IV 



In re-mapping the Lower Greensand Mr. Strahan has taken 

 advantage o£ certain broad lithological characters, which beino- 

 traceable across the Island, permitted of a convenient subdivision 

 of that formation into groups whose respective limits could be 

 shown on the Map. This subdivision, for which a new scheme 

 of colouring has been adopted, is only intended for the Isle of 

 Wight, where it is of considerable local service. Mr. Strahan 

 found that an upper subgroup of the Lower Greensand, correspond- 

 ing to the Folkestone Beds, existed on the Island, capable of sub- 

 division into an upper ferruginous and slightly conglomeratic rock, 

 the Carstone, which passes up into the Gault, and a lower sand- 

 rock resembling in lithological characters the Folkestone Beds, and 

 passing downwards into ferruginous sands. Another subgroup, 

 exhibiting both the lithological and palaeontological features of the 

 Sandgate Beds, has been placed with these underlying sands (the 

 Uythe Beds) under the name of the Ferruginous Sands. The 

 position and extent of the Atherfield Clay remain nearly as in the 

 first edition of the Map. 



^ A few fossils have been added to the small fauna hitherto 

 yielded by the Gault. A line has been engraved on the Map to 

 mark the position of the bold topographical feature formed by 

 the Chert beds of the Upper Greensand in the central parts of the 

 Island. 



The subdivisions of the Chalk which can be traced on the 

 ground have now been inserted on the Map. The Chalk-rock 

 is so shown, but the Melbourn-rock, though frequently recoo-- 

 nised in place, is not represented on the Map for want of space. 



In the preparation of the following Chapters it has been found 

 necessary entirely to re-measure the cliff sections of the Secondary 

 Kocks. This has been done in Compton Bay from the Upper 

 Greensand downwards, in Atherfield Bay from the Chalk-marl 

 downwards, and in Sandown Bay from the Chalk-rock down- 

 wards. The total thickness of strata measured at the last-named 

 locality was 1,218 feet. The results of this detailed re-exami- 

 nation are shown graphically in Plate II., which represents the 

 coast-section from Compton Bay to Blackgang, and in Plate III., 

 which contains a series of comparative Vertical Sections showino- 

 the varying thickness of the Secondary formations in different 

 parts of the Island and on the adjacent coast of Dorsetshire. 



In revising the Tertiary area of the Island, Mr. Reid found 

 that only slight chariges were required in the Eocene lines of the 

 Map. In the Sections and Memoir he has somewhat modified 

 the boundaries of the Bracklesham and Barton Beds in con- 

 formity with the recent researches of the Rev. Osmond Fisher 

 and Mr. Keeping. The so-called "Upper Bagshot Sands" of 

 the Isle of Wight are not improbably considerably higher than 

 the division of that name in the actual Bagshot district. Hence, 

 until the position of the glass-sands of the Island has beeri 

 definitely ascertained, it has been thought desirable not to speak 

 of these deposits as " Upper Bagshot," but to revert to the older 

 name of " Headcn Hill tSanrls." 



