The Formation of Vegetable Mould through the Action of Worms

about 100 inches of rain fall) into the little water-course, and thence into the plains lying below at a depth of 3000 or 4000 feet. Castings ejected before or during dry weather become hard, sometimes surprisingly hard, from the particles of earth having been cemented together by the intestinal secre- tions. Frost seems to be less effective in their disintegration than might have been expected. Nevertheless they readily disin- tegrate into small pellets, after being alter- nately moistened with rain and again dried. Those which have flowed during rain down a slope, disintegrate in the same manner. Such pellets often roll a little down any sloping surface; their descent being sometimes much aided by the wind. The whole bottom of a broad dry ditch in my grounds, where there were very few fresh castings, was completely covered with these pellets or disintegrated castings, which had rolled down the steep s.des, inclined at an angle of 27°.

Near Nice, in places where the great cylin- drical castings, previously described, abound, the soil consists of very fine arenaceo-cal- these castings are extremely liable to crumble during dry weather into small fragments, which are soon acted on by rain, and then sink down so as to be no longer distinguish- able from the surrounding soil. He sent me a mass of such disintegrated castings, collected on the top of a bank, where none could have rolled down from above. They must have been ejected within the previous five or six months, but they now consisted of more or less rounded fragments of all sizes, from 2 of an inch in diameter to minute grains and mere dust. . Dr. King witnessed the crumbling process whilst drying some perfect castings, which he afterwards sent me. Mr. Scott also remarks on the crumbling of the castings near Calcutta and on the mountains of Sikkim during the hot and dry season.

When the castings near Nice had been ejected on an inclined surface, the disinteg- rated fragments rolled downwards, without losing their distinctive shape; and in some places could “ be collected in basketfuls.” Dr. King observed a striking instance of this fact on the Corniche road, where a drain, about 24 feet wide and 9 inches deep, had been made to catch the surface drainage from the adjoin- ing hill-side. The bottom of this drain was covered for a distance of several hundred yards, to a depth of from 13 to 3 inches, by a layer of broken castings, still retaining their characteristic shape. Nearly all these in- numerable fragments had rolled down from above, for extremely few castings had been ejected in the drain itself. ‘The hill-side was steep, but varied much in inclination, which Dr. King estimated at from 30° to 60° with the horizon. He climbed up the slope, and “found every here and there little embank- “ments, formed by fragments of the castings “that had been arrested in their downward “progress by irregularities of the surface, ‘by stones, twigs, &c. One little group of “plants of Anemone hortensis had acted in this ‘‘manner, and quite a small bank of soil had ‘“‘eollected round it. Much of this soil had “crumbled down, but a great deal of it still “retained the form of castings.” Dr. King © dug up this plant, and was struck with the thickness of the soil which must have recently accumulated over the crown of the rhizoma, as shown by the length of the

bleached petioles, in comparison with those of other plants of the same kind, where there had been no such accumulation. The earth thus accumulated had no doubt been secured (as I have everywhere seen) by the smaller roots of the plants. After describing this and other analogous cases, Dr. King con- cludes: “IT can have no doubt that worms “help greatly in the process of denudation.” Ledges of earth on steep hill-sides.—Little horizontal ledges, one above another, have been observed on steep grassy slopes in many parts of the world. Their formation las been attributed to animals travelling repeatedly along the slope in the same horizontal lines while grazing, and that they do thus move and use the ledges is certain; but Professor Hens- low (a most careful observer) told Sir J. Hooker that he was convineed that this was not the sole cause of their formation. Sir J. Hocker saw such ledges on the Himalayan and Atlas ranges, where there were no domesticated animals and not many wild ones ; but these latter would, it is probable, use the ledges at night while grazing like our domesticated animals. A friend observed for me the ledges

