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Chapter V
THE ACTION OF WORMS IN THE DENUDATION OF THE LAND.
Evidence of the amount of denudation which the land has undergone--
Sub-aerial denudation--The deposition of dust--Vegetable mould, its
dark colour and fine texture largely due to the action of worms--
The disintegration of rocks by the humus-acids --Similar acids
apparently generated within the bodies of worms--The action of
these acids facilitated by the continued movement of the particles
of earth--A thick bed of mould checks the disintegration of the
underlying soil and rocks. Particles of stone worn or triturated
in the gizzards of worms--Swallowed stones serve as mill-stones--
The levigated state of the castings--Fragments of brick in the
castings over ancient buildings well rounded. The triturating
power of worms not quite insignificant under a geological point of
view.
No one doubts that our world at one time consisted of crystalline
rocks, and that it is to their disintegration through the action of
air, water, changes of temperature, rivers, waves of the sea,
earthquakes and volcanic outbursts, that we owe our sedimentary
formations. These after being consolidated and sometimes
recrystallized, have often been again disintegrated. Denudation
means the removal of such disintegrated matter to a lower level.
Of the many striking results due to the modern progress of geology
there are hardly any more striking than those which relate to
denudation. It was long ago seen that there must have been an
immense amount of denudation; but until the successive formations
were carefully mapped and measured, no one fully realised how great
was the amount. One of the first and most remarkable memoirs ever
published on this subject was that by Ramsay, {57} who in 1846
showed that in Wales from 9000 to 11,000 feet in thickness of solid
rock had been stripped off large tracks of country. Perhaps the
plainest evidence of great denudation is afforded by faults or
cracks, which extend for many miles across certain districts, with
the strata on one side raised even ten thousand feet above the
corresponding strata on the opposite side; and yet there is not a
vestige of this gigantic displacement visible on the surface of the
land. A huge pile of rock has been planed away on one side and not
a remnant left.
Until the last twenty or thirty years, most geologists thought that
the waves of the sea were the chief agents in the work of
denudation; but we may now feel sure that air and rain, aided by
streams and rivers, are much more powerful agents,--that is if we
consider the whole area of the land. The long lines of escarpment
which stretch across several parts of England were formerly
considered to be undoubtedly ancient coast-lines; but we now know
that they stand up above the general surface merely from resisting
air, rain and frost better than the adjoining formations. It has
rarely been the good fortune of a geologist to bring conviction to
the minds of his fellow-workers on a disputed point by a single
memoir; but Mr. Whitaker, of the Geological Survey of England, was
so fortunate when, in 1867, he published his paper "On sub-aerial
Denudation, and on Cliffs and Escarpments of the Chalk." {58}
Before this paper appeared, Mr. A. Tylor had adduced important
evidence on sub-aerial denudation, by showing that the amount of
matter brought down by rivers must infallibly lower the level of
their drainage basins by many feet in no immense lapse of time.
This line of argument has since been followed up in the most
interesting manner by Archibald Geikie, Croll and others, in a
series of valuable memoirs. {59} For the sake of those who have
never attended to this subject, a single instance may be here
given, namely, that of the Mississippi, which is chosen because the
amount of sediment brought down by this great river has been
investigated with especial care by order of the United States
Government. The result is, as Mr. Croll shows, that the mean level
of its enormous area of drainage must be lowered 1/4566 of a foot
annually, or 1 foot in 4566 years. Consequently, taking the best
estimate of the mean height of the North American continent, viz.
748 feet, and looking to the future, the whole of the great
Mississippi basin will be washed away, and "brought down to the
sea-level in less than 4,500,000 years, if no elevation of the land
takes place." Some rivers carry down much more sediment relatively
to their size, and some much less than the Mississippi.
