and, as I 

 have before mentioned, it affords only slight evidence of the })artially 

 marine conditions of the lower part, ending however, as at White 

 Cliff Bay, with well-characterized freshwater marls ami earthy lime-* 

 stones with Limna^a and Planorbis. . 