on the Alps of Switzerland, and states that they ran at 3 or 4 ft. one above the other and were about a foot in breadth. They had been deeply pitted by the feet of grazing cows. Similar ledges were observed by the same friend on our Chalk downs, and on an old talus of chalk-fragments (thrown out of a quarry) which had become clothed with turf. My son Francis examined a Chalk escarp- ment near Lewes; and here on a part which was very steep, sloping at 40° with the horizon, about 30 flat ledges extended hori- zontally for more than 100 yards, at an average distance of about 20 inches, one beneath the other. They were from 9 to 10 inches in breadth. When viewed from a distance they presented a striking appearance, owing to their parallelism; but when examined closely, they were seen to be somewhat sinuous, and one often ran into another, giving the appearance of one ledge having forked into two. They are formed of light-coloured earth, which on the outside, where thickest, was in one case 9 inches, and in another case between 6 and 7 inches in thickness. Above the ledges, the

the former case 4 and in the latter only 3 inches. The grass grew more vigorously on the outer edges of the ledges than on any other part of the slope, and here formed a tufted fringe. Their middle part was bare, but whether this had been caused by the trampling of sheep, which sometimes frequent the ledges, my son could not ascertain. Nor could he feel sure how much of the earth on the middle aud bare parts, consisted of disintecrated worm-castings which had rolled down from above; but he felt convinced that some had thus originated ; and it was manifest that the ledges with their grass-fringed edges would arrest, any small object rolling down from above.

At one end or side of the oank bearing these ledges, the surface consisted in parts of bare chalk, and here the ledges were very irregular. At the other end of the bank, the slope suddenly became less steep, and here the ledges ceased rather abruptly; but little em- bankments only a foot or two in length were still present. The slope became steeper lower down the hill, and the regular ledges then re- appeared. Another of my sons observed, on the inland side of Beachy Head, where the surface sloped at about 25°, many short little embankments like those just mentioned. They extended horizontally and were from a few inches to two or three feet in length, They supported tufts of grass growing vigorously. The average thickness of the mould of which they were formed, taken from nine measurements, was 4°5 inches; while that of the mould above and beneath them was on an average only 3°2 inches, and on each side, on the same level, 3°1 inches. On the upper parts of the slope, these em- bankments showed no signs of having been trampled on by sheep, but in the lower parts such signs were fairly plain. No long con- tinuous ledges had here been formed.

If the little embankments above the Cor- niche road, which Dr. King saw in the act of formation by the accumulation of dis- integrated and rolled worm-castings, were to become confluent along horizontal lines, ledges would be formed. EKach embankment would tend to extend laterally by the lateral extension of the arrested castings; and animals grazing on a steep slope would almost certainly make use of every prominence at nearly the same level, and would indent the turf between them; and such intermediate indentations would again arrest the castings. An irregular ledge when once formed would also tend to become more regular and horizontal by some of the castings rolling laterally from the higher to the lower parts, which would thus be raised. Any pro- jection beneath a ledge would not afterwards receive distintegrated matter from above, and would tend to be obliterated by rain and other atmospheric agencies. ‘There is some analogy between the formation, as here sup- posed, of these ledges, and that of the ripples of wind-drifted sand as described by Lyell.* The steep, grass-covered sides of a mountainous valley in Westmoreland, called Grisedale, was marked in many places with innumerable, almost horizontal, little ledges, or rather lines of miniature cliffs. Their formation was in no way connected with the action of worms, for castings could not any- where be seen (and their absence is an inexplicable fact) although the turf .ay in many places over a considerable thickness of

boulder-clay and moraine rubbish. Nor, as far as I could judge, was the formation of these little cliffs at all closely connected with the trampling of cows or sheep. It appeared as if the whole superficial, somewhat argil- laceous earth, while partially held together by the roots of the grasses, had slided a little way down the mountain sides; and in thus sliding, had yielded and cracked in horizontal Castings blown to leeward by the wind—We have seen that moist castings flow, and that disintegrated castings roll down any inclined surface ; and we shall now see that castings, recently ejected on level grass-covered surfaces, are blown during gales of wind ac- companied by rain to leeward, This has bees observed by me many times on many fields during several successive years. After such gales, the castings present a gently inclined and smooth, or sometimes furrowed, surface to windward, while they are steeply inclined or precipitous to leeward, so that they resem- ble on a miniature scale glacier-ground hiilocks of rock. They are often cavernous on the

