MACKIE — THOUGHTS ON DOVER CLIFFS. 289 



Downs forbid us to suppose, or they have been swept off from it 

 since. 



To this latter conclusion we must inevitably come ; but then the 

 question arises whether the Cretaceous and Upper Wealden beds ex- 

 tended over this area in their full thickness ; if so, from off this re- 

 gion the rocks must have been swept away to the extent in vertical 

 height of more than 1400 feet. 



Wealden Area. 



Fig. 3. — Dome-shaped strata over the Weald, of equal thickness throughout. 



Some geologists, however, and with good reason, consider that the 

 beds became thinner as they overlapped the Wealden district ; and 

 it is certain a great submarine ridge has existed at this spot from 

 the Carboniferous era, if not from an even earlier date. The sea 

 might therefore naturally be supposed to have shoaled over this tract, 

 and consequently the deposit would not have been so thick as in the 

 deeper parts of the sea. 



Wealden Area. 



Fig. 4, — Strata diminishing in thickness over the central or Wealden area. 

 a, Upper Chalk ; b, Lower Chalk ; c, Gault and Lower Greensand ; 

 d, Weald clay or Old Wealden land. 



The proof of this surmise would be in the diminished collective 

 thickness of the groups of strata (as between h and g), as one after 

 another abutted against the submerged slopes of the ridge and was 



£ _ \ 



Fig. 5. — Ideal section of strata abutting against a ridge. 

 a, Upper Chalk ; b, Lower Chalk ; c, Gault and Lower Greensand ; 

 d, Neocomian beds ; c, Old Wealden land. 



terminated; or in the difference of diminished dip of the superior 

 over the inferior strata. 



YOL. vi. 2 p 



