292 EOCENE PERTOD. [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 Beachy 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. 67. 
a, Chalk with flints, b, Chalk without flints, 
c, Upper green sand, or firestone. d, Gault. 
terrace varying in breadth from a quarter of a mile to three 
miles, and following the sinuosities of the chalk escarpment*. 
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 f , 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 strata, ridges where harder crop out. 
— It is evident that the Gault No. 2 (see the map) could 
not have opposed any effectual resistance to the denuding force 
* Mr. Murchison, Geol. Sketch of Sussex, &c, Geol. Trans., 2nd Series, vol. ii. p. 98. 
f See p. Ill, and wood-cut No, 24. 