Disintegrated matter is carried away by the wind as well as by
running water. During volcanic outbursts much rock is triturated
and is thus widely dispersed; and in all arid countries the wind
plays an important part in the removal of such matter. Wind-driven
sand also wears down the hardest rocks. I have shown {60} that
during four months of the year a large quantity of dust is blown
from the north-western shores of Africa, and falls on the Atlantic
over a space of 1600 miles in latitude, and for a distance of from
300 to 600 miles from the coast. But dust has been seen to fall at
a distance of 1030 miles from the shores of Africa. During a stay
of three weeks at St. Jago in the Cape Verde Archipelago, the
atmosphere was almost always hazy, and extremely fine dust coming
from Africa was continually falling. In some of this dust which
fell in the open ocean at a distance of between 330 and 380 miles
from the African coast, there were many particles of stone, about
1/1000 of an inch square. Nearer to the coast the water has been
seen to be so much discoloured by the falling dust, that a sailing
vessel left a track behind her. In countries, like the Cape Verde
Archipelago, where it seldom rains and there are no frosts, the
solid rock nevertheless disintegrates; and in conformity with the
views lately advanced by a distinguished Belgian geologist, De
Koninck, such disintegration may be attributed in chief part to the
action of the carbonic and nitric acids, together with the nitrates
and nitrites of ammonia, dissolved in the dew.
In all humid, even moderately humid, countries, worms aid in the
work of denudation in several ways. The vegetable mould which
covers, as with a mantle, the surface of the land, has all passed
many times through their bodies. Mould differs in appearance from
the subsoil only in its dark colour, and in the absence of
fragments or particles of stone (when such are present in the
subsoil), larger than those which can pass through the alimentary
canal of a worm. This sifting of the soil is aided, as has already
been remarked, by burrowing animals of many kinds, especially by
ants. In countries where the summer is long and dry, the mould in
protected places must be largely increased by dust blown from other
and more exposed places. For instance, the quantity of dust
sometimes blown over the plains of La Plata, where there are no
solid rocks, is so great, that during the "gran seco," 1827 to
1830, the appearance of the land, which is here unenclosed, was so
completely changed that the inhabitants could not recognise the
limits of their own estates, and endless lawsuits arose. Immense
quantities of dust are likewise blown about in Egypt and in the
south of France. In China, as Richthofen maintains, beds appearing
like fine sediment, several hundred feet in thickness and extending
over an enormous area, owe their origin to dust blown from the high
lands of central Asia. {61} In humid countries like Great Britain,
as long as the land remains in its natural state clothed with
vegetation, the mould in any one place can hardly be much increased
by dust; but in its present condition, the fields near high roads,
where there is much traffic, must receive a considerable amount of
dust, and when fields are harrowed during dry and windy weather,
clouds of dust may be seen to be blown away. But in all these
cases the surface-soil is merely transported from one place to
another. The dust which falls so thickly within our houses
consists largely of organic matter, and if spread over the land
would in time decay and disappear almost entirely. It appears,
however, from recent observations on the snow-fields of the Arctic
regions, that some little meteoric dust of extra mundane origin is
continually falling.
The dark colour of ordinary mould is obviously due to the presence
of decaying organic matter, which, however, is present in but small
quantities. The loss of weight which mould suffers when heated to
redness seems to be in large part due to water in combination being
dispelled. In one sample of fertile mould the amount of organic
matter was ascertained to be only 1.76 per cent.; in some
artificially prepared soil it was as much as 5.5 per cent., and in
the famous black soil of Russia from 5 to even 12 per cent. {62}
In leaf-mould formed exclusively by the decay of leaves the amount
is much greater, and in peat the carbon alone sometimes amounts to
64 per cent.; but with these latter cases we are not here
concerned. The carbon in the soil tends gradually to oxidise and
to disappear, except where water accumulates and the climate is
cool; {63} so that in the oldest pasture-land there is no great
excess of organic matter, notwithstanding the continued decay of
the roots and the underground stems of plants, and the occasional
addition of manure. The disappearance of the organic matter from
mould is probably much aided by its being brought again and again
to the surface in the castings of worms.
