[ 231 ] 



XXX. Proceedings of Learned Societies, 



GEOLOGICAL SOCIETY. 

 [Continued from p. 152.] 



February 4, 1880.— Henry Clifton Sorby, Esq., LL J)., F.K.S., 



President, in the Chair. 

 rPHE following communication was read : — 

 -*- " On the Oligocene Strata of the Hampshire Basin." By Prof. 

 John W. Judd, F.B.S., Sec.G.S. 



The study of the succession of strata in the fluvio-marine 

 series of the Isle of Wight and the New Forest is attended with 

 considerable difficulties, partly on account of the inconstant character 

 of the beds composing estuarine formations, and partly because of 

 the thick superficial deposits which everywhere cover them. By 

 "Webster a Lower Freshwater Series, a Middle Marine, and an 

 Upper Freshwater Series were recognized ; but Mr. Prestwich 

 showed, in the year 1846, that at Hamstead Cliff we have both 

 freshwater and marine strata lying above all these ; and in 1853 

 Edward Forbes proved that the marine and freshwater strata seen 

 at Bembridge Ledge were not, as had previously been supposed, the 

 equivalent of those of Headon Hill, but occupy a distinct and 

 higher horizon. Hitherto, however (in spite of some suggestions to 

 the contrary which were made by Dr. Wright and Prof. Hebert), the 

 strata exposed at the base of Headon Hill have been believed to be 

 a repetition, through an anticlinal fold, of those seen at Colwell and 

 Totland Bays. 



In the present memoir it is shown, both by stratigraphical 

 and paleeontological evidence, that the Colwell- and Totland-Bay 

 beds are distinct from and overlie those at the base of Headon Hill. 

 The distinctness and importance of the purely marine series ex- 

 posed at White cliff Bay, Colwell Bay, and several localities in the 

 New Forest is pointed out ; and it is shown that, among the 200 

 forms of Mollusca which they contain, only one fifth are found in 

 the Barton Clay below. For this important division of the strata 

 the name of the Brock enhurst Series is proposed. 



In consequence of the detection of an error in the accepted order 

 of succession of the strata, a rectification of the classification of the 

 fluvio-marine series is rendered necessary; and it is proposed to 

 divide them as follows : — 



1. The Hempstead Series (marine and estuarine), 100 feet. 



2. The Bembridge Group (estuarine), 300 feet. 



3. The Brockenhurst Series (marine), 25 to 100 feet. 



4. The Headon Group (estuarine), 400 feet. 



By this new grouping the strata of the Hampshire Basin are 

 brought into exact correlation with those of France, Belgium, 

 North Germany, and Switzerland ; and the whole series of fluvio- 

 marine beds in the Isle of Wight, which are shown to have a thick- 

 ness of between 800 and 900 feet, are proved to be the represen- 

 tatives of the Lower and Middle Oligocene of those countries. The 



