OLIGOCENE INTRODUCTION. 125 



Formations ; and Headon Hill was considered to comprise a com- 

 plete section of the whole of the Fliivio-marine series. Although 

 the calcareous strata in the upper part of Headon Hill were 

 noticed, the limestones of other parts of the Island were referred 

 to some of the thick beds of Lower Headon limestone displayed 

 at Headon Hill, and all the marine shells of the Fluvio-marine 

 series to his " Upper Marine " formation, or the Middle Headon 

 beds of Professor Forbes. Hence the limestones of Gurnard 

 Bay, East and West Cowes, and Binstead were referred to the 

 " Lower Freshwater " formations, while the " blocks of calcareous 

 stone containing Limnsea lying on the top, in a detritus of blue 

 clay," seen along the shore eastward of the latter locality, as also 

 the limestones of Dodpits and Bembridge, were considered iden- 

 tical with those of the " Upper Freshwater " formation, or the 

 thick limestones which are displayed in the Upper Headon beds 

 at Headon Hill. 



Mr. G. B. Sowerby visited Headon Hill in 1821 and inferred 

 that the Upper Marine formation had been deposited under 

 estuarine rather than under marine conditions, in consequence of 

 observing the occurrence together of shells of marine and fresh- 

 water genera."^ 



Professor Sedgwick, in a paper published in May 1822,t 

 referred all the strata exposed in the cliffs between Bembridge 

 Ledge and Ryde, between Ryde and Gurnard Bay, and also the 

 argillaceous beds between Yarmouth and Hamstead, to the 

 Lower Freshwater formation of Professor Webster; while the 

 oyster bed and marine marls overlying the Bembridge Limestone, 

 and the upper argillaceous beds of Hamstead, were regarded as 

 the equivalents of the Upper Marine formation of that author. 



Professor Prestwich showed, J in 1846, that there were no 

 grounds for the supposition of a want of conformity between the 

 series in Alum Bay and that in Headon Hill, and expressed an 

 opinion that no well-marked divisions could be drawn there, as 

 proposed by Webster,§ inasmuch as marine shells of the Barton 

 clays re-appear among the overlying freshwater strata in White- 

 clifd Bay, and that the same freshwater species ranged through 

 nearly the whole thickness of the Headon Hill deposits; the 

 phenomena being such as might be purely local, the result of an 

 accidental irruption of brackish water into a freshwater area. 



With respect to the age of the fluvio-marine series of the Isle 

 of Wight, and their synchronism with the deposits of the Paris 

 basin, Mr. Prestwich stated that he felt considerable hesitation in 

 hazarding an opinion ; but, guided by the circumstance that all 

 French and English geologists were agreed in referring the 

 Barton group to the Calcaire grossier, as also by the consideration 

 of the upward range of the Barton species, he was disposed to 



* Ou the Geological Formatious of Headon Hill. . . , Aim. Phil., ser. 2 

 vol. ii. p. 216. 



f On the Geology of the Isle oF Wight. Ann. PIuL, vol. xix. p. 32'J. 



X Quart. Journ. Geol. Soc, vol. ii. pp. 223-259. 



§ Lower freshwater Upper marine, Upper freshwater. 



