STRATA OF THE HAMPSHIEE BASIN. 169 



includes strata both above and below the " Bembridge series " of 

 Edward Forbes. It includes not only the Bembridge limestone and 

 marls of that author, but also beds referred by him to the base of 

 the Hempstead, the Osborne and St. Helens, and to the Upper 

 Headon. These great changes in the classification and uomencla- 

 ture of the strata are rendered absolutely necessary by the discovery 

 that has been made of the error in the existing views of the succes- 

 sion of the strata. 



With respect to Porbes's division of the " Osborne and St. Helen's 

 series," I think that it had better be given up altogether, and on the 

 following grounds : — Firsts there are no good characters, either phy- 

 sical or palseontological, by which this division can be defined. 

 Secondly^ the separation of this division from those above and below 

 it has been found so difficult, that even in the difi'erent publications 

 of the Geological Survey very serious discrepancies exist as to the 

 limits of the series ; and thirdly^ under this name beds lying below 

 the Brockenhurst series, as at Headon Hill, have been confounded 

 with others on a totally different horizon, above the Brockenhurst 

 series. 



VII. Thickness of the Strata and their Development in 



DIFFERENT ArEAS. 



Immediately above the richly fossiliferous Barton Clay we find a 

 series of sandy strata with subordinate argillaceous beds. At Alum 

 Bay these strata attain a very considerable thickness, which has 

 been variously estimated at from 100 feet to 200 feet ; it is probably 

 not less than 150 feet. These Headon-Hill sands are usually called 

 the Upper Bagshot beds ; but it appears to me that it cannot but 

 be a source of confusion to base our classification of the Upper 

 Eocene strata on the poorly fossiliferous deposits of the London 

 basin rather than on the richly fossiliferous deposits of the Hamp- 

 shire basin. It is true that at Alum Bay the Headon-Hill sands 

 have not yielded any fossils ; but the equivalent beds at HordweU 

 Cliff contain a by no means scanty fauna, in which we find the same 

 admixture of marine and freshwater forms which cbaracterizes the 

 overlying Headon strata. As, moreover, we detect in these beds 

 the eminently characteristic fossil Cerithium concavum, it seems 

 clear that we must regard them as constituting the lowest member 

 of the Headon group. 



The Headon group, as exhibited at the western extremity of the 

 Isle of Wight, is about 400 feet in thickness. It consists in the 

 upper part of freshwater and terrestrial beds, — beds of limestone 

 containing Limnoia, Paludina, Planorhis, and other pulmoniferous 

 Gasteropods, alternating with sands and clays containing freshwater 

 fossils, while beds of lignite, sometimes a foot or two in thickness, 

 indicate old terrestrial surfaces. But in all the lower part of the 

 series we find a tendency to the recurrence of brackish-water condi- 

 tions ; and in these intercalated fluvio-marine bands we find numerous 

 Cerithia^ Cyrence, and dwarfed Ostrece. 

 Q. J. G. S. No. 142. N 



