Ch. XIX.] WEALD, WHEN DENUDED. 283 



so near, the white chalk would not have remained unsoiled, or without 

 intermixture of mud and sand ; nor would organic remains of terrestrial, 

 fluviatile, or littoral origin have been so entirely wanting in the strata of 

 the North and South Downs, where the chalk terminates abruptly in the 

 escarpments. It is admitted that the fossils now found there belong ex- 

 clusively to classes wbich inhabit a deep sea. Moreover, the uppermost 

 beds of the Wealden group, as Mr. Prestwich has remarked, would not 

 have been so strictly conformable with the lowest beds of the Lower 

 Greensand had the strata of the Wealden undergone upheaval before the 

 deposition of the incumbent cretaceous series. 



But, although we must assume that the white chalk was once contin- 

 uous, over what is now the Weald, it by no means follows that the first 

 denudation was subsequent to the entire Cretaceous era. Most probably 

 it commenced before a large portion of the Maestricht beds were formed, 

 or while they were in progress. I have already stated (p. 238, above), 

 that in parts of Belgium I observed rolled pebbles of chalk-flints very 

 abundant in the lowest Maestricht beds, where these last overlie the white 

 chalk, showing at how early a date the chalk was upraised from deep 

 water and exposed to aqueous abrasion. 



Guided by the amount of change in organic life, we may estimate the 

 interval between the Maestricht beds and the Thanet Sands to have been 

 nearly equal in duration to the time which elapsed between the depo- 

 sition of those same Thanet Sands and the Glacial period. If so, it 

 would be idle to expect to be able to make ideal restorations of the innu- 

 merable phases in physical geography through which the southeast of 

 England must have passed since the Weald began to be denuded. In 

 less than half the same lapse of time the aspect of the whole European 

 area has been more than once entirely changed. Nevertheless, it may be 

 useful to enumerate some of the known fluctuations in the physical con- 

 formation of the Weald and the regions immediately adjacent during the 

 period alluded to. 



First, we have to carry back our thoughts to those very remote move- 

 ments which first brought up the white chalk from a deep sea into 

 exposed situations where the waves could plane off certain portions, as 

 expressed in diagram (fig. 329), before the British Lower Eocene beds 

 originated. 



Secondly, we have to take into account the gradual wear and tear of 

 the chalk and its flints, to which the Thanet sands bear witness, as well as 

 the subsequent Woolwich and Blackheath shingle-beds, occasionally 50 

 feet thick, and composed of rolled flint-pebbles. 



Thirdly, at a later period a great subsidence took place, by which the 

 shallow-water and freshwater beds of Woolwich and other Lower Eocene 

 deposits were depressed (see above, p. 221) so as to allow the London 

 Clay and Bagshot series, of deep-sea origin, to accumulate over tbem. 

 The amount of this subsidence, according to Mr. Prestwich, exceeded 800 

 feet in the London, and 1800 feet in the Hampshire or Isle of Wight 

 basin ; and if so, the intervening area of the Weald could scarcely fail to 



