The Formation of Vegetable Mould through the Action of Worms

von Haast has described * a section near the coast, consisting of mica-schist, “covered by “5 or 6 feet of loess, above which about 12 “inches of vegetable soil had accumulated.” Between the loess and the mould there was a layer from 3 to 6 inches in thickness, consisting of “cores, implements, flakes, and “chips, all manufactured from hard basaltic “rock.” It is therefore probable that the aborigines, at some former period, had left these objects on the surface, and that they had afterwards been slowly covered up by the castings of worms. ,

Farmers in England are well aware that objects of all kinds, left on the surface of ‘pasture-land, after a time disappear, or, as they say, work themselves downwards. How powdered lime, cinders, and heavy stones, can work down, and at the same rate, through the matted roots of a grass-covered surface, is a question which has probably never occurred to them. { Mr. Lindsay Carnagie, in a letter (June 1838) to Sir C Lyell, remarks that Scotch farmers are afraid of putting lime on

ploughed land until just before it is laid down for pasture, from a belief that it has some tendency to sink. He adds: “Some The Sinking of great Stones through the Action of Worms.—When a stone of large size and of irregular shape is left on the surface of the ground, it rests, of course, on the more protuberant parts; but worms soon fill up with their castings all the hollow spaces on the lower side; for, as Hensen re- marks, they like the shelter of stones, As soon as the hollows are filled up, the worms eject the earth which they have swallowed beyond the circumference of the stones; and thus the surface of the ground is raised all round the stone. As the burrows ex- eavated directly beneath the stone after a time collapse, the stone sinks a little* Hence it is, that boulders which at some ancient

years since, in autumn, I laid lime on an oat-stubble and ploughed it down ; thus bringing it into immediate contact with the dead vegetable matter, and securing its thorough mixture through the means of all the subsequent operations of fallow. In consequence of the above prejudice, I was considered to have committed a great fault; but the result was eminently successful, and the practice was partially followed. By means of Mr. Darwin’s observations, I think the prejudice will be removed.”

* This conclusion, which, as we shall immediately see, is fully jastified, is of some little importance, as the so-called bench-stones, which surveyors fix in the ground as a record of their levels, may in time become false standards. My son Horace intends at some future period to ascertain how far this has occurred. period have rolled down from a rocky moun- tain or cliff on to a meadow at its base, are always somewhat embedded in the soil; and, when removed, leave an exact impression of their lower surfaces in the under-lying fine mould. If, however, a boulder is of such huge dimensions, that the earth beneath is kept dry, such earth will not be inhabited by worms, and the boulder will not sink into the ground.

A lime-kiln formerly stood in a grass-field near Leith Hill Place in Surrey, and was pulled down 35 years before my _ visit; all the loose rubbish had been carted away, excepting three large stones of quartzose sandstone, which it was thought might here- after be of some use. An old workman re- membered that they had been left on a bare surface of broken bricks and mortar, close to the foundations of the kiln; but the whole surrounding surface is now covered with turf and mould. The two largest of these stones had never since been moved; nor could this easily have been done, as, when I had them removed, it was the work of two men with levers. One of these stones, and not the

largest, was 64 inches long, 17 inches broad, and from 9 to 10 inches in thickness. Its lower surface was somewhat protuberant in the middle; and this part still rested on broken bricks and mortar, showing the truth of the old workman’s account. Beneath the brick rubbish the natural sandy soil, full of fragments of sandstone was found; and this could have yielded very little, if at all, to the weight of the stone, as might have been expected if the sub-soil had been clay. The surface of the field, for a distance of about 9 inches round the stone, gradually sloped up to it, and close to the stone stood in most places about 4 inches above the surrounding ground. The base of the stone was buried from 1 to 2 inches beneath the general level, and the upper surface projected about 8 inches above this level, or about 4 inches above the sloping border of turf. After the removal of the stone it became evident that one of its poited ends must at first have stood clear above the ground by some inches, but its upper surface was now on a level with the surrounding turf. When the stone was removed, an exact cast of its lower

side, forming a shallow crateriform hollow, was left, the inner surface of which consisted of fine black mould, excepting where the more protuberant parts rested on the brick- rubbish. A transverse section of this stone, together with its bed, drawn from measure- ments made after it had been displaced, is here given on a scale of 3 inch to a foot Transverse section across a large stone, which had lain on a grass-field for 85 years. A A, general level of the field. The underlying brick rubbish has not been represented. Scale 3 inch to one foot,

sloped up to the stone, consisted of fine vegetable mould, in one part 7 inches in thickness. This evidently consisted of worm- castings, several of which had been recently ejected. The whole stone had sunk in the thirty-five years, as far as I could judge, about 1$ inch; and this must have been due to the brick-rubbish beneath the more pro- tuberant parts having been undermined by worms. At this rate the upper surface of the stone, if it had been left undisturbed, would have sunk to the general level of the field in 247 years; but before this could have occurred, some earth would have been washed down by heavy rain from the castings on the raised border of turf over the upper surface of the stone.

