286 IMPERFECTION OF THE Chaf. IX. 



oneself the great dome of rocks which must have covered 

 up the Weald within so limited a period as since the 

 latter part of .the Chalk formation. The distance from 

 the northern to the southern Downs is about 22 miles, 

 and the thickness of the several formations is on an ave- 

 rage about 1100 feet, as I am informed by Prof. Eamsay. 

 But if, as some geologists suppose, a range of older rocks 

 underlies the Weald, on the flanks of which the over- 

 lying sedimentary deposits might have accumulated in 

 thinner masses than elsewhere, the above estimate would 

 be erroneous ; but this source of doubt probably would 

 not greatly affect the estimate as applied to the western 

 extremity of the district. If, then, we knew the rate at 

 which the sea commonly wears away a line of cliff of 

 any given height, we could measure the time requisite 

 to have denuded the Weald. This, of course, cannot 

 be done ; but we may, in order to form some crude 

 notion on the subject, assume that the sea would eat 

 into cliffs 500 feet in height at the rate of one inch 

 in a century. This will at first appear much too 

 small an allowance ; but it is the same as if we were to 

 assume a cliff one yard in height to be eaten back 

 along a whole line of coast at the rate of one yard in 

 nearly every twenty-two years. I doubt whether any 

 rock, even as soft as chalk, would yield at this rate ex- 

 cepting on the most exposed coasts ; though no doubt the 

 degradation of a lofty cliff would be more rapid from the 

 breakage of the fallen fragments. On the other hand, 

 I do not believe that any line of coast, ten or twenty 

 miles in length, ever suffers degradation at the same time 

 along its whole indented length ; and we must remember 

 that almost all strata contain harder layers or nodules, 

 which from long resisting attrition form a breakwater 

 at the base. Hence, under ordinary circumstances, I 

 conclude that for a cliff 500 feet in height, a denudation 



