256 ON THE ORIGIN OF SPECIES 



mind almost in the same manner as does the vain 

 endeavour to grapple with the idea of eternity. 



I am tempted to give one other case, the well-known 

 one of the denudation of the Weald. Though it must 

 be admitted that the denudation of the Weald has been 

 a mere trifle, in comparison with that which has 

 removed masses of our palaeozoic strata, in parts ten 

 thousand feet in thickness, as shown in Prof. Ramsay's 

 masterly memoir on this subject : yet it is an admir- 

 able lesson to stand on the intermediate hilly country 

 and look on the one hand at the North Downs, and 

 on the other hand at the South Downs ; for, remem- 

 bering that at no great distance to the west the 

 northern and southern escarpments meet and close, one 

 can safely picture to 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 average about 1100 

 feet, as I am informed by Prof. Ramsay. But if, as 

 some geologists suppose, a range of older rocks under- 

 lies the Weald, on the flanks of which the overlying 

 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 