The second stone was larger than the one just described, viz., 67 inches in length, 39 in breadth, and 15 in thickness. The lower surface was nearly flat, so that the worms must soon have been compelled to eject their castings beyond its circumference. The stone as a whole had sunk about 2 inches into the ground. At this rate it would. have required 262 years for its upper surface to have sunk to the general level of the field. The up- wardly sloping, turf-covered border round the stone was broader than in the last case, viz, from 14 to 16 inches; and why this should be so, I could see no reason. In most parts this border was not so high as in the last case, viz., from 2 to 24 inches, but in one

place it was as much as 54. Its average height close to the stone was probably about 3 inches, and it thinned out to nothing. If so, a layer of fine earth, 15 inches in breadth and 14 inch in average thickness, of sufficient length to surround the whole of the much elongated slab, must have been brought up by the worms in chief part from beneath the stone in the course of 35 years. This amount would be amply sufficient to account for its having sunk about 2 inches into the ground; more especially if we bear in mind that a good deal of the finest earth would have been washed by heavy rain from the castings ejected on the sloping border down to the level of the field. Some fresh castings were seen close to the stone. Nevertheless, on digging a large hole to a depth of 18 inches where the stone had lain, only two worms and a few burrows were seen, although the soil was damp and seemed favourable for worms. There were some large colonies of ants beneath the stone, and possibly since their establishment the worms had decreased in number.

large as the others; and two strong boys could together have rolled it over. I have no doubt that it had been rolled over at a moderately recent time, for it now lay at some distance from the two other stones at the bottom of a little adjoining slope. It rested also on fine earth, instead of partly on brick-rubbish. In agreement with this con- clusion, the raised surrounding border of turf was only 1 inch high in some parts, and 2 inches in other parts. There were no colonies of ants beneath this stone, and on digging a hole where it had lain, several burrows and worms were found.

At Stonehenge, some of the outer Druidical stones are now prostrate, having fallen at a remote but unknown period; and these have become buried to a moderate depth in the ground. They are surrounded by sloping borders of turf, on which recent castings were seen. Close to one of these fallen stones, which was 17 ft. long, 6 ft. broad, and 284 inches thick, a hole was dug; and here the vegetable mould was at least 94 inches in thickness. At this depth a flint was found, and a little higher up on one side of the hole

a fragment of glass. The base of the stone lay about 94 inches beneath the level of the surrounding ground, and its upper surface 19 inches above the ground. A hole was also dug close to a second huge stone, which in falling had broken into two pieces; and this must have happened long ago, judging from the weathered aspect of the fractured ends. The base was buried to a depth of 10 inches, as was ascertained by driving an iron skewer horizontally into the eround beneath it. The vegetable mould forming the turf-covered sloping border round the stone, on which many castings had re- cently been ejected, was 10 inches in thick- ness; and most of this mould must have been brought up by worms from beneath its base. At a distance of 8 yards from the stone, the mould was only 53 inches in thickness (with a piece of tobacco pipe at a depth of 4 inches), and this rested on broken flint and chalk which could not have easily yielded to the pressure or weight of the stone.

A straight rod was fixed horizontally (by the aid of a spirit-level) across a third fallen contour of the projecting parts and of the ad- joining ground, which was not quite level, was thus ascertained, as shown in the ac- companying diagram (Fig. 7) on a scale of Section through one of the fallen Druidical stones at Stonehenge, showing how much it had sunk into the ground. Scale 4 inch to 1 foot. Zinch to a foot. The turf-covered border sloped up to the stone on one side to a height of 4 inches, and on the opposite side to only 24 inches above the general level. A hole was dug on the eastern side, and the base of the stone was here found to lie at a depth of 4 inches beneath the general level of the ground, and of 8 inches beneath the top of the sloping turf-covered border.

