IMPERFECTION OF GEOLOGICAL RECORD 257 



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 

 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 cliff 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 imperfect, 



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