286 
IMPEEFECTION OF THE 
Chap. IX. 
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. Eamsay. 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 thin¬ 
ner 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 wEole 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* 
excepting 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 