leeward side, from the upper part having curled over the lower part. During one un- usually heavy south-west gale with torrents of rain, many castings were wholly blown to leeward, so that the mouths of the burrows were left naked and exposed on the windward side. Recent castings naturally flow down an inclined surface, but on a grassy field, which sloped between 10° and 15°, several were found after a heavy gale blown up the slope. This likewise occurred on another occasion on a part of my lawn where the slope was somewhat less. Ona third occasion, the castings on the steep, grass-covered sides of a valley, down which a gale had blown, were directed obliquely instead of straight down the slope; and this was obviously due to the combined action of the wind and gravity. Four castings‘on my lawn, where the downward inclination was 0° 45’, 1°, 3° and 3° 30’ (mean 2° 45’) towards the north-east, after a heavy south-west gale with rain, were divided across the mouths of the burrows and weighed in the manner formerly described. The mean weight of the earth below the mouths of burrows and to leeward, was to that

above the mouths and on the windward side as 23 to 1; whereas we have seen that with several castings which had flowed down slopes having a mean inclination of 9° 26’, and with three castings where the inclination was above 12°, the proportional weight of the earth below to that above the burrows was as only 2 to 1. These several cases show how efficiently gales of wind accompanied by rain act in displacing recently-ejected castings. We may therefore conclude that even a moderately strong wind will produce some slight effect. on them.

Dry and indurated castings, after their dis- integration into small fragments or pellets, are sometimes, probably often, blown by a strong wind to leeward. This was observed on four occasions, but I did not sufficiently attend to this point. One old casting on a gently slop- ing bank was blown quite away by a strong south-west wind. Dr. King believes that the wind removes the greater part of the old crumbling castings near Nice. Several old castings on my lawn were marked with pins and protected from any disturbance. They were examined after an interval of 10

weeks, during which time the weather had been alternately dry and rainy. Some, which were of a yellowish colour had been washed almost completely away, as could be seen by the colour of the surrounding ground. Others had completely disappeared, and these no doubt had been blown away. Lastly, others still remained and would long remain, as blades of grass had grown through them. On poor pasture land, which has never been rolled and has not been much trampled on by animals, the whole surface is sometimes dotted with little pimples, through and on which grass grows; and these pimples con- sist of old worm-castings.

In ail the many observed cases of soft cast- ings blown to leeward, this had been effected by strong winds accompanied by rain. <As such winds in England generally blow from the south and south-west, earth must on the whole tend to travel over our fields in a north and north-east direction. This fact is interesting, because it might be thought that none could be removed from a level, grass- covered surface by any means. In thick and level woods, protected from the wind, castings

will never be removed as long as the wood lasts; and mould will here tend to accumulate to the depth at which worms can work. I tried to procure evidence as to how much mould is blown, whilst in the state of caste ings, by our wet southern gales to the north- east, over open and flat land, by looking to the level of the surface on opposite sides of old trees and hedge-rows; but I failed owing to the unequal growth of the roots of trees and to most pasture-land having been formerly ploughed.

On an open plain near Stonehenge, there exist shallow circular trenches, with a low embankment outside, surrounding level spaces 50 yards in diameter. These rings appear very ancient, and are believed to be contem- poraneous with the Druidical stones. Castings ejected within these circular spaces, if blown to the north-east by south-west winds would form a layer of mould within the trench, thicker on the north-eastern than on any other side. But the site was not favourable for the action of worms, for the mould over the surrounding Chalk formation with flints, was only 3°37 inches in thickness, from a mean of

