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 Wadhurst ; 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 land, and other newer deposits formed 

 of their debris. Of such origin are the Thanet 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. 



Sea-level. 



Fig. 12. — Section of the present strata across Kent and Sussex, 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, b, 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 boues of elephants, deer, hippopotami, and oxen 

 that ranged across the narrow isthmus which then connected our land 

 with Prance, 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 raised and how denuded ? When were the British Isles dis- 