Sufficient evidence has now been given showing that small objects left on the surface of the land where worms abound soon get buried, and that large stones sink slowly downwards through the same means. Every step of the process could be followed, from the accidental deposition of a single casting on a small object lying loose on the surface, to its being entangled amidst the matted roots of the turf, and lastly to its being embedded in the mould at various depths beneath the surface. When the same field was re-ex- amined after the interval of a few years, such objects were found at a greater depth than before. The straightness and regularity of the lines formed by the embedded objects, and their parallelism with the surface of the land, are the most striking features of the case; for this parallelism shows how equably the worms must have worked; the result being, however, partly the effect of the wash- ing down of the fresh castings by rain. The specific gravity of the objects does not affect their rate of sinking, as could be seen by porous cinders, burnt marl, chalk and quartz pebbles, having all sunk to the same depth within the same time. Considering the nature of the substratum, which at Leith Hill

Place was sandy soil including many bits of rock, and at Stonehenge, chalk-rubble with broken flints; considering, also, the presence of the turf-covered sloping border of mould round the great fragments of stone at both these places, their sinking does not appear to have been sensibly aided by their weight, though this was considerable.* On the number of worms which live within a given space-—We will now show, firstly, what a vast number of worms live unseen by us beneath our feet, and, secondly, the actual weight of the earth which they bring up to the surface within a given space and within a given time. Hensen, who has published so full and interesting an account of the habits of worms,f calculates, from the number which he found in a measured space, that there must exist 133,000 living worms in a hectare of

* Mr. R. Mallet remarks (‘ Quarterly Journal of Geolog. Soc., vol. xxxiii., 1877, p. 745) that “ the extent to which the ground beneath the foundations of ponderous architectural structures, #.ch as cathedral towers, has been known to become compressed, is as remarkable as it is instructive and curious. ‘The amount of depression in some cases may be measured by feet.” He instances the Tower of Pisa, but adds that it was founded on “ dense clay.” land, or 53,767 in an acre. This latter number of worms would weigh 356 pounds, taking Hensen’s standard of the weight of a single worm, namely, one gram. It should, however, be noted that this calculation is founded on the numbers found in a garden, and Hensen believes that worms are here twice as numerous as in corn-fields. The above result, astonishing though it be, seems to me credible, judging from the number of worms which I have sometimes seen, and from the number daily destroyed by birds without the species being exterminated. Some barrels of bad ale were left on Mr. Miller’s land,* in the hope of making vinegar, but the vinegar proved bad, and the barrels were upset. It should be premised that acetic acid is so deadly a poison to worms that Perrier found that a glass rod dipped into this acid and then into a considerable body of water in which worms were immersed, in- variably killed them quickly. On the morn- ing after the barrels had been upset, ‘ the “heaps of worms which lay dead on the

* See Mr. Dancer’s paper in ‘ Proc, Phil. Soc. of Manchester, “ground were so amazing, that if Mr. Miller “had not seen them, he could not have ‘thought it possible for such numbers to “have existed in the space.” As further evi- dence of the large number of worms which live in the ground, Hensen states that he found in a garden sixty-four open burrows in a space of 14% square feet, that is, nine in 2 square feet. But the burrows are some- times much more numerous, for when digging in a grass-field near Maer Hall, I found a cake of dry earth, as large as my two open hands, which was penetrated by seven bur- rows, as large as goose-quills,

Weight of the earth ejected from a single burrow, and from all the burrows within a given space.—With respect to the weight of the earth daily ejected by worms, Hensen found that it amounted, in the case of some worms which he kept in confinement, and which he appears to have fed with leaves, to only 0°5 gram, or less than 8 grains per diem. But a very much larger amount must be ejected by worms in their natural state, at the periods when they consume earth as food instead of leaves, and when they are

making deep burrows. This is rendered almost certain by the following weights of the castings thrown up at the mouths of single burrows; the whole of which appeared to have been ejected within no long time, as was certainly the case in several instances. The castings were dried (excepting in one specified instance) by exposure during many days to the sun or before a hot fire. (1.) Down, Kent (sub-soil red clay, full of flints, over- lying the chalk). ‘The largest casting which I could find on the flanks of a steep valley, the sub-soil being here shallow. In this one case, the casting was not well dried

(2.) Down.—Lar vest casting which I ‘could find (con- sisting chiefly of calcareous matter), on extremely poor pasture land at the bottom of the ves mentioned under (1.) (8.) Down.—A large casting, but not of unusual size, from a nearly level field, poor pasture, laid down in? 1°22 grass about 35 years before.. (4.) Down.—Averave weight of 11 not large casting gs) ejected ona sloping surface on my lawn, after they had suffered some loss of weight from being exposed during a considerable length ‘of time to rain

castings of ordinary dimensions, collected by Dr. King on land which bad not been mown for a long time and where worms abounded, viz., a lawn pro- tected by shrubberies, near the sea; soil sandy ande 1°37 calcareous ; these castings had been exposed for some (6.) The heaviest of the above twelve castings os. AG (7.) Lower Bengal.—Avi rage weight of 22 castings, collected by Mr. J. Scott, and stated by him to have} 1°24 been thrown up in the course of one or two nights

had been “exposed to the rain of the last monsoon, — and must have lost some weight .. = ae (10.) The heaviest of the above 5 castings .. . 4°34 In this table we see that castings which had been ejected at the mouth of the same burrow, and which in most cases appeared fresh and always retained their vermiform configuration, generally exceeded an ounce in weight after being dried, and sometimes nearly equalled a quarter of a pound. On the Nilgiri moun- tains one casting even exceeded this latter weight. The largest castings in England were found on extremely poor pasture-land; and these, as far as I have seen, are generally larger than those on land producing a rich vegetation. It would appear that worms have to swallow a greater amount of earth on poor than on rich land, in order to obtain sufficient nutriment.

