OLIGOCENE — INTRODUCTION. 127 



into Headon Group (cstuarine), Brockenhurst Series (marine), and 

 Benibridge Group (estuarine). 



Subsequently Messrs. Keeping and Tawney maintained that the 

 correlation of the marine beds of FTeadon Hill and CoKvell Bay 

 made by Forbes and the Survey was correct, and that the faunas 

 at the two spots were practically identical, the slight variations 

 being accounted for by the somewhat different conditions under 

 which the beds were deposited.^ 



Forbes' correlation is followed in this Memoir, for though there 

 are some minor points on which Prof Judd's criticisms are uo 

 doubt just, yet with regard to the main difference the recent re- 

 examination of the Island and mapping of the beds on the scale 

 of 6 inches to the mile have not supported Prof. Judd's contention, 

 but rather shown that Forbes' correlation must still be accepted. 



As already observed, the subdivision and grouping of the beds 

 in such a variable series of strata are, in the absence of any real 

 breaks, so entirely a matter of convenience, that without stronger 

 evidence it would be most unadvisable to upset the established 

 nomenclature, and introduce a new mode of grouping, founded on 

 that adopted in other districts. Here also the original nomencla- 

 ture and grouping used by Forbes have been adopted. 



The principal alteration in this new edition of the Memoir is 

 in the use of the term Oligocene for the whole of the Fluvio- 

 marine beds formerly known partly as Upper Eocene and partly 

 as Middle Eocene.t This term is universally adopted on the 

 continent, and the change of conditions at the base of the Fluvio- 

 marine series is so marked in the Isle of Wight, that the division 

 of our Lower Tei'tiary Beds into two, instead of into three 

 series, and the acceptation of the Headon Beds as the base of the 

 upper group is very convenient. Of course the rarity of fossils 

 in the underlying Headon Hill Sands leaves it still somewhat 

 uncertain to which group they should belong, but the marked 

 change of lithological character at the base of the overlying 

 beds, and the fact, recorded by Forbes, that the Sands contain 

 marine fossils of Barton species, is certainly in favour of their 

 being grouped with the Barton Clay. 



Table of the Oligocene Beds of the Isle of Wight. 



Feet. 



Hamstead Series - - - _ about 260 



Bembridge Marls - - - - „ 100 



„ Limestone - - - „ 10 



Osborne Series - - - - ,, 100 



Upper Headon Series - - - "] 



Middle Headon Series (marine) - - > „ 150 



Lower Headon Series - - -J 



Total ... 620 



* Quart. Journ. Geol, Soc, vol. xxxvii. p. 85. (1881.) 

 f Lyell referred the highest portion to the Miocene. 