Worms, on the other hand, add largely to the organic matter in the
soil by the astonishing number of half-decayed leaves which they
draw into their burrows to a depth of 2 or 3 inches. They do this
chiefly for obtaining food, but partly for closing the mouths of
their burrows and for lining the upper part. The leaves which they
consume are moistened, torn into small shreds, partially digested,
and intimately commingled with earth; and it is this process which
gives to vegetable mould its uniform dark tint. It is known that
various kinds of acids are generated by the decay of vegetable
matter; and from the contents of the intestines of worms and from
their castings being acid, it seems probable that the process of
digestion induces an analogous chemical change in the swallowed,
triturated, and half-decayed leaves. The large quantity of
carbonate of lime secreted by the calciferous glands apparently
serves to neutralise the acids thus generated; for the digestive
fluid of worms will not act unless it be alkaline. As the contents
of the upper part of their intestines are acid, the acidity can
hardly be due to the presence of uric acid. We may therefore
conclude that the acids in the alimentary canal of worms are formed
during the digestive process; and that probably they are nearly of
the same nature as those in ordinary mould or humus. The latter
are well known to have the power of de-oxidising or dissolving per-
oxide of iron, as may be seen wherever peat overlies red sand, or
where a rotten root penetrates such sand. Now I kept some worms in
a pot filled with very fine reddish sand, consisting of minute
particles of silex coated with the red oxide of iron; and the
burrows, which the worms made through this sand, were lined or
coated in the usual manner with their castings, formed of the sand
mingled with their intestinal secretions and the refuse of the
digested leaves; and this sand had almost wholly lost its red
colour. When small portions of it were placed under the
microscope, most of the grains were seen to be transparent and
colourless, owing to the dissolution of the oxide; whilst almost
all the grains taken from other parts of the pot were coated with
the oxide. Acetic acid produced hardly any effect on his sand; and
even hydrochloric, nitric and sulphuric acids, diluted as in the
Pharmacopoeia, produced less effect than did the acids in the
intestines of the worms.
Mr. A. A. Julien has lately collected all the extant information
about the acids generated in humus, which, according to some
chemists, amount to more than a dozen different kinds. These
acids, as well as their acid salts (i.e., in combination with
potash, soda, and ammonia), act energetically on carbonate of lime
and on the oxides of iron. It is also known that some of these
acids, which were called long ago by Thenard azohumic, are enabled
to dissolve colloid silica in proportion to the nitrogen which they
contain. {64} In the formation of these latter acids worms
probably afford some aid, for Dr. H. Johnson informs me that by
Nessler's test he found 0.018 per cent. of ammonia in their
castings.
It may be here added that I have recently been informed by Dr.
Gilbert "that several square yards on his lawn were swept clean,
and after two or three weeks all the worm-castings on the space
were collected and dried. These were found to contain 0.35 of
nitrogen. This is from two to three times as much as we find in
our ordinary arable surface-soil; more than in our ordinary pasture
surface-soil; but less than in rich kitchen-garden mould.
Supposing a quantity of castings equal to 10 tons in the dry state
were annually deposited on an acre, this would represent a manuring
of 78 lbs. of nitrogen per acre per annum; and this is very much
more than the amount of nitrogen in the annual yield of hay per
acre, if raised without any nitrogenous manure. Obviously, so far
as the nitrogen in the castings is derived from surface-growth or
from surface-soil, it is not a gain to the latter; but so far as it
is derived from below, it is a gain."
The several humus-acids, which appear, as we have just seen, to be
generated within the bodies of worms during the digestive process,
and their acid salts, play a highly important part, according to
the recent observations of Mr. Julien, in the disintegration of
various kinds of rocks. It has long been known that the carbonic
acid, and no doubt nitric and nitrous acids, which are present in
rain-water, act in like manner. There is, also, a great excess of
carbonic acid in all soils, especially in rich soils, and this is
dissolved by the water in the ground. The living roots of plants,
moreover, as Sachs and others have shown, quickly corrode and leave
their impressions on polished slabs of marble, dolomite and
phosphate of lime. They will attack even basalt and sandstone.
