OCR Text |
Show 250 IMPERFECTION OF THE [CHAP. IX. best evidence of the lapse of time. I rememb~r having been much struck with the evidence of denudation, when viewing volcanic islands, which have ?een wo~n by the waves and pared all ro~nd iJ?to perl?endicular chffs of one or two thousand feet In h.m~ht; for ~he .gentle slope of the lava-streams, due to thmr formerly hquid state, showed at a glance how far the hard, rocky bed~ ha~ once exte~ded into the open ocean. The sa1ne story IS still.more plmnly told by faults -those great cracks along which the strata have been upheaved on one side, or thrown down on the other, to the height or depth of thousands of feet ; for since the crust cracked, the surface of the land has been t~o completely planed d~wn bJ: the. action of the .s~a, that no trace of these vast dislocations IS externally visible. The Craven fault, for instance, extends for upwards of 30 1niles, and along this line the vertical, displacement of the strata has varied from 600 to 3000 feet. Prof. Ran1- say has published an account of a downthrow in .A~1glesea of 2300 feet · and he informs me that he fully beheves there is one in Merionethshire of 12,000 feet; yet in these cases there is nothing on the surface to show such prodigious movements ; the pile of rocks on the one or _other side havinO' been smoothly swept away. The consideration of the~e facts impresses my mind almost in the san1e 1nanner 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 Inust be admitted that the denudation of the Weald has been a mere trifle in comparison ·with that which has removed masses of 'our palreozoic strata, in parts ten thousand feet in thickness, as shown in Prof. Ramsay's Inasterly memoir on this subject. Yet it is an admirable lesson to stand on the North Downs and to look at the distant 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 ~oll?e of rocks which must have covered up the Weald Withm so limited a period. as since the latter part of the Chalk formation. The distance from the northern to the south· CHAP. IX.] GEOLOGICAL RECORD. 251 ern Downs is about 22 miles, and the thickness of the sev~ral 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 th~ flanks of which the overlying sedimentary dep~sits m1ght have ac?umulated in thinner masses than elsewhere, the above estimate would be erroneous; but this source of dou~t probably would not greatly affect the estimate as apphed to the western extremity of the district. If, then, we. knew t~1e rate at which the sea commonly wears awa:y: a hne o~ ~hff 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 crud~ noti?n on the subject, as~ume that the sea :voul~ eat Into chffs 500 feet in height at the rate of one Inch In a century. This .w~ll at first app~ar much too small an all_owance ; but It IS the same as If we were to assume ·a .chff one yard in height to be eaten back along a whole hne 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 expose~ coasts; though no doubt the degradation of a lofty chff would be more rapid from the breakage of the fallen fragments. On the other hand I do not believe that an;y line of coast, ten or twenty miles in length, ~ver suffers degradation at the same time along its whole mdented length ; and we must remember that almost all strata contain harder layers or nodules ,vhich from long resisting attrition form a breakwater at the base Hen.ce, under or~inar~ circumstances, I conclude that fo; a chff 500 feet 1n hmght; a denudation of one inch per century for t!1e whole length would be an ample allow~ ance. At this rate, on the above data the denudation of the Weald must have required 306 66.2 400 years · or say three hundred million years. ' ' ' The ac~io~ of fresh water on the gently inclined Wealden d!striCt, when upraised, could hardly have been ~reat, but 1t would somewhat reduce the above estimate. b the o~her hand, during oscillations of level, which we ow this area has undergone, the surface may have |