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, {210} 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.” {211} 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. {212} 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 {213} 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. {215} 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. {217a} 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; {217b} 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 Thénard
azohumic, are enabled to dissolve colloid silica in proportion to the
nitrogen which they contain. {220} 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. {222} 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. {223} 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. {224a} 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. {224b} 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, {225a} as “une
véritable armature.” The gizzard is surrounded by powerful transverse
muscles, which, according to Claparède, 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. {225b} 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. {226}
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,
{227} 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
tenuissimâ terrâ, veluti in pulverem redactâ.” {228a} Perrier also
speaks of “l’état de pâte excessivement fine à laquelle est réduite la
terre qu’ils rejettent,” &c. {228b}
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 tesseræ on one of the buried floors of the Roman villa at
Abinger, there were many particles (from ½ 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 “pâte excessivement fine,”—of which the castings largely
consist, is in part due to the mechanical action of the gizzard; {234}
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.” {236} 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.