364 PROMINENCE OF HARDER STRATA. [Oh. XIX 



The quantity of denudation, or removal by water, of stratified masses 

 assumed to have once reached continuously from the North to the South 

 Downs is so enormous, that the reader may at first be startled by the 

 boldness of the hypothesis. But the difficulty will disappear when once 

 sufficient time is allowed for the gradual rising and sinking of the 

 strata at many successive geological periods, during which the waves 

 and currents of the ocean, and the power of rain, rivers, and land-floods, 

 might slowly accomplish operations which no sudden diluvial rush of 

 waters could possibly effect. 



Among other proofs of the action of water, it may be stated that the 

 great longitudinal valleys follow the outcrop of the softer and more 

 incoherent beds, while ridges or lines of cliff usually occur at those 

 points where the strata are composed of harder stone. Thus, for ex- 

 ample, the chalk with flints, together with the subjacent upper green- 

 sand, which is often used for building, under the provincial name 

 of " firestone," have been cut into a steep cliff on that side on which 

 the sea encroached. This escarpment bounds a deep valley, exca- 

 vated chiefly out of the soft argillaceous bed, termed gault (No. 3, 

 map, p. 357). In some places the upper greensand is in a loose 

 and incoherent state, and there it has been as much denuded as 

 the gault ; as, for example, near Beachy Head ; but farther to the 

 westward it is of great thickness, and contains hard beds of blue 

 chert and calcareous sandstone or firestone. 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 terrace, varying in breadth from a quar 

 ter of a mile to three miles, and following the sinuosities of the chalk 

 escarpment* 



Fig. 363. 



A 



a. Chalk with flints. &. Chalk without flints. 



c. Upper greensand, or firestone. d. Gault. 



It is impossible to desire a more satisfactory proof that the escarp- 

 ment is due to the excavating power of water during the rise of the 

 strata, or during their rising and sinking at successive periods ; for 

 I have shown, in my account of the coast of Sicily (p. *76), in what 

 manner the encroachments of the sea tend to efface that succession 

 of terraces which must otherwise result from the intermittent - up- 

 heaval of a coast preyed upon by the waves. During the inter- 



^ Sir R. Murchison, Geol. Sketch of Sussex, <fec , Geol. Trans., Second Series, 

 vol. ii. p. 93. 



