THE CHALK FORMATION. 67 



the Great Western, from Maidenhead to beyond 

 Wallingford. After passing through the Chalk, 

 the lower subdivisions of the Cretaceous formation, 

 viz. the Marl, Firestone, Gait, and Greensand, 

 successively appear ; and these are followed by 

 the Oolite on the Great Western and Birmingham 

 lines, and by the Wealden on the South-eastern ; 

 but on the South-western, by which we shall 

 travel to Southampton, the Chalk and the Tertiary 

 strata only are exposed. 



The Chalk, as is well known, constitutes the pre- 

 valent geological feature of this part of England. 

 The upper or white chalk rises into chains of 

 hills, called downs, which are remarkable for their 

 smooth and undulated outline, and form a well 

 defined geographical boundary. The southern 

 range extends from Hampshire, through Sussex to 

 the sea-coast, terminating in the bold promontory 

 of Beachy Head ; the northern passes from Hamp- 

 shire through Surrey and Kent to the British 

 Channel, and ends in the line of cliffs near Dover. 



The lower cretaceous strata compose ranges 

 of hills less uniform in elevation and extent, 

 which skirt the inner margin and escarpments of 

 the Downs, and in some parts of Surrey and 

 Western Sussex attain an altitude equal to that 

 of any portion of the North or South Downs. 



