Chap. IX. 
GEOLOGICAL KECOED. 
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 care¬ 
fully observed the shape of old fallen fragments at the 
base of cliffs, will admit any near approach to such 
rapid wearing away. Hence, under ordinary circum¬ 
stances, I should infer that for a cliif 500 feet in height, 
a denudation of one inch per century for the whole 
length would be a sufficient allowance. At this rate, 
on the above data, the denudation of the Weald must 
have required 306,662,400 years; or say three hundred 
million years. But perhaps it would be safer to allow 
two or three inches per century, and this would reduce 
the number of years to one hundred and fifty or one 
hundred million years. 
The action of fresh water on the gently inclined 
Wealden district, when upraised, could hardly have 
been great, but it would somewhat reduce the above 
estimate. On the other hand, during oscillations of 
level, which we know this area has undergone, the sur¬ 
face may have existed for millions of years as land, and 
thus have escaped the action of the sea: when deeply 
submerged for perhaps equally long periods, it would, 
likewise, have escaped the action of the coast-waves. 
So that it is not improbable that a longer period than 
300 million years has elapsed since the latter part of 
the Secondary period. 
I have made these few remarks because it is highly 
important for us to gain some notion, however imper¬ 
fect, of the lapse of years. During each of these years, 
