the formation of vegetable mould-第30部分
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arenaceo…calcareous loam； and Dr。 King informs me that these
castings are extremely liable to crumble during dry weather into
small fragments； which are soon acted on by rain； and then sink
down so as to be no longer distinguishable from the surrounding
soil。 He sent me a mass of such disintegrated castings； collected
on the top of a bank； where none could have rolled down from above。
They must have been ejected within the previous five or six months；
but they now consisted of more or less rounded fragments of all
sizes； from 0。75 of an inch in diameter to minute grains and mere
dust。 Dr。 King witnessed the crumbling process whilst drying some
perfect castings； which he afterwards sent me。 Mr。 Scott also
remarks on the crumbling of the castings near Calcutta and on the
mountains of Sikkim during the hot and dry season。
When the castings near Nice had been ejected on an inclined
surface； the disintegrated fragments rolled downwards； without
losing their distinctive shape； and in some places could 〃be
collected in basketfuls。〃 Dr。 King observed a striking instance of
this fact on the Corniche road； where a drain； about 2。5 feet wide
and 9 inches deep； had been made to catch the surface drainage from
the adjoining hill…side。 The bottom of this drain was covered for
a distance of several hundred yards； to a depth of from 1。5 to 3
inches； by a layer of broken castings； still retaining their
characteristic shape。 Nearly all these innumerable fragments had
rolled down from above； for extremely few castings had been ejected
in the drain itself。 The hill…side was steep； but varied much in
inclination； which Dr。 King estimated at from 30 degrees to 60
degrees with the horizon。 He climbed up the slope； and 〃found
every here and there little embankments； formed by fragments of the
castings that had been arrested in their downward progress by
irregularities of the surface； by stones； twigs； &c。 One little
group of plants of Anemone hortensis had acted in this manner； and
quite a small bank of soil had collected round it。 Much of this
soil had crumbled down； but a great deal of it still retained the
form of castings。〃 Dr。 King dug up this plant； and was struck with
the thickness of the soil which must have recently accumulated over
the crown of the rhizoma； as shown by the length of the bleached
petioles； in comparison with those of other plants of the same
kind； where there had been no such accumulation。 The earth thus
accumulated had no doubt been secured （as I have everywhere seen）
by the smaller roots of the plants。 After describing this and
other analogous cases； Dr。 King concludes： 〃I can have no doubt
that worms help greatly in the process of denudation。〃
Ledges of earth on steep hill…sides。Little horizontal ledges； one
above another； have been observed on steep grassy slopes in many
parts of the world。 The formation has been attributed to animals
travelling repeatedly along the slope in the same horizontal lines
while grazing； and that they do thus move and use the ledges is
certain； but Professor Henslow （a most careful observer） told Sir
J。 Hooker that he was convinced that this was not the sole cause of
their formation。 Sir J。 Hooker saw such ledges on the Himalayan
and Atlas ranges； where there were no domesticated animals and not
many wild ones； but these latter would； it is probable； use the
ledges at night while grazing like our domesticated animals。 A
friend observed for me the ledges on the Alps of Switzerland； and
states that they ran at 3 or 4 ft。 one above the other； and were
about a foot in breadth。 They had been deeply pitted by the feet
of grazing cows。 Similar ledges were observed by the same friend
on our Chalk downs； and on an old talus of chalk…fragments （thrown
out of a quarry） which had become clothed with turf。
My son Francis examined a Chalk escarpment near Lewes； and here on
a part which was very steep； sloping at 40 degrees with the
horizon； about 30 flat ledges extended horizontally for more than
100 yards； at an average distance of about 20 inches； one beneath
the other。 They were from 9 to 10 inches in breadth。 When viewed
from a distance they presented a striking appearance； owing to
their parallelism； but when examined closely； they were seen to be
somewhat sinuous； and one often ran into another； giving the
appearance of the ledge having forked into two。 They are formed of
light…coloured earth； which on the outside； where thickest； was in
one case 9 inches； and in another case between 6 and 7 inches in
thickness。 Above the ledges； the thickness of the earth over the
chalk was in the former case 4 and in the latter only 3 inches。
The grass grew more vigorously on the outer edges of the ledges
than on any other part of the slope； and here formed a tufted
fringe。 Their middle part was bare； but whether this had been
caused by the trampling of sheep； which sometimes frequent the
ledges； my son could not ascertain。 Nor could he feel sure how
