MACKIE— THOUGHTS ON DOVER CLIFFS. 
291 
trict possessing a contorted oval shape by an equally-pressing power 
from below, such as an expanding volume of gas or steam, or the ex- 
pansion of a heated rock-mass, would be to fracture the superincum- 
bent rigid beds in lines partly concentric, partly straight, partly ra- 
diating and partly curved, according to their position with respect 
to the centre or sides of the region upraised. 
Such are really the characters of the lines of the hills and valleys 
of the Weald and the Chalk gorges. The comparatively straight line 
of the St. Leonard's and Ashdown hills passes by Horsham, Crow- 
borough, and AVadhurst ; the concentric ridges by Lewes and Dork- 
ing, while the diverging radiating lines are the great cross-fractures 
through which the rivers of Kent, Surrey, and Sussex find their outlets 
to the sea. (See Map, PI. XVI.) 
To return again to our history. Since the sediments of the Creta- 
ceous ocean were consolidated into rock, they have been furrowed by 
another ocean as they rose into laud, and other newer deposits formed 
of their debris. Of such origin are the Thauet sands and the plastic 
clays of Reading and the Isle of Wight. These and many other of 
the Tertiary beds of the surrounding country are nearly wholly 
formed of the greensand, clay, chalk, and flints of the wasted Wealden 
land, and have been accumulated on its ancient successive shores. 
Wealden Dome. 
Fig. 12. — Section of the present strata aeross Kent and Snssex, showing the suc- 
cessive ancient sea-levels at which the former Wealden dome was periodically cut 
away as it rose above the sea: the Upper Chalk, 1, being first cut away; then 
the Middle Chalk, 2 ; then the Grey Chalk, 3 ; then the Upper Greensand, 4 ; 
then the Gault, 5 ; and finally, the Lower Greensand, 6, and Neocomian, 7- These 
beds forming consecutively the corresponding Tertiary beds. No. 1 to 7 ; on the 
older secondary strata, a to g. These are a, h, Neocomian and Lower Greensand ; 
c, Gault ; d. Upper Greensand; e, Grey Chalk ; /, Middle Chalk ; g. Upper Chalk. 
The Tertiary period passed away, and the age of the great mammoths 
succeeded. On our hills and plains, and in the alluvia of our river- 
valleys, are spread the bones of elephants, deer, hippopotami, and oxen 
that ranged across the narrow isthmus which then connected our land 
with France, and which still leaves traces of its former position in 
the ribbon of shallow water between Folkestone and Boulogne. 
Interesting indeed are the inquiries — When was the Wealden dome 
first raiaed and how denuded ? When were the British Isles dis- 