six observations made ata distance of 10 yards outside the embankment. The thickness of the mould within two of the circular trenches was measured every 5 yards all round, on the inner sides near the bottom. My son Horace protracted these measurements on paper ; and though the curved line representing the thick- ness of the mould was extremely irregular, yet in both diagrams it could be seen to be thicker on the north-eastern side than elsewhere. When a mean of all the measurements in both the trenches was laid down and the line smoothed, it was obvious that the mould was thickest in the quarter of the circle between north-west and north-east; and thinnest in the quarter between south-east and south- west, especially at this latter point. Besides the foregoing measurements, six others were taken near together in one of the circular trenches, on the north-east side; and the mould here had a mean thickness of 2°29 inches; while the mean of six other measure- ments on the south-west side was only 1°46 inches. These observations indicate that the castings had been blown by the south-west winds from the circular enclosed space into

the trench on the north-east side ; but many more measurements in other analogous cases would be requisite for a trustworthy result. The amount of fine earth brought to the surface under the form of castings, and after- wards transported by the winds accompanied by rain, or that which flows and rolls down an inclined surface, no doubt is small in the course of a few scores of years; for otherwise all the inequalities in our pasture fields would be smoothed within a much shorter period than appears to be the case. But the amount which is thus transported in the course of thousands of years cannot fail to be consider- able and deserves attention. E. de Beaumont looks at the vegetable mould which every- where covers the land as a fixed line or zero, from which the amount of denudation may be measured.* He ignores the continued formation of fresh mould by the disintegra- tion of the underlying rocks and fragmeuts of rock; and it 1s curious to find how much more philosophical were the views, main-

* “Lecons de Géologie pratique, 1845; cinquieme Legon.’ All Elie de Beaumont’s arguments are admirably controverted by Prof. A. Geikie in his essay in Transact. Geolog. Soc. of Glasgow, vol. iii. p. 153, 1868. tained long ago, by Playfair, who, in 1802, wrote, “in the permanence of a coat of “vegetable mould on the surface of the earth, “we have a demonstrative proof of the cun- “tinued destruction of the rocks.”* Ancient encampments and tumuli. de Beaumont adduces the present state of many ancient encampments and tumuli and of old ploughed fields, as evidence that the surface of the land undergoes hardly any degradation. But it does not appear that he ever examined the thickness of the mould over different parts of such old remains. He relies chiefly on indirect, but apparently trustworthy, evi- dence that the slopes of the old embankments are the same as they originally were; and it is obvious that he could know nothing about their original heights. In Knole Park a mound had been thrown up behind the rifle- targets, which appeared to have been formed of earth originally supported by square blocks of turf. The sides sloped, as nearly as I could estimate them, at an angle of 45° or 50° with the horizon, and they were covered, especially on the northern side, with long coarse grass,

* ‘Tllustrations of the Huttonian Theory of the Earth,’ p. 107. beneath which many worm-castings were found. These had flowed bodily downwards, and others had rolled down as pellets. Hence it is certain that as long as a mound of this kind is tenanted by worms, its height will be continually lowered. The fine earth which flows or rolls down the sides of such a mound accumulates at its base in the form of a talus. A bed, even a very thin bed, of fine earth is eminently favourable for worms; so that a greater number of castings would tend to be ejected ona talus thus formed than elsewhere : and these would be partially washed away by every heavy shower and be spread over the adjoming level ground. The final result would be the lowering of the whole mound, whilst the inclination of the sides would not be greatly lessened. The same result would assuredly follow with ancient embankments and tumuli; except where they had been formed of gravel or of nearly pure sand, as such matter is unfavourable for worms. Many old fortifications and tumuli are believed to be at icast 2000 years old; and we should bear in mind that in many places about one inch of mould is brought to the suriace in 5 years or

