Ga XIX.] PERIODS OF DENUDATION IN THE WEALD. 367 



At what periods the Weald valley was denuded. — We may next 

 inquire at "what time the denudation of the "Weald was effected, and 

 we shall find, on considering all the facts brought to light by recent 

 investigation, that it was accomplished in the course of so long a 

 series of ages, that the greatest revolutions m the physical geography 

 of the globe, yet known to us, have taken place within the same 

 lapse of time. It has now been ascertained, that part of the denu- 

 dation of the Weald was completed before the British Eocene strata, 

 aud consequently before the nummulitic rocks of Europe and Asia were 

 formed. The date, therefore, of part of the changes now under contem- 

 plation was long antecedent to the existence of the Alps, Pyrenees, and 

 many other European and Asiatic mountain-chains, and even to the 

 accumulation of large portions of their component materials beneath 

 the sea. 



M. Elie de Beaumont suggested, in 1833, that there was an island 

 in the Eocene sea in the area now occupied by the French and 

 English Wealden strata, and he gave a map or hypothetical restora- 

 tion of the ancient geography of that region at the era alluded to.* 

 Mr. Prestwich has since shown that the materials of which the lower 

 tertiary beds of England are made up, and their manner of resting 

 on the chalk, imply, that such an island, or several islands and shoals, 

 composed of Chalk, Upper Greensand, Gault, and probably of some 

 of the Lower Cretaceous rocks, did exist somewhere between the present 

 North and South Downs. The undermined cliffs and shores of those 

 lands suj)plied the flints, which the action of the waves rounded into 

 pebbles, such as now form the Woolwich, and Blackheath shingle- 

 beds below the London Clay. It is supposed, that the land referred 

 to was drained by rivers flowing into the Eocene sea, and whence 

 the brackish and freshwater deposits of Woolwich and other contem- 

 poraneous strataf were derived. The large size of some of the rolled 

 flints (eight inches and upwards in diameter) qf the Blackheath shingle 

 demonstrates the proximity of land. Such heavy masses could not 

 have been transported from great distances, whether they owe their 

 shape to waves breaking on a sea-beach, or to rivers descending a steep 

 slope. 



In the annexed diagram (fig. 364) Mr. Prestwich has represented 

 a section from near Saffron Walden, in Essex, to the Weald, passing 

 north and south through Godstone, in which we see how the chalk, 

 c, had been disturbed and denuded before the lower Eocene beds, 6, 

 were deposited. Some small patches of the last-mentioned beds, &', 

 consisting of clay and sand, extend occasionally, as in this instance, 

 to the very edge of the escarpment of the North Downs, proving that 

 the surface of the white chalk, now covered with tertiary strata, is 

 the same which oriainallv constituted the bottom of the Eocene sea. 



* Mem. de ia Soc. Geol. de France, vol. i. part i. p. Ill, pi. 7, fig. 5. 

 f See p. 296, above. 



