the formation of vegetable mould-第34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！
filled up with brick…rubbish at the same time； viz。； seven years
ago； and was now covered with turf； and here the castings contained
very many particles of brick； all more or less rounded； and this
brick…rubbish； after being shot into the hole； could not have
undergone any attrition。 Again； old bricks very little broken；
together with fragments of mortar； were laid down to form walks；
and were then covered with from 4 to 6 inches of gravel； six little
fragments of brick were extracted from castings collected on these
walks； three of which were plainly worn。 There were also very many
particles of hard mortar； about half of which were well rounded；
and it is not credible that these could have suffered so much
corrosion from the action of carbonic acid in the course of only
seven years。
Much better evidence of the attrition of hard objects in the
gizzards of worms； is afforded by the state of the small fragments
of tiles or bricks； and of concrete in the castings thrown up where
ancient buildings once stood。 As all the mould covering a field
passes every few years through the bodies of worms； the same small
fragments will probably be swallowed and brought to the surface
many times in the course of centuries。 It should be premised that
in the several following cases； the finer matter was first washed
away from the castings； and then all the particles of bricks； tiles
and concrete were collected without any selection； and were
afterwards examined。 Now in the castings ejected between the
tesserae on one of the buried floors of the Roman villa at Abinger；
there were many particles （from to 2 mm。 in diameter） of tiles and
concrete； which it was impossible to look at with the naked eye or
through a strong lens； and doubt for a moment that they had almost
all undergone much attrition。 I speak thus after having examined
small water…worn pebbles； formed from Roman bricks； which M。 Henri
de Saussure had the kindness to send me； and which he had extracted
from sand and gravel beds； deposited on the shores of the Lake of
Geneva； at a former period when the water stood at about two metres
above its present level。 The smallest of these water…worn pebbles
of brick from Geneva resembled closely many of those extracted from
the gizzards of worms； but the larger ones were somewhat smoother。
Four castings found on the recently uncovered； tesselated floor of
the great room in the Roman villa at Brading； contained many
particles of tile or brick； of mortar； and of hard white cement；
and the majority of these appeared plainly worn。 The particles of
mortar； however； seemed to have suffered more corrosion than
attrition； for grains of silex often projected from their surfaces。
Castings from within the nave of Beaulieu Abbey； which was
destroyed by Henry VIII。； were collected from a level expanse of
turf； overlying the buried tesselated pavement； through which worm…
burrows passed； and these castings contained innumerable particles
of tiles and bricks； of concrete and cement； the majority of which
had manifestly undergone some or much attrition。 There were also
many minute flakes of a micaceous slate； the points of which were
rounded。 If the above supposition； that in all these cases the
same minute fragments have passed several times through the
gizzards of worms； be rejected； notwithstanding its inherent
probability； we must then assume that in all the above cases the
many rounded fragments found in the castings had all accidentally
undergone much attrition before they were swallowed； and this is
highly improbable。
On the other hand it must be stated that fragments of ornamental
tiles； somewhat harder than common tiles or bricks； which had been
swallowed only once by worms kept in confinement； were with the
doubtful exception of one or two of the smallest grains； not at all
rounded。 Nevertheless some of them appeared a little worn； though
not rounded。 Notwithstanding these cases； if we consider the
evidence above given； there can be little doubt that the fragments；
which serve as millstones in the gizzards of worms； suffer； when of
a not very hard texture； some amount of attrition； and that the
smaller particles in the earth； which is habitually swallowed in
such astonishingly large quantities by worms； are ground together
and are thus levigated。 If this be the case； the 〃terra
tenuissima；〃the 〃pate excessivement fine；〃of which the castings
largely consist； is in part due to the mechanical action of the
gizzard； ｛75｝ and this fine matter； as we shall see in the next
chapter； is that which is chiefly washed away from the innumerable
castings on every field during each heavy shower of rain。 If the
softer stones yield at all； the harder ones will suffer some slight
amount of wear and tear。
The trituration of small particles of stone in the gizzards of
worms is of more importance under a geological point of view than
may at first appear to be the case； for Mr。 Sorby has clearly shown
that the ordinary means of disintegration； namely； running water
and the waves of the sea； act with less and less power on fragments
of rock the smaller they are。 〃Hence；〃 as he remarks； 〃even making
no allowance for the extra buoying up of very minute particles by a
current of water； depending on surface cohesion； the effects of
wearing on the form of the grains must vary directly as their
diameter or thereabouts。 If so； a grain of 1/10 an inch in
diameter would be worn ten times as much as one of an inch in
diameter； and at least a hundred times as much as one of 1/100 an
inch in diameter。 Perhaps； then； we may conclude that a grain 1/10
of an inch in diameter would be worn as much or more in drifting a
mile as a grain 1/1000 of an inch in being drifted 100 miles。 On
the same principle a pebble one inch in diameter would be worn
relatively more by being drifted only a few hundred yards。〃 ｛76｝
Nor should we forget； in considering the power which worms exert in
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 fac
