292 EOCENE PERIOD. [Ch. XXI. 



green-sand (' firestone/ or ' malm rock,' as it is sometimes called) 

 is almost absent, in the tract here alluded to. It is, in fact, 

 seen at Beacby Head to thin out to an inconsiderable stratum 

 of loose green-sand ; but farther to the westward it is of great 

 thickness, and contains hard beds of blue chert and limestone. 

 Here, accordingly, we find that it produces a corresponding 

 influence on the scenery of the country, for it runs out like a 

 step beyond the foot of the chalk-hills, and constitutes a lower 



No. G7. 



a, Chalk with flints. I, Chalk without flints. 



c t Upper green sand, or firestone. </, Gault. 



terrace varying in breadth from a quarter of a mile to three 

 miles, and following the sinuosities of the chalk escarpment' 14 . 



It is impossible to desire a more satisfactory proof that the 

 escarpment is due to the excavating power of water during the 

 gradual rise of the strata. For we have shown, in our account 

 of the coast of Sicily ]', in what manner the encroachments of 

 the sea tend to efface that succession of terraces which must 

 otherwise result from the successive rises of a coast preyed 

 upon by the waves. During the interval between two eleva- 

 tory movements, the lower terrace will usually be destroyed, 

 wherever it is composed of incoherent materials; whereas the 

 sea will not have time entirely to sweep away another part of the 

 same terrace, or lower platform, which happens to be composed 

 of rocks of a harder texture and capable of offering a firmer 

 resistance to the erosive action of water. 



Valleys where softer slrata, ridycs iclicre harder crop out. 

 It is evident that the Gault No. 2 (sec the map) could 

 not have opposed any effectual resistance to the denuding force 



* Mr. Murchison, Gcol. Sketch of Sussex, c.,Geol. Trans., 2nd Series, vol. ii. p. 98. 

 f See p. Ill, and wood-cut No. 21, 



