do not occur until after long intervals,
when | when 1869 |
| due to the occurrence of physical 1872 |
| in the physical conditions 1869 |
| OMIT 1872 |
| immigration have occurred; and individual differences or variations of the right nature, by which some of the inhabitants might be better fitted to their new places under the altered circumstances, might not at once occur. 1869 |
| the immigration of new forms. 1872 |
| 2 blocks not present in 1859 1860 1861 1866 1869; present in 1872 | | Moreover variations or individual differences of the right nature, by which some of the inhabitants might be better fitted to their new places under the altered circumstances, would not always occur at once.
Unfortunately we have no means of determining, according to the standard of years, how long a period it takes to modify a species; but to the subject of time we must return.
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| 6 blocks not present in 1869 1872; present in 1859 1860 1861 1866 | | But the amount of denudation which the strata have in many places suffered, independently of the rate of accumulation of the degraded matter, probably offers the best evidence of the lapse of time.
I remember having been much struck with the evidence of denudation, when viewing volcanic islands, which have been worn by the waves and pared all round into perpendicular cliffs of one or two thousand feet in height; for the gentle slope of the lava-streams, due to their formerly liquid state, showed at a glance how far the hard, rocky beds had once extended into the open ocean.
The same story is still more plainly 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 so completely planed down by the action of the sea,
that no trace of these vast dislocations is externally visible.
The Craven fault, for instance, extends for upwards of 30 miles, and along this line the vertical displacement of the strata has varied from
600 to 3000 feet.
Prof. Ramsay has published an account of a downthrow in Anglesea of 2300 feet; and he informs me that he fully believes 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 having been smoothly swept away.
The consideration of these facts impresses my
mind almost in the same manner as does the vain endeavour to grapple with the idea of eternity.
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| 9 blocks not present in 1861 1866 1869 1872; present in 1859 1860 | | I am tempted to give one other case, the well-known one of the denudation of the Weald.
Though it must be admitted that the denudation of the Weald has been a mere trifle, in comparison with that which has removed masses of our palæozoic strata, in parts ten thousand feet in thickness, as shown in Prof. Ramsay's masterly 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 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 must remember that almost all strata contain harder layers or nodules, which from long resisting attrition form a breakwater at the base.
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| 1 blocks not present in 1859 1861 1866 1869 1872; present in 1860 | | 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 base of cliffs, will admit any near approach to such rapid wearing away.
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