the formation of vegetable mould-第31节
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！
triturating particles of rock； that there is good evidence that on
each acre of land； which is sufficiently damp and not too sandy；
gravelly or rocky for worms to inhabit； a weight of more than ten
tons of earth annually passes through their bodies and is brought
to the surface。 The result for a country of the size of Great
Britain； within a period not very long in a geological sense； such
as a million years； cannot be insignificant； for the ten tons of
earth has to be multiplied first by the above number of years； and
then by the number of acres fully stocked with worms； and in
England； together with Scotland； the land which is cultivated and
is well fitted for these animals； has been estimated at above 32
million acres。 The product is 320 million million tons of earth。
CHAPTER VITHE DENUDATION OF THE LANDcontinued。
Denudation aided by recently ejected castings flowing down inclined
grass…covered surfacesThe amount of earth which annually flows
downwardsThe effect of tropical rain on worm castingsThe finest
particles of earth washed completely away from castingsThe
disintegration of dried castings into pellets； and their rolling
down inclined surfacesThe formation of little ledges on hill…
sides； in part due to the accumulation of disintegrated castings
Castings blown to leeward over level landAn attempt to estimate
the amount thus blownThe degradation of ancient encampments and
tumuliThe preservation of the crowns and furrows on land
anciently ploughedThe formation and amount of mould over the
Chalk formation。
We are now prepared to consider the more direct part which worms
take in the denudation of the land。 When reflecting on sub…aerial
denudation； it formerly appeared to me； as it has to others； that a
nearly level or very gently inclined surface； covered with turf；
could suffer no loss during even a long lapse of time。 It may；
however； be urged that at long intervals； debacles of rain or
water…spouts would remove all the mould from a very gentle slope；
but when examining the steep； turf…covered slopes in Glen Roy； I
was struck with the fact how rarely any such event could have
happened since the Glacial period； as was plain from the well…
preserved state of the three successive 〃roads〃 or lake…margins。
But the difficulty in believing that earth in any appreciable
quantity can be removed from a gently inclined surface； covered
with vegetation and matted with roots； is removed through the
agency of worms。 For the many castings which are thrown up during
rain； and those thrown up some little time before heavy rain； flow
for a short distance down an inclined surface。 Moreover much of
the finest levigated earth is washed completely away from the
castings。 During dry weather castings often disintegrate into
small rounded pellets； and these from their weight often roll down
any slope。 This is more especially apt to occur when they are
started by the wind； and probably when started by the touch of an
animal； however small。 We shall also see that a strong wind blows
all the castings； even on a level field； to leeward； whilst they
are soft； and in like manner the pellets when they are dry。 If the
wind blows in nearly the direction of an inclined surface； the
flowing down of the castings is much aided。
The observations on which these several statements are founded must
now be given in some detail。 Castings when first ejected are
viscid and soft； during rain； at which time worms apparently prefer
to eject them； they are still softer； so that I have sometimes
thought that worms must swallow much water at such times。 However
this may be； rain； even when not very heavy； if long continued；
renders recently…ejected castings semi…fluid； and on level ground
they then spread out into thin； circular； flat discs； exactly as
would so much honey or very soft mortar； with all traces of their
vermiform structure lost。 This latter fact was sometimes made
evident； when a worm had subsequently bored through a flat circular
disc of this kind； and heaped up a fresh vermiform mass in the
centre。 These flat subsided discs have been repeatedly seen by me
after heavy rain； in many places on land of all kinds。
On the flowing of wet castings； and the rolling of dry
disintegrated castings down inclined surfaces。When castings are
ejected on an inclined surface during or shortly before heavy rain；
they cannot fail to flow a little down the slope。 Thus； on some
steep slopes in Knole Park； which were covered with coarse grass
and had apparently existed in this state from time immemorial； I
found （Oct。 22； 1872） after several wet days that almost all the
many castings were considerably elongated in the line of the slope；
and that they now consisted of smooth； only slightly conical
masses。 Whenever the mouths of the burrows could be found from
which the earth had been ejected； there was more earth below than
above them。 After some heavy storms of rain （Jan。 25； 1872） two
rather steeply inclined fields near Down； which had formerly been
ploughed and were now rather sparsely clothed with poor grass； were
visited； and many castings extended down the slopes for a length of
5 inches； which was twice or thrice the usual diameter of the
castings thrown up on the level parts of these same fields。 On
some fine grassy slopes in Holwood Park； inclined at angles between
8 degrees and 11 degrees 30 seconds with the horizon； where the
surface apparently had never been disturbed by the hand of man；
castings abounded in extraordinary numbers： and a space 16 inches
in length transversely to the slope and 6 inches in the line of the
slope； was completely coated； between the blades of grass； with a
uniform sheet of confluent and subsided castings。 Here also in
many places the castings had flowed down the slope； and now formed
smooth narrow patches of earth； 6； 7； and 7。5 inches in length。
Some of these consisted of two castings； one above the other； which
had become so completely confluent that they could hardly be
distinguished。 On my lawn； clothed with very fine grass； most of
the castings are black； but some are yellowish from earth having
been brought up from a greater depth than usual； and the flowing…
down of these yellow castings after heavy rain； could be clearly
seen where the slope was 5 degrees； and where it was less than 1
degree some evidence of their flowing down could still be detected。
On another occasion； after rain which was never heavy； but which
lasted for 18 hours； all the castings on this same gently inclined
lawn had lost their vermiform structure； and they had flowed； so
that fully two…thirds of the ejected earth lay below the mouths of
the burrows。
These observations led me to make others with more care。 Eight
castings were found on my lawn； where the grass…blades are fine and
close together； and three others on a field with coarse grass。 The
inclination of the surface at the eleven places where these
castings were collected varied between 4 degrees 30 seconds and 17
degrees 30 seconds； the mean of the eleven inclinations being 9
degrees 26 seconds。 The length of the castings in the direction of
the slope was first measured with as much accuracy as their
irregularities would permit。 It was found possible to make these
measurements within about of an inch； but one of the castings was
too irregular to admit of measurement。 The average length in the
direction of the slope of the remaining ten castings was 2。03
inches。 The castings were then divided with a knife into two parts
along a horizontal line passing through the mouth of the burrow；
which was discovered by slicing off the turf； and all the ejected
earth was separately collected； namely； the part above the hole and
the part below。 Afterwards these two parts were weighed。 In every
case there was much more earth below than above； the mean weight of
that above being 103 grains； and of that below 205 grains； so that
the latter was very nearly double the former。 As on level ground
castings are commonly thrown up almost equally round the mouths of
the burrows； this difference in weight indicates the amount of
ejected earth which had flowed down the slope。 But very many more
observations would be requisite to arrive at any general result；
for the nature of the vegetation and other accidental
circumstances； such as the heaviness of the rain； the direction and
force of the wind； &c。； appear to be more important in determining
the quantity of the earth which flows down a slope than its angle。
Thus with four castings on my lawn （included in the above eleven）
where the mean slope was 7 degrees 19 seconds； the difference in
the amount of earth above and below the burrows was greater than
with three other castings on the same lawn where the mean slope was
12 degrees 5 seconds。
We may； however； take the above eleven cases； which are accurate as
far as they go； and calculate the weight of the ejected earth which
annually flows down a slope having a mean inclination of 9 degrees
26 seconds。 This was done by my
