Ch. XIX.] DEXUDATIOX OF THE WEALD. 365 



val between two elevatory 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. As the yielding clay termed gault would 

 be readily washed away, we find its outcrop marked everywhere by a 

 valley which skirts the base of the chalk-hills, and which is usually 

 bounded on the opposite side by the lower greensand ; but as the 

 upper beds of this last formation are most commonly loose and inco- 

 herent, they also have usually disappeared and increased the breadth 

 of the valley. In those districts, however, where chert, limestone, 

 and other solid materials enter largely into the composition of this 

 formation (Xo. 4, map, p. 357), they give rise to a range of hills 

 parallel to the chalk, which sometimes rival the escarpment of the 

 chalk itself in height, or even surpass it, as in Leith Hill, near Dork- 

 ing. This ridge often presents a steep escarpment toward the 

 soft argillaceous deposit called the TVeald clay (as above, No. 5, 

 rig. 356, p. 358), which usually forms a broad valley, separating the 

 lower greensand from the Hastings sands or Forest Eidge ; but where 

 subordinate beds of sandstone of a firmer texture occur, the uniform- 

 ity of the plain of No. 5 is broken by waving irregularities and 

 hillocks. 



Pluvial action. — In considering, however, the comparative destruc- 

 tibility of the harder and softer rocks, we must not underrate the 

 power of rain. The chalk-downs, even on their summits, are usually 

 covered with unrounded chalk-flints, such as might remain after mass- 

 es of white chalk had been softened and removed by water. This 

 superficial accumulation of the hard or siliceous materials of dis- 

 integrated strata may be due in no small degree to pluvial action ; for 

 during extraordinary rains a rush of water charged with calcareous 

 matter, of a milk-white color, may be seen to descend even gently 

 sloping hills of chalk. If a layer no thicker than the tenth of an inch 

 be removed once in a century, a considerable mass may in the course 

 of indefinite ages melt away, leaving nothing save a stratum of flinty 

 nodules to attest its former existence. A bed of fine clay some- 

 times covers the surface of slight depressions in the white chalk, 

 which may represent the aluminous residue of the rock, after the 

 pure carbonate of lime has been dissolved by rain-water, charged with 

 excess of carbonic acid derived from decayed vegetable matter. The 

 acidulous waters sometimes descend through "sand-pipes" and "swal- 

 low-holes " in the chalk, so that the surface may be undermined, and 

 cavities may be formed or enlarged, even by that part of the drainao-e 

 which is subterranean.* 



* See above, p. 82, 83, " Sand-pipes in Chalk ; " and Prestwich, Geol. Quart. 

 Journ., vol. x. p. 222. 



