The Formation of Vegetable Mould through the Action of Worms

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 leads of glass and fragments of brick and of hard tiles were scattered over the surface * Morren, in speaking of the earth in the alimentary canals of worms, says, “* preesepe cum lapillis commixtam vidi:” ‘ De i: umbrici terrestris,’ &., 1829, p. 16.

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, intes- tines, 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 dif- ferent kinds of leaves. It is therefore mauifest 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,* and

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 con- clusion is supported by the appearance of freshly ejected castings, for these often re- minded 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. speaks of “l'état de pate excessivement fine a laquelle est réduite la terre qu’ils rejettent,” &e.F As the amount of trituration which the particles of earth undergo in the gizzards of worms possesses some interest (as we shall hereafter see), [ 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 re- peatedly 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-mollusce (as ascertained by their microscopical structure), which latter were not only rounded but somewhat: polished. The calcareous concre- tions 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 frag- ments 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 cen- turies. 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 con- erete were collected without any selection, and were afterwards examined. Now in the cast- ings ejected between the tesserae on one of the buried floors of the Roman villa at Abinger, there were many particles (from 4 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 un- covered, 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 VIIL, were collected from a level expanse of turf, overlying the buried tessclated 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 under- gone some or much attrition. There were also many minute flakes of a micaceous slate, the points of which were rounded. Ifthe 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 ex- ception of one or two of the smallest grains, 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;* 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. Ifthe softer stones yield at all, the harder ones will suffer some slight amount of wear and tear.

* This conclusion reminds me of the vast amount of extremely fine chalky mud which is found within the lagoons of many atolls, where the sea is tranqnil and waves cannot triturate the blocks of coral. This mud must, as I believe (‘ The Structure and Distribution of Coral-Reefs,’ 2nd edit. 1874, p. 19), be attributed to the innumerable annelids and other animals which burrow into the dead coral, ard to the fishes, Holothurians, &c., which browse on the living corals

The trituration of small particles of stone in the gizzards of worms is of more import- ance 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 yy “of an inch in diameter would be worn ten “times as much as one ,3, of an inch in “diameter, and at least a hundred times as “much as one ygl5y of an inch in diameter. “Perhaps, then, we may conclude that a “grain +, of an inch in diameter would be “worn as much or more in drifting a mile as “a orain yyp of an inch in being drifted 100 miles. On the same principle a pebble “one inch in diameter would be worn re- “Jatively more by being drifted only a few

“hundred yards.”* 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 ot 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.

* Anniversary Address: ‘The Quarterly Journal of the Geological Soc.’ May 1880, p. 59. Denudation aided by recently ejected castings flowing down aclined grass-covered surfaces—The amount of earth which annually flows downwards—The effect of tropical rain on worm castings—The finest particles of earth washed com- pletely away from castings—The disintegration of dried caste ings into pellets, and their rolling down inclined surfaces— The formation of little ledges on hill-sides, in part due to the accumulation of disintegrated castings—Castings blown to leeward over level land—An attempt to estimate the amount thus blown—The degradation of ancient encampments and tumuli—The preservation of the crowns and furrows on land anciently ploughed—The formation and amount of mould over the Chalk formation.

We are now prepared to consider the more direct part which worms take in the denuda- tion 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 ex- amining 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 in- clined 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 levi- gated 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 scen by

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 Knowle 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 con- siderably 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 come 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 oetween 8° and 11° 30’ 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 74 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, ard the flowing-down of these yellow castings after heavy rain, could be clearly seen where the slope was 5°; and where it was less than 1° 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. Hight 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° 30’ and 17° 30’; the mean of the eleven inclinations being 9° 26’. 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 itsangle. Thus with four castings on my lawn (included in the above eleven) where the mean slope was 7° 19’, the difference in the amount of earth above and below the burrows was greater than with three other

We may, however, take the above eleven eases, 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° 26’, This was done by my son George. It has been shown that almost exactly two-thirds of the ejected earth is found below the mouth of the burrow and one-third above it. Now if the two-thirds which is below the hole be divided into two equal parts, the upper half of this two-thirds exactly counterbalances the one- third which is above the hole, so that as far as regards the one-thiid above and the upper half of the two-thirds below, there is no flow of earth down the hill-side. The earth con- stituting the lower half of the two-thirds is, however, displaced through distances which are different for every part of it, but which may be represented by the distance between the middle point of the lower half of the two-thirds and the hole. So that the average distance of displacement is a half of the whole length of the worm-casting, Now the

average length of ten out of the above eleven castings was 2°03 inches, and half. of this we may take as being one inch. It may therefore be concluded that one-third of the whole earth brought to the surface was in these cases carried down the slope through one inch. It was shown in the third chapter that on Leith Hill Common, dry earth weighing at least 7°453 lbs. was brought up by worms to the surface on a square yard in the course of a year. If a square yard be drawn on a hill-side with two of its sides horizontal, then it is clear that only 3; part of the earth brought up on that square yard would be near enough to its lower side to cross it, supposing the displacement of the earth to be through one inch, But it appears that only 4 of the earth brought up can be con- sidered to flow downwards; hence 4 of 3’; or sax Of 7453 lbs. will cross the lower side of our square yard in a year. Now yg of 7453 lbs. is l-l oz. Therefore 1:1 oz. of dry earth will annually cross each linear yard run- ning horizontally along a slope having the above inclination; or very nearly 7 Ibs, will

annually cross a horizontal line, 100 yards in length, on a hill-side having this inclination. A more accurate, though still very rough, calculation can be made of the bulk of earth, which in its natural damp state annually flows down the same slope over a yard-line drawn horizontally across it. From the several cases given in the third chapter, it is known that the castines annually brought to the surface on a square yard, if uniformly spread out would form a layer -2 of an inch in thickness: it therefire follows by a calculation similar to the one already given, that + of :2 x 36, or 2°4 cubic inches of damp earth will annually cross a horizontal line one yard in length on a hill-side with the above inclination. This bulk of damp castings was found to weigh 1°85 oz. Therefore 11°56 lbs. of damp earth, instead of 7 lbs. of dry earth as by-the former calculation, would annually cross a line 100 yards in longth on our inclined surface.

