144 



GEOLOGY OF THE ISLE OF WIGHT. 



to a Inrgc extent the admixture oF freshwater shells which is so 

 conspicuous at the west end of the Island. The tufaceous fresh- 

 water limestones have all died out, and most of the purely 

 freshwater beds seem to be largely replaced by beds of estuarine 

 origin. However, the occurrence of derivative frao-ments of the 

 underlying freshwater clays at the base of the marine beds, shows 

 that the thinning out of the lower series may be due to actual 

 erosion, and not to a replacement by contemporaneous beds of 

 marine origin. Messrs. Keeping and Tawney record the occurrence 

 of a similar line of erosion at the base of the Brockenhurst Beds 

 in the New Forest. 



In Whitecliff Bay two principal horizons in the marine beds 

 yield most of the fossils. The lowest zone is about 30 feet from 

 the base of the Headon Series and the greater part of the fossils 

 are crowded into a seam a few inches thick. The most abundant 



species are the Ostrea, Nucida, Gav 

 dita acuticosta, (Jytherea incrassata 

 (Fig. 37). Pleurotoma, and Valuta 

 spinosa. 



The other bed is a shaly clay about 

 90 feet higher. This latter seems to 

 correspond with the " Venus Bed " 

 of Colwell Bay, and contains a similar 

 assemblao;e of fossils. Amono; the 

 common species are Gytherea incras- 

 sata, Gorhula. dcltoidea, Ostrea, Sa.ii- 

 guinolaria, Cerithium pseudo-cinctum, 

 Voluta spinosa, &c. 

 A large number of the marine mollusca of the Headon Beds 

 range downwards into the Barton Clay, but about half are peculiar 

 to the Oligocene. This apparent break between the Eocene and 

 tliO Oligocene will probably disappear when the marine fossils of 

 the Lower Headon Beds and of the Headon Hill Sands are better 

 known, but at present it is sufficiently marked. 



Gytherea incrassata^ 



Fig. 37. 



Cytherea incrassata, Desh 



Fig. 38. 

 Ostrea Jiahellula, Lam. 



though especially abun- 

 dant in the Middle Hea- 

 don Series, has a some- 

 what extended range, 

 from the Barton Clay to 

 the Bembridge Beds. It 

 gives the name to the 

 well-known "Venus bed " 

 of collectors, the Gytherea. 

 having formerly been 

 known as Venus incras- 

 sata. Among the other 

 abundant marine bivalves 

 may be mentioned the 

 Ostrea velata, which forms thick banks in Colwell Bay, and the 

 Ostrea Jlahelhda (Fig. 38), a much scarcer species which ranges 



