GEOLOGICAL STRUCTURE OF THE ISLAND. 93 



from north to south given in the map (PL XX), in 

 which the different members of the chalk forma- 

 tion are denned by variations in the lines of shading, 

 as explained in the index of colours. 



The promontory of the Undercliff is flanked 

 both on the east and west by extensive bays, 

 which have been excavated in the clays and sands 

 of the wealden and inferior cretaceous deposits, 

 by the long continued encroachments of the sea. 

 The wealden (coloured sienna on the map) occu- 

 pies an inconsiderable extent of surface ; but in 

 Sandown bay on the east, and in Brixton, Brook, 

 and Compton bays on the west, the cliffs, which 

 are formed of the upper clays and sands of this for- 

 mation, are exposed to unremitting destruction 

 from the action of the waves. The sea-shore is, 

 therefore, strewn with the detritus of these fluvia- 

 tile strata, and the shingle contains innumerable 

 water-worn fragments of the bones of reptiles, and 

 other organic remains. 



That the general reader may obtain an accurate 

 idea of the phenomena above described, the dia- 

 gram, PL VII, for which I am indebted to my 

 friend Professor John Phillips, is subjoined. In 

 this plan the geographical features of the island 

 are intentionally exaggerated, that the most im- 

 portant physical characters may be clearly under- 