much of the earth on the middle and bare parts； consisted of
disintegrated worm…castings which had rolled down from above； but
he felt convinced that some had thus originated； and it was
manifest that the ledges with their grass…fringed edges would
arrest any small object rolling down from above。
At one end or side of the bank bearing these ledges； the surface
consisted in parts of bare chalk； and here the ledges were very
irregular。 At the other end of the bank； the slope suddenly became
less steep； and here the ledges ceased rather abruptly； but little
embankments only a foot or two in length were still present。 The
slope became steeper lower down the hill； and the regular ledges
then reappeared。 Another of my sons observed； on the inland side
of Beachy Head； where the surface sloped at about 25 degrees； many
short little embankments like those just mentioned。 They extended
horizontally and were from a few inches to two or three feet in
length。 They supported tufts of grass growing vigorously。 The
average thickness of the mould of which they were formed； taken
from nine measurements； was 4。5 inches； while that of the mould
above and beneath them was on an average only 3。2 inches； and on
each side； on the same level； 3。1 inches。 On the upper parts of
the slope； these embankments showed no signs of having been
trampled on by sheep； but in the lower parts such signs were fairly
plain。 No long continuous ledges had here been formed。
If the little embankments above the Corniche road； which Dr。 King
saw in the act of formation by the accumulation of disintegrated
and rolled worm…castings； were to become confluent along horizontal
lines； ledges would be formed。 Each embankment would tend to
extend laterally by the lateral extension of the arrested castings；
and animals grazing on a steep slope would almost certainly make
use of every prominence at nearly the same level； and would indent
the turf between them； and such intermediate indentations would
again arrest the castings。 An irregular ledge when once formed
would also tend to become more regular and horizontal by some of
the castings rolling laterally from the higher to the lower parts；
which would thus be raised。 Any projection beneath a ledge would
not afterwards receive disintegrated matter from above； and would
tend to be obliterated by rain and other atmospheric agencies。
There is some analogy between the formation； as here supposed； of
these ledges； and that of the ripples of wind…drifted sand as
described by Lyell。 ｛78｝
The steep； grass…covered sides of a mountainous valley in
Westmoreland； called Grisedale； was marked in many places with
innumerable lines of miniature cliffs； with almost horizontal；
little ledges at their bases。 Their formation was in no way
connected with the action of worms； for castings could not anywhere
be seen （and their absence is an inexplicable fact）； although the
turf lay in many places over a considerable thickness of boulder…
clay and moraine rubbish。 Nor； as far as I could judge； was the
formation of these little cliffs at all closely connected with the
trampling of cows or sheep。 It appeared as if the whole
superficial； somewhat argillaceous earth； while partially held
together by the roots of the grasses； had slided a little way down
the mountain sides； and in thus sliding； had yielded and cracked in
horizontal lines； transversely to the slope。
Castings blown to leeward by the wind。We have seen that moist
castings flow； and that disintegrated castings roll down any
inclined surface； and we shall now see that castings； recently
ejected on level grass…covered surfaces； are blown during gales of
wind accompanied by rain to leeward。 This has been observed by me
many times on many fields during several successive years。 After
such gales； the castings present a gently inclined and smooth； or
sometimes furrowed； surface to windward； while they are steeply
inclined or precipitous to leeward； so that they resemble on a
miniature scale glacier…ground hillocks of rock。 They are often
cavernous on the leeward side； from the upper part having curled
over the lower part。 During one unusually heavy south…west gale
with torrents of rain； many castings were wholly blown to leeward；
so that the mouths of the burrows were left naked and exposed on
the windward side。 Recent castings naturally flow down an inclined
surface； but on a grassy field； which sloped between 10 degrees and
15 degrees； several were found after a heavy gale blown up the
slope。 This likewise occurred on another occasion on a part of my
lawn where the slope was somewhat less。 On a third occasion； the
castings on the steep； grass…covered sides of a valley； down which
a gale had blown； were directed obliquely instead of straight down
the slope； and this was obviously due to the combined action of the
wind and gravity。 Four castings on my lawn； where the downward
inclination was 0 degrees 45 seconds； 1 degree； 3 degrees and 3
degrees 30 seconds （mean 2 degrees 45 seconds） towards the north
