the formation of vegetable mould-第38章
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
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…
east； after a heavy south…west gale with rain； were divided across
the mouths of the burrows and weighed in the manner formerly
described。 The mean weight of the earth below the mouths of
burrows and to leeward； was to that above the mouths and on the
windward side as 2。75 to 1； whereas we have seen that with several
castings which had flowed down slopes having a mean inclination of
9 degrees 26 seconds； and with three castings where the inclination
was above 12 degrees； the proportional weight of the earth below to
that above the burrows was as only 2 to 1。 These several cases
show how efficiently gales of wind accompanied by rain act in
displacing recently ejected castings。 We may therefore conclude
that even a moderately strong wind will produce some slight effect
on them。
Dry and indurated castings； after their disintegration into small
fragments or pellets； are sometimes； probably often； blown by a
strong wind to leeward。 This was observed on four occasions； but I
did not sufficiently attend to this point。 One old casting on a
gently sloping bank was blown quite away by a strong south…west
wind。 Dr。 King believes that the wind removes the greater part of
the old crumbling castings near Nice。 Several old castings on my
lawn were marked with pins and protected from any disturbance。
They were examined after an interval of 10 weeks； during which time
the weather had been alternately dry and rainy。 Some； which were
of a yellowish colour had been washed almost completely away； as
could be seen by the colour of the surrounding ground。 Others had
completely disappeared； and these no doubt had been blown away。
Lastly； others still remained and would long remain； as blades of
grass had grown through them。 On poor pasture…land； which has
never been rolled and has not been much trampled on by animals； the
whole surface is sometimes dotted with little pimples； through and
on which grass grows； and these pimples consist of old worm…
castings。
In all the many observed cases of soft castings blown to leeward；
this had been effected by strong winds accompanied by rain。 As
such winds in England generally blow from the south and south…west；
earth must on the whole tend to travel over our fields in a north
and north…east direction。 This fact is interesting； because it
might be thought that none could be removed from a level； grass…
covered surface by any means。 In thick and level woods； protected
from the wind； castings will never be removed as long as the wood
lasts； and mould will here tend to accumulate to the depth at which
worms can work。 I tried to procure evidence as to how much mould
is blown； whilst in the state of castings； by our wet southern
gales to the north…east； over open and flat land； by looking to the
level of the surface on opposite sides of old trees and hedge…rows；
but I failed owing to the unequal growth of the roots of trees and
to most pasture…land having been formerly ploughed。
On an open plain near Stonehenge； there exist shallow circular
trenches； with a low embankment outside； surrounding level spaces
50 yards in diameter。 These rings appear very ancient； and are
believed to be contemporaneous with the Druidical stones。 Castings
ejected within these circular spaces； if blown to the north…east by
south…west winds would form a layer of mould within the trench；
thicker on the north…eastern than on any other side。 But the site
was not favourable for the action of worms； for the mould over the
surrounding Chalk formation with flints； was only 3。37 inches in
thickness； from a mean of six observations made at a distance of 10
yards outside the embankment。 The thickness of the mould within
two of the circular trenches was measured every 5 yards all round；
on the inner sides near the bottom。 My son Horace protracted these
measurements on paper； and though the curved line representing the
thickness of the mould was extremely irregular； yet in both
diagr