In these calculations it has been assumed that the castings flow a short distance down- wards during the whole year, but this occurs only with those ejected during or shortly before rain; so that the above results are thus far exaggerated. On the other hand, during rain much of the finest earth is washed to a considerable distance from the castings, even where the slope is an ex- tremely gentle one, and is thus wholly lost as far as the above calculations are concerned. Castings ejected during dry weather and which have set hard, lose in the same manner a considerable quantity of fine earth. Dried castings, moreover, are apt to disinte- grate into little pellets, which often roll or are blown down any inclined surface. There- fore the above result, namely that 2°4 cubic inches of earth (weighing 1°85 oz. whilst damp) annually crosses a yard-line of the specified kind, is probably not much if at all exaggerated,

This amount is small; but we should bear in mind how many branching valleys inter- sect most countries, the whole length of which must be very great; and that earth is steadily travelling down both turf-covered sides of each valley. For every 100 yards in length in a valley with sides sloping as in the foregoing cases, 480 cubic inches of damp earth, weighing above 23 pounds, will annually reach the bottom. Here a thick: bed of alluvium will accumulate, ready to be washed away in the course of centuries, as the stream in the middle meanders from side to side.

If it could be shown that worms generally excavate their burrows at right angles to an inclined surface, and this would be their shortest course for bringing up earth from beneath, then as the old burrows col- lapsed from the weight of the superincum- bent soil, the collapsing would inevitably cause the whole bed of vegetable mould to sink or slide slowly down the inclined sur- face. But to ascertain the direction of many burrows was found too difficult and trouble- some. A straight piece of wire was, how- ever, pushed into twenty-five burrows on several sloping fields, and in eight cases the burrows were nearly at right angles to the slope; whilst in the remaining cases they were indifferently directed at various angles, either upwards or downwards with respect to the slope.

as in the tropics, the castings appear, as might have been expected, to be washed down in a greater degree than in England. Mr. Scott informs me that near Calcutta the tall columnar castings (previously described). the diameter of which is usually between 1 and 13 inch, subside on a level surface, after heavy rain, into almost circular, thin, flat discs, between 3 and 4 and sometimes 5 inches in diameter. Three fresh castings, which had been ejected in the Botanic Gardens “on a slightly inclined, grass- “ covered, artificial bank of loamy clay,” were carefully measured, and had a mean height of 2°17, and a mean diameter of 1°43 inches ; these after heavy rain, formed elongated patches of earth, with a mean length in the direction of the slope of 5°83 inches. As the earth had spread very litile up the slope, a large part, judging from the original diameter of these castings, must have flowed bodily downwards about 4 inches. Moreover some of the finest earth of which they were com- posed must have been washed completely away to a still greater distance. In drier sites near Calcutta, a species of worm ejects

its castings, not In vermiform masses, but in little pellets of varying sizes: these are very numerous in some places, and Mr. Scott says that they “are washed away by every *¢ shower.” I was led to believe that a considerable quantity of fine earth is washed quite away from castings during rain, from the surfaces of old ones being often studded with coarse particles. Accordingly a little fine precipi- tated chalk, moistened with saliva or gum- water, so as to be slightly viscid and of the same consistence as a fresh casting, was placed on the summits of several castings and gently mixed with them. These castings were then watered through a very fine rose, the drops from which were closer together than those of rain, but not nearly so large as those in a thunder storm ; nor did they strike the ground with nearly so much force as drops during heavy rain. A casting thus treated subsided with surprising slowness, owing as I suppose to its viscidity. It did not flow bodily down the grass-covered sur- face of the lawn, which was here inclined at an angle of 16° 20’; nevertheless many par-

ticles of the chalk were found three inches below the casting. The experiment was re- peated on three other castings on different parts of the lawn, which sloped at 2° 30’, 3° and 6°; and particles of chalk could be seen between 4 and 5 inches below the cast- ing ; and after the surface had become dry, particles were found in two cases at a distance of 5 and 6 inches. Several other castings with precipitated chalk placed on their summits were left to the natural action of the rain. In one case, after rain which was not heavy, the casting was longitudinally streaked with white. In two other cases the surface of the ground was rendered some- what white for a distance of one inch from the casting ; and some soil collected at a dis- tance of 23 inches, where the slope was 7°, effervesced slightly when placed ia acid. After one or two weeks, the chalk was wholly or almost wholly washed away from all the castings on which it had been placed, and these had recovered their natural colour.

It may be here remarked that after very heavy rain shallow pools may be seen on level or nearly level fields, where the soil is not very porous, and the water in them is often slightly muddy; when such little pools have dried, the leaves and blades of grass at their bottoms are generally coated with a thin layer of mud. This mud I believe is derived in large part from recently ejected castings. Dr. King informs me that the majority of the before described gigantic castings, which he found on a fully exposed, bare, gravelly knoll on the Nilgiri Mountains in India, had been more or less weathered by the previous north-east monsoon ; and most of them pre- sented a subsided appearance. The worms here eject their castings only during the rainy season ; and at the time of Dr. King’s visit no rain had fallen for 110 days. He carefully examined the ground between the place where these huge castings lay, and a little water-course at the base of the knoll, and nowhere was there any accumulation of fine earth, such as would necessarily have been left by the disintegration of the castings if they had not been wholly removed. He therefore has no hesitation in asserting that the whole of these huge castings are annually washed during the two monsoons (when