144 PROF. J. W. JTJDD OK THE OLIGOCENE 



culties felt by the geological surveyors are brought out very pro- 

 minently if we carefully compare the several publications which they 

 have issued at successive dates. Thus, in the sections prepared to 

 illustrate the memoir published in 1856*, 48 feet of strata, which 

 in the text of the memoir are described as belonging to the '• Osborne 

 and St. Helen's Series," are placed in the " Upper Headon ;" but in 

 the sheet of 'Vertical Sections,' issued in 1858 1, the first reading 

 is adopted, while in the general memoir on the Isle of Wight, which 

 appeared in 1862 J, a return is made to the second reading. A com- 

 parison of these several publications of the Geological Survey must 

 lead every one to the conclusion that, by the assumption of the iden- 

 tity of the marine beds at Colwell Eay and Headon Hill, the authors 

 of these works had been led into inextricable difficulties and confu- 

 sion. All these difficulties are removed when we recognize the fact 

 that the Colwell- and Totland-Bay beds altogether overlie those ex- 

 posed at the base of Headon Hill. 



There are not wanting indications, however, that several authors 

 were almost upon the point of recognizing the true succession of 

 beds at the western extremity of the Isle of Wight. Thus Dr. 

 Wright, in his admirable section of the strata between Cliff End 

 and Headon Hill§, clearly points out that the Limnsean limestone 

 of How Ledge, and the sandy rock of Warden Point, are distinctly 

 recognizable high up in the north-eastern face of Headon Hill, 

 This being the case, it is clear that the " Yenus-bed" and Oyster- 

 bands of Colwell Bay, which are undoubtedly above the Limnsean 

 limestone and sandy rock, cannot possibly be represented by the 

 brackish-water beds which occur just above the sea-level in the same 

 part of Headon Hill. Unfortunately, however, Dr. Wright did not 

 follow up this important clue to the true succession of beds which 

 he had discovered, but permitted himself to be carried away by the 

 usually accepted opinion that there exists only one marine stratum 

 with freshwater deposits above and below it. 



Dr. Wright's valuable observations were published in 1851 ; and 

 in the year following Prof. Hebert declared his conviction, as the 

 result of a personal examination of the Isle-of- Wight sections, that 

 the ColweU-Bay beds are upon an altogether higher horizon than 

 those of Headon Hill. Prof. Hebert, whose exact acquaintance with 

 the Lower Tertiaries of the Paris basin lends the greatest weight to 

 his opinions, pointed out that the marine beds of Headon Hill and 

 Hordwell Cliff contain precisely the same fauna as " the upper fos- 

 siliferous zone of Mortefontaine, Monneville," &c., while the ColweU- 

 Bay beds agree in their fossils with the lower part of the Pontaine- 



* Mem. Geol. Surv. ' On the Tertiary MuTio-marine Formation of the Isle 

 of Wight; by Prof. Edward Forbes, F.E.S. Compare plate 10 and p. 81. 



t Vertical Sections, Geol. Survey, sheet 25. 



% Mem. Geol. Surv. 'The Geology of the Isle of Wight' (sheet 10), by H. 

 W. Bristow, F.G.S., plate 4. 



§ "A Stratigraphical Account of the Section from Eound-Tower Point to 

 Alum Bay, on the North-west Coast of the Isle of Wight," by Dr. T. Wright, 

 Ann. and Mag. Nat. Hist. ser. 2, vol. rii. p. 14, and Proc. Cotteswold Nat. 

 Club, vol. i. p. 87. 



