memoir on this subject: yet it is an admirable
lesson to stand on the intermediate hilly country and look on the one hand
at the North Downs, and {286} on the other hand at the South Downs; for,
remembering 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
underlies 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 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 {287} must remember that almost all
strata contain harder layers or nodules, which from long resisting
attrition form a breakwater at the base. We may at least confidently
believe that no rocky coast 500 feet in height commonly yields at the rate
of a foot per century; for this would be the same in amount as a cliff one
yard in height retreating twelve yards in twenty-two years; and no one, I
think, who has carefully observed the shape of old fallen fragments at the