Ch. XIX.] WEALD, WHEN DENUDED. 371 



London clay, Woolwich, and Thanet series, that, before the waters of 

 the Upper Miocene sea spread over this region and south of the 

 Thames, all those Eocene strata had been much wasted and often re- 

 duced to mere isolated outliers scattered over the chalk. After the 

 ferruginous sands were thrown down the bed of the sea must have 

 been again raised 500 or 600 feet, in order that the North Downs 

 might attain their present elevation. 



We learn from these discoveries how impossible it may often be 

 to demonstrate the former presence of the sea on any given area by 

 organic remains, or by sea-beaches. Long and diligent inquiries had 

 been made before the year 1856, for sea-shells of recent or crag 

 species, and for the signs of old sea-margins within the area of the 

 North and South Downs and the Wealden, or on Nos. 2, 3, 4, 5, 6, 

 and 7 of the map (p. 357) ; but'in vain, until at last a few shells and 

 casts of others prove incontestibly the sojourn of the Older Pliocene 

 or .Tipper Miocene sea in those very spaces. We must now, there- 

 fore, admit the retreat of its waters to have been an event as modern 

 as the Upper Miocene, if not the Pliocene period. It follows that in 

 many cases the land may have sunk and have emerged again without 

 retaining on its surface any monuments of the kind usually demanded 

 as indispensable to warrant our speculating on marine denudation as 

 a great modifying cause in the physical geography of the globe. 



Eighthly. But we have still to consider another vast interval of 

 time, that which separated the end of the Miocene from the end of 

 the Newer Pliocene era— a lapse of ages which, if measured by the 

 fluctuations experienced in the marine fauna, may have sufficed to 

 submerge and reelevate whole continents by a process as slow as 

 that which is now operating to upraise Sweden and depress Greenland. 



Lastly. The reader must recall to mind what was said, in Chap- 

 ters XI. and XII. respecting the vast geographical changes of Post- 

 pliocene date, especially those relating to the glacial drift and its 

 far-transported materials. A wide extent of the British Isles appears 

 to have been under the sea during some part or other of that epoch. 

 Most of the submerged areas were afterwards converted into dry 

 land, now several hundred and in Wales more than thirteen hundred 

 feet high, as proved by marine fossil shells. It seems highly prob- 

 able that the Wealden area was dry land when the most charac- 

 teristic northern drift originated, no traces of northern erratics 

 having been met with farther south than Highgate, near London. 

 But it by no means follows that the area of the Weald was stationary 

 during all these ages. It may have been raised and depressed, and its 

 surface may have been modified by rain, rivers, and floods caused 

 by the sudden melting of deep snow again and again during the 

 Glacial era.* 



* In my Geological Evidences of the Antiquity of Man, pp. 2*76, 2Y8, I Lave 

 given maps illustrating the changes in physical geography which have taken place 



