Ch. XXII.] 



AMOUNT OF DENUDATION. 313 



of large districts of the chalk and other denuded secondary 

 rocks. But these facts do not, we think, militate against the 

 theory above proposed, for we have assumed a long-continued 

 series of elevatory movements in a region where the degrada- 

 tion and reproduction of strata were in progress. 



If this be granted, it is evident that the great antagonist 

 powers, the igneous and the aqueous, would, throughout the 

 whole period, be brought into play in their fullest energy, the 

 igneous labouring continually to produce the greatest inequality 

 of surface, by uplifting certain lines of country and depressing 

 others ; the aqueous no less incessantly engaged in reducing the 

 whole to a level, by cutting off the summits of the upraised 

 tracts, and throwing the materials thence removed into the 

 adjoining hollows. If the volcanic forces eventually prevail, 

 so as to convert the whole region into land, we must expect 

 that some of the materials drifted into the hollows, and forming 

 the newer strata, will be brought up to view, while the de- 

 nuded districts are raised at the same time. If these last 

 continue, in general, to occupy a higher position above the 

 level of the sea, it is all that can be expected after the levelling 

 operations before alluded to. 



Now the tracts occupied by our Eocene formations are low, 

 not so much with reference to the secondary rocks which re- 

 main, as to these masses which must be supposed by our theory 

 to have disappeared, having been carried away by denudation. 

 Let the portions removed from the space intervening between 

 the North and South Downs, and which are expressed by faint 

 lines in our section, wood-cut No. 63, be restored, and we may 

 readily conceive that those masses may have formed shoals 

 and dry land for ages before any part of our tertiary basins 

 emerged. 



The estimate of Mr. Martin is not, perhaps, exaggerated, 

 when he computes the probable thickness of strata removed 

 from the highest part of the Forest ridge to be about 1900 

 feet. So that if we restore to Crowborough Hill, in Sussex, 

 the beds of Weald clay. Lower green-sand, Gault, and chalk, 



