MACKIE — THOUGHTS ON DOVER CLIFFS. 
2S9 
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 tliis 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 Greensaud ; 
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, 
«, Upper Chalk ; h. 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. 
VOL. VI. 2 P 