{65} But we are not here concerned with agencies which are wholly
independent of the action of worms.
The combination of any acid with a base is much facilitated by
agitation, as fresh surfaces are thus continually brought into
contact. This will be thoroughly effected with the particles of
stone and earth in the intestines of worms, during the digestive
process; and it should be remembered that the entire mass of the
mould over every field, passes, in the course of a few years,
through their alimentary canals. Moreover as the old burrows
slowly collapse, and as fresh castings are continually brought to
the surface, the whole superficial layer of mould slowly revolves
or circulates; and the friction of the particles one with another
will rub off the finest films of disintegrated matter as soon as
they are formed. Through these several means, minute fragments of
rocks of many kinds and mere particles in the soil will be
continually exposed to chemical decomposition; and thus the amount
of soil will tend to increase.
As worms line their burrows with their castings, and as the burrows
penetrate to a depth of 5 or 6, or even more feet, some small
amount of the humus-acids will be carried far down, and will there
act on the underlying rocks and fragments of rock. Thus the
thickness of the soil, if none be removed from the surface, will
steadily though slowly tend to increase; but the accumulation will
after a time delay the disintegration of the underlying rocks and
of the more deeply seated particles. For the humus-acids which are
generated chiefly in the upper layer of vegetable mould, are
extremely unstable compounds, and are liable to decomposition
before they reach any considerable depth. {66} A thick bed of
overlying soil will also check the downward extension of great
fluctuations of temperature, and in cold countries will check the
powerful action of frost. The free access of air will likewise be
excluded. From these several causes disintegration would be almost
arrested, if the overlying mould were to increase much in
thickness, owing to none or little being removed from the surface.
{67} In my own immediate neighbourhood we have a curious proof how
effectually a few feet of clay checks some change which goes on in
flints, lying freely exposed; for the large ones which have lain
for some time on the surface of ploughed fields cannot be used for
building; they will not cleave properly, and are said by the
workmen to be rotten. {68} It is therefore necessary to obtain
flints for building purposes from the bed of red clay overlying the
chalk (the residue of its dissolution by rain-water) or from the
chalk itself.
Not only do worms aid directly in the chemical disintegration of
rocks, but there is good reason to believe that they likewise act
in a direct and mechanical manner on the smaller particles. All
the species which swallow earth are furnished with gizzards; and
these are lined with so thick a chitinous membrane, that Perrier
speaks of it, {69} as "une veritable armature." The gizzard is
surrounded by powerful transverse muscles, which, according to
Claparede, are about ten times as thick as the longitudinal ones;
and Perrier saw them contracting energetically. Worms belonging to
one genus, Digaster, have two distinct but quite similar gizzards;
and in another genus, Moniligaster, the second gizzard consists of
four pouches, one succeeding the other, so that it may almost be
said to have five gizzards. {70} In the same manner as
gallinaceous and struthious birds swallow stones to aid in the
trituration of their food, so it appears to be with terricolous
worms. The gizzards of thirty-eight of our common worms were
opened, and in twenty-five of them small stones or grains of sand,
sometimes together with the hard calcareous concretions formed
within the anterior calciferous glands, were found, and in two
others concretions alone. In the gizzards of the remaining worms
there were no stones; but some of these were not real exceptions,
as the gizzards were opened late in the autumn, when the worms had
ceased to feed and their gizzards were quite empty. {71}
When worms make their burrows through earth abounding with little
stones, no doubt many will be unavoidably swallowed; but it must
not be supposed that this fact accounts for the frequency with
which stones and sand are found in their gizzards. For beads of
glass and fragments of brick and of hard tiles were scattered over
the surface of the earth, in pots in which worms were kept and had
already made their burrows; and very many of these beads and
fragments were picked up and swallowed by the worms, for they were
found in their castings, intestines, and gizzards. They even
swallowed the coarse red dust, formed by the pounding of the tiles.
