148 PROF. J. W. JCDD ON THE OLIGOCENE 



given by Prof. Edward Forbes in 'bis memoir on tbe Tertiary 

 Eluvio-marine Pormation of the Isle of Wight (p. 89). In fig. 2 

 the relative heights are given in accordance with the measurements 

 of the Ordnance Survey ; but the vertical scale is different from 

 the horizontal ; and the section illustrates diagrammatically the view 

 of the succession of beds which I now put forward. The section 

 fig. 3 on the same Plate is drawn upon the true scale, both vertical 

 and horizontal, the heights and distances being taken from the new 

 maps of the Ordnance Survey. 



lY. PALiEONTOLOGICAL EVIDENCE. 



I shall now proceed to show that the palseontological evidence in 

 favour of the correlation which I propose is not less complete and 

 satisfactory than the stratigraphical. According to the usually 

 received interpretation, a series of marine strata 100 feet thick in 

 Whitecliff Bay, the well-known and distinctly marked marine beds 

 of ColweU Bay, and the marine beds of the New Porest exposed at 

 Brockenhurst, Roy don, and Lyndhurst are represented at Headon 

 Hill by the brackish-water Middle Headon beds, and at Hordwell 

 Cliff by a band a few inches thick, containing both marine and fresh- 

 water forms. 



But against this correlation several very serious objections may be 

 urged. In the first place, it must be noticed that at Whitecliff Bay, 

 at ColweU Bay, and in the several New-Porest localities, the strata 

 are of ])urely marine origin, and contain no admixture of fresh- 

 water shells, while in the last-mentioned reef-building corals 

 abound*, and to the existence of these an influx of fresh water is 

 known to be highly inimical. The so-called " Middle Marine " beds 

 of Headon Hill and Hordwell Cliff are of totally different character, 

 exhibiting clear evidence of the prevalence of estuarine or brackish- 

 water conditions only. At both localities we find some shells be- 

 longing to marine genera, such as Ancillaria, Area, Bulla, Cancel- 

 laria, Ghemnitzia, Corhula, Cytherea, Fusus, Lucina, Murex, Natica, 

 Nucula, Ostrea, Pleurotoma, Psammohia, Scalaria, and Voluta, with 

 Balani and Serpulas. But these marine forms are almost always 

 dwarfed in size, and exhibit clear evidence of having lived under 

 unfavourable conditions; while mingled with them we find shells 

 belonging to genera which usually frequent brackish water, such as 

 Cerithium, Cyrena, Hydrohia, and Odostomia, with other purely 

 freshwater forms, such as Limncea, Melania, Melaiiojpsis, Potamomya^ 

 and Planorhis. 



The marine beds of Whitecliff Bay attain a thickness of 100 feet ; 

 at ColweU Bay they are about 25 feet ; while in the JSTew Porest 

 their entire thickness, although it has never been determined, is 

 probably very considerable. These beds contain a very large and 



* See the important memoir by Dr. Duncan " on the Physical Geography of 

 Western Europe during the Mesozoic and Cainozoic Periods, elucidated by their 

 Coral Faunas," Quart. Journ. Geol. Soc. vol. xxvi. (1870), p. 51. 



