the formation of vegetable mould-第31部分
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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
diagrams it could be seen to be thicker on the north…eastern side
than elsewhere。 When a mean of all the measurements in both the
trenches was laid down and the line smoothed； it was obvious that
the mould was thickest in the quarter of the circle between north…
west and north…east； and thinnest in the quarter between south…east
and south…west； especially at this latter point。 Besides the
foregoing measurements； six others were taken near together in one
of the circular trenches； on the north…east side； and the mould
here had a mean thickness of 2。29 inches； while the mean of six
other measurements on the south…west side was only 1。46 inches。
These observations indicate that the castings had been blown by the
south…west winds from the circular enclosed space into the trench
on the north…east side； but many more measurements in other
analogous cases would be requisite for a trustworthy result。
The amount of fine earth brought to the surface under the form of
castings； and afterwards transported by the winds accompanied by
rain； or that which flows and rolls down an inclined surface； no
doubt is small in the course of a few scores of years； for
otherwise all the inequalities in our pasture fields would be
smoothed within a much shorter period than appears to be the case。
But the amount which is thus transported in the course of thousands
of years cannot fail to be considerable and deserves attention。 E。
de Beaumont looks at the vegetable mould which everywhere covers
the land as a fixed line； from which the amount of denudation may
be measured。 ｛79｝ He ignores the continued formation of fresh
mould by the disintegration of the underlying rocks and fragments
of rock； and it is curious to find how much more philosophical were
the views maintained long ago； by Playfair； who； in 1802； wrote；
〃In the permanence of a coat of vegetable mould on the surface of
the earth； we have a demonstrative proof of the continued
destruction of the rocks。〃 ｛80｝
Ancient encampments and tumuli。E。 de Beaumont adduces the present
state of many ancient encampments and tumuli and of old ploughed
fields； as evidence that the surface of the land undergoes hardly
any degradation。 But it does not appear that he ever examined the
thickness of the mould over different parts of such old remains。
He relies chiefly on indirect； but apparently trustworthy； evidence
that the slopes of the old embankments are the same as they
originally were； and it is obvious that he could know nothing about
their original heights。 In Knole Park a mound had been thrown up
behind the rifle…targets； which appeared to have been formed of
earth originally supported by square blocks of turf。 The sides
sloped； as nearly as I could estimate them； at an angle of 45
degrees or 50 degrees with the horizon； and they were covered；
especially on the northern side； with long coarse grass； beneath
which many worm…castings were found。 These had flowed bodily
downwards； and others had rolled down as pellets。 Hence it is
certain that as long as a mound of this kind is tenanted by worms；
its height will be continually lowered。 The fine earth which flows
or rolls down the sides of such a mound accumulates at its base in
the form of a talus。 A bed； even a very thin bed； of fine earth is
eminently favourable for worms； so that a greater number of
castings would tend to be ejected on a talus thus formed than
elsewhere； and these would be partially washed away by every heavy
shower and be spread over the adjoining level ground。 The final
result would be the lowering of the whole mound； whilst the
inclination of the sides would not be greatly lessened。 The same
result would assuredly follow with ancient embankments and tumuli；
except where they had been formed of gravel or of nearly pure sand；
as such matter is unfavourable for worms。 Many old fortifications
and tumuli are believed to be at least 2000 years old； and we
should bear in mind that in many places about one inch of mould is
brought to the surface in 5 years or two inches in 10 years。
Therefore in so long a period as 2000 years； a large amount of
earth will have been repeatedly brought to the surface on most old
embankments and tumuli； especially on the talus round their bases；
and much of this earth will have been washed completely away。 We
may therefore conclude that all ancient mounds； when not formed of
materials unfavourable to worms； will have been somewhat lowered in
the course of centuries； although their inclinations may not have
been greatly changed。
Fields formerly ploughed。From a very remote period and in many
countries； land has been ploughed； so that convex beds； called
crowns or ridges； usually about 8 feet across and separated by
furrows； have been thrown up。 The furrows are directed so as to
carry off the surface water。 In my attempts to ascertain how long
a time these crowns and furrows last； when ploughed land has been
converted into pasture； obstacles of many kinds were encountered。
It is rarely known when a field was last ploughed； and some fields
which were thought to have been in pasture from time immemorial
were afterwards discovered to have been ploughed only 50 or 60
years before。 During the early part of the present century； when
the price of corn was very high； land of all kinds seems to have
been ploughed in Britain。 There is； however； no reason to doubt
that in many cases the old crowns and furrows have been preserved
from a very ancient period。 ｛81｝ That they should have been
preserved for very unequal lengths of time would naturally follow
from the crowns； when first thrown up； having differed much in
height in different districts； as is now the case with recently
ploughed land。
In old pasture fields； the mould； wherever measurements were made；
was found to be from 0。5 to 2 inches thicker in the furrows than on
the crowns； but this would naturally follow from the finer earth
having been washed from the crowns into the furrows before the land
was well clothed with