near Nice (Nos. 5 and 6 in the above table), Dr. King often found five or six of them on a square foot of surface; and these, judging from their average weight, would have weighed together 74 ounces; so that the weight of those on a square yard would have been 41b. 3402. Dr. King collected, near the close of the year 1872, all the castings which still retained their vermiform shape, whether broken down or not, from a square foot, in a place abounding with worms, on the summit of a bank, where no castings could have rolled down from above. These castings must have been ejected, as he judged from their appearance in- reference to the rainy and dry periods near Nice, within the previous five or six months; they weighed 94 0z., or 51b. 5402. per square yard. After an interval of four months, Dr. King collected all the castings subsequently ejected on the same square foot of surface, and they weighed 240z., or llb. 6402. per square yard. Therefore within about ten months, or we will say for safety’s sake within a year, 12 oz. of castings were thrown up on this one square foot, or 6°75 pounds on the square

In a field at the bottom of a valley in the chalk (see No. 2 in the foregoing table), a square yard was measured at a spot where very large castings abounded ; they appeared, however, almost equally numerous in a few other places. These castings, which retained perfectly their vermiform shape, were col- lected; and they weighed when partially dried, 1 lb. 133 oz. This field had been rolled with a heavy agricultural roller fifty-two days before, and this would certainly have flattened every single casting on the land. The weather had been very dry for-.two or three weeks before the day of collection, so that not one casting appeared fresh or had been recently ejected. We may therefore assume that those which were weighed had been ejected within, we will say, forty days from the time when the field was rolled,— that is, twelve days short .of the whole inter- vening period. I had examined the same part of the field shortly before it was rolled, and it then abounded with fresh castings. Worms do not work in dry weather during

the summer, or in winter during severe frosts. If we assume that they work for only half the year—though this is too low an estimate —then the worms in this field would eject during the year, 8°387 pounds per square yard; or. 18°12 tons per acre, assuming the whole surface to be equally productive in castings. In the foregoing cases some of the necessary data had to be estimated, but in the two following cases the results are much more trustworthy. A lady, on whose ac- curacy I can implicitly rely, offered to collect during a year all the castings thrown up on two separate square yards, near Leith Hill Piace, in Surrey. The amount collected was, however, somewhat less than that originally ejected by the worms; for, as I have repeatedly observed, a good deal of the finest earth is washed away, whenever castings are thrown up during or shortly before heavy rain. Small portions also adhered to the surrounding blades of grass, and it required too much time to detach every one of them. Qn sandy soil, as in the present instance, castings are liable to crumbie after dry weather, and

particles were thus o ten lost. The lady also occasionally left home for a week or two, and at such times the castings must have suffered still greater loss from exposure to the weather. These losses were, however, compensated to some extent by the collections having been made on one of the squares for four days, and on the other square for two days more than the year. A space was selected (October 9th, 1870) on a broad, grass-covered terrace, which had been mowed and swept during many years. It faced the south, but was shaded during part of the day by trees. It had been formed at least a century ago by a great accumulation of small and large fragments of sandstone, together with some sandy earth, rammed down level. It is probable that it was at first protected by being covered with turf. This terrace, judging from the number of castings on it, was rather unfavourable for the existence of worms, in comparison with the neighbouring fields and an upper terrace. It was indeed surprising that as many worms could live here as were seen; for on digging a hole in this terrace, the black vegetable

mould together with the turf was only four inches in thickness, beneath which lay the level surface of light-coloured sandy soil, with many fragments of sandstone. Before any castings were collected all the previously existing ones were carefully removed. The last day’s collection was on October 14th, 1871. The castings were then well dried before a fire; and they weighed exactly 33 lbs. This would give for an acre of similar land 756 tons of dry earth annually ejected by worms.

The second square was marked on un- enclosed common land, at a height of about 700 ft. above the sea, at some little distance from Leith Hill Tower. The surface was clothed with short, fine turf, and had never been disturbed by the hand of man. The spot selected appeared neither particularly favourable nor the reverse for worms; but I have often noticed that castings are especially abundant on common land, and this may, perhaps, be attributed to the poorness of the soil. The vegetable mould was here between three and four inches in thickness, As this spot was at some distance from the