Nor can it be supposed that they mistook the beads and fragments
for food; for we have seen that their taste is delicate enough to
distinguish between different kinds of leaves. It is therefore
manifest that they swallow hard objects, such as bits of stone,
beads of glass and angular fragments of bricks or tiles for some
special purpose; and it can hardly be doubted that this is to aid
their gizzards in crushing and grinding the earth, which they so
largely consume. That such hard objects are not necessary for
crushing leaves, may be inferred from the fact that certain
species, which live in mud or water and feed on dead or living
vegetable matter, but which do not swallow earth, are not provided
with gizzards, {72} and therefore cannot have the power of
utilising stones.
During the grinding process, the particles of earth must be rubbed
against one another, and between the stones and the tough lining
membrane of the gizzard. The softer particles will thus suffer
some attrition, and will perhaps even be crushed. This conclusion
is supported by the appearance of freshly ejected castings, for
these often reminded me of the appearance of paint which has just
been ground by a workman between two flat stones. Morren remarks
that the intestinal canal is "impleta tenuissima terra, veluti in
pulverem redacta." {73} Perrier also speaks of "l'etat de pate
excessivement fine a laquelle est reduite la terre qu'ils
rejettent," &c. {74}
As the amount of trituration which the particles of earth undergo
in the gizzards of worms possesses some interest (as we shall
hereafter see), I endeavoured to obtain evidence on this head by
carefully examining many of the fragments which had passed through
their alimentary canals. With worms living in a state of nature,
it is of course impossible to know how much the fragments may have
been worn before they were swallowed. It is, however, clear that
worms do not habitually select already rounded particles, for
sharply angular bits of flint and of other hard rocks were often
found in their gizzards or intestines. On three occasions sharp
spines from the stems of rose-bushes were thus found. Worms kept
in confinement repeatedly swallowed angular fragments of hard tile,
coal, cinders, and even the sharpest fragments of glass.
Gallinaceous and struthious birds retain the same stones in their
gizzards for a long time, which thus become well rounded; but this
does not appear to be the case with worms, judging from the large
number of the fragments of tiles, glass beads, stones, &c.,
commonly found in their castings and intestines. So that unless
the same fragments were to pass repeatedly through their gizzards,
visible signs of attrition in the fragments could hardly be
expected, except perhaps in the case of very soft stones.
I will now give such evidence of attrition as I have been able to
collect. In the gizzards of some worms dug out of a thin bed of
mould over the chalk, there were many well-rounded small fragments
of chalk, and two fragments of the shells of a land-mollusc (as
ascertained by their microscopical structure), which latter were
not only rounded but somewhat polished. The calcareous concretions
formed in the calciferous glands, which are often found in their
gizzards, intestines, and occasionally in their castings, when of
large size, sometimes appeared to have been rounded; but with all
calcareous bodies the rounded appearance may be partly or wholly
due to their corrosion by carbonic acid and the humus-acids. In
the gizzards of several worms collected in my kitchen garden near a
hothouse, eight little fragments of cinders were found, and of
these, six appeared more or less rounded, as were two bits of
brick; but some other bits were not at all rounded. A farm-road
near Abinger Hall had been covered seven years before with brick-
rubbish to the depth of about 6 inches; turf had grown over this
rubbish on both sides of the road for a width of 18 inches, and on
this turf there were innumerable castings. Some of them were
coloured of a uniform red owing to the presence of much brick-dust,
and they contained many particles of brick and of hard mortar from
1 to 3 mm. in diameter, most of which were plainly rounded; but all
these particles may have been rounded before they were protected by
the turf and were swallowed, like those on the bare parts of the
road which were much worn. A hole in a pasture-field had been
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.
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