the formation of vegetable mould-第39章
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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 turf； and it is impossible to tell what part
worms may have played in the work。 Nevertheless from what we have
seen； castings would certainly tend to flow and to be washed during
heavy rain from the crowns into the furrows。 But as soon as a bed
of fine earth had by any means been accumulated in the furrows； it
would be more favourable for worms than the other parts； and a
greater number of castings would be thrown up here than elsewhere；
and as the furrows on sloping land are usually directed so as to
carry off the surface water； some of the finest earth would be
washed from the castings which had been here ejected and be carried
completely away。 The result would be that the furrows would be
filled up very slowly； while the crowns would be lowered perhaps
still more slowly by the flowing and rolling of the castings down
their gentle inclinations into the furrows。
Nevertheless it might be expected that old furrows； especially
those on a sloping surface； would in the course of time be filled
up and disappear。 Some careful observers； however； who examined
fields for me in Gloucestershire and Staffordshire could not detect
any difference in the state of the furrows in the upper and lower
parts of sloping fields； supposed to have been long in pasture； and
they came to the conclusion that the crowns and furrows would last
for an almost endless number of centuries。 On the other hand the
process of obliteration seems to have commenced in some places。
Thus in a grass field in North Wales； known to have been ploughed
about 65 years ago； which sloped at an angle of 15 degrees to the
north…east； the depth of the furrows （only 7 feet apart） was
carefully measured； and was found to be about 4。5 inches in the
upper part of the slope； and only 1 inch near the base； where they
could be traced with difficulty。 On another field sloping at about
the same angle to the south…west； the furrows were scarcely
perceptible in the lower part； although t
