250 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 25, 



To view in its strongest light the palaeontological evidence in fa- 

 vour of the supposed synchronism of the Isle of Wight freshwater 

 or fluvio-marine strata with the freshwater strata of the Paris biasin, 

 let us now, without breaking the fossils into groups according to the 

 several divisions .of these deposits, consider generally all the fossils of 

 this series in the two countries, and see what identity can be esta- 

 blished between them. About 400 species are known in these groups 

 in France, and we have found about eighty in England. Now out of 

 this number it appears that not more than ten or twelve species are 

 common in these strata to the two countries; that not one-half of this 

 small number can be regarded as characteristic of the beds over the 

 calcaire grossier, while the majority of these common species range 

 downwards, some into the calcaire grossier and some as low as the 

 plastic clays : whereas a comparison of the fauna of these Isle of 

 Wight fluvio-marine strata with that of the calcaire grossier has 

 shown that out of the thirty-one species having French analogues, as 

 many as twenty-four are found in and below the calcaire grossier, and 

 some of these are characteristic forms. It has also been shown that as 

 many as twenty-two or twenty-three species of this fluvio-marine 

 fauna have been found in and below the London clay ; that several 

 of the species most common in the upper group at White Cliff Baj"- 

 are met with in the middle group; and that no positive line of separa- 

 tion can be drawn there between the London clay and overlying beds, 

 but that they pass palaeontologically one into the other. 



From this evidence, I am therefore inclined to consider the fresh- 

 water and estuary strata of the Isle of Wight to be synchronous 

 or nearly so with the upper calcaire grossier. It may be objected, 

 that, as these strata contain a considerable number of fossils pe- 

 culiar to them, such cannot be the case. I have already stated 

 why we should refuse to receive these new species as evidence of 

 change of formation. Might we not, in fact, expect that the different 

 geographical position and more freshwater condition of the strata in 

 this locality would cause material modifications in the fauna, although 

 still, as a whole, it may assimilate far more to that of the calcaire gros- 

 sier and lower beds than to that of the more recent beds of the Paris 

 basin, notwithstanding the prevalence during the accumulation of 

 the latter of hydrographical conditions very similar to those existing 

 during the formation of the Isle of Wight upper Eocene group? 



That a parallelism of strata accompanied by identity of organic 

 remains should exist over a sea-bottom subject to constant and vari- 

 able but slow changes of subsidence and elevation is not probable : 

 the greater deepening in one part than in another would allow both 

 a greater thickness of deposit and a greater development of testacea ; 

 a littoral fauna might continue in one place, exhibiting little or no 

 change, and at a distance it might gradually pass into others re- 

 presenting various zones of depth ; or again, a rich and nearly constant 

 marine fauna might exist in one part, whilst in another part a slow 

 movement of subsidence or elevation of the same sea-bottom would 

 destroy some genera and species and introduce others, either de- 

 creasing their number and bringing in genera of a wider geographical 