two inches in 10 years. Therefore in so long a period as 2000 years, a large amount of earth will have been repeatedly brought to the surface on most old embankments and tumuli, especially on the talus round their bases, and much of this earth will have been washed completely away. We may therefore conclude that all ancient mounds, when not formed of materials unfavourable to worms, will have been somewhat lowered in the course of centuries, although their inclina- tions may not have been greatly changed. Fields formerly ploughed.—From a very remote period and in many countries, land has been ploughed, so that convex beds, called crowns or ridges, usually about 8 feet across and separated by furrows, have been thrown up. The furrows are directed so as to carry off the surface water. In my attempts to ascertain how long a time these crowns and furrows last, when ploughed land has been converted into pasture, obstacles of many kinds were encountered. It is rarely known when a field was last ploughed; and some fields which were thought to have been in pasture from time immemorial were after-

wards discovered to have been ploughed only 50 or 60 years before. During the early part of the present century, when the price of corn was very high, land of all kinds seems to have been ploughed in Britain. There is. however, no reason to doubt that in many cases the old crowns and furrows have been preserved from a very ancient period.* That they should have been preserved for very unequal lengths of time would naturally follow from the crowns, when first thrown up, having differed much in height in dif- ferent districts, as is now the case with recently ploughed land.

In old pasture fields, the mould, wherever measurements were made, was found to be from 4 to 2 inches thicker in the furrows than * Mr. E. Tylor in his Presidential address (‘Journal of the Anthropological Institute,’ May 1880, p. 451) remarks: “ It appears from several papers of the Berlin Society as to the German ‘high-fields’ or ‘ heathen-fields’ (Hochiicker, and Heidenicker) that they correspond much in their situation on hills and wastes with the ‘ elf-furrows’ of Scotland, which popular mythology accounts for by the story of the fields having been put under a Papal interdict, so that people took to cultivating the hills. There seems reason to suppose that, like the tilled plots in the Swedish forests which tradition ascribes to the old ‘hackers,’ the German heathen-fields represent tillage by an ancient and barbaric population.”

on the crowns; but this would naturally follow from the finer earth having been washed from the crowns into the furrows before the land was well clothed with turf; and it is impossible to tell what part worms may have played in the work. Nevertheless from what we have seen, castings would certainly tend to flow and to be washed during heavy rain from the crowns into the furrows. But as soon as a bed of fine earth had by any means been accumulated in the furrows, it would be more favourable for worms than the other parts, and a greater number of castings would be thrown up here than elsewhere ; and as the furrows on sloping land are usually directed so as to carry off the surface water, some of the finest earth would be washed from the castings which had been here ejected and be carried completely away. The result would be that the furrows would be filled up very slowly, while the crowns would be lowered perhaps still more slowly by the flowing and rolling of the castings down their gentle inclinations into the furrows. Nevertheless it might be expected that old furrows, especially those on a sloping surface,

would in the course of time be filled up and disappear. Some careful observers, however, who examined fields for me in Gloucestershire and Staffordshire, could not detect any dif- ference in the state of the furrows in the upper and lower parts of sloping fields, sup- posed to have been long in pasture; and they came to the conclusion that the crowns and furrows would last for an almost endless number of centuries. On the other hand the process of obliteration seems to have com- menced in some places. Thus in a grass field in North Wales, known to have been ploughed about 65 years ago, which sloped at an angle of 15° to the north-east, the depth of the furrows (only 7 feet apart) was care- fully measured, and was found to be about 4} inches in the upper part of the slope, and only 1 inch near the base, where they could be traced with difficulty. On another field sloping at about the same angle to the south- west, the furrows were scarcely perceptible in the lower part; although these same furrows when followed on to some adjoining level ground were from 24 to 34 inches in depth. A third and closely similar case was

observed. In a fourth case, the mould in a furrow in the upper part of a sloping field was 24 inches, and in the lower part 44 inches in thickness. On the Chalk Downs at about a mile dis- tance from Stonehenge, my son William ex- amined a grass-covered, furrowed surface, sloping at from 8° to 10°, which an old shep- herd said had not been ploughed within the memory of man. The depth of one furrow was measured at 16 points in a length of 68 paces, and was found to be deeper where the slope was greatest and where less earth would naturally tend to accumulate, and at the base it almost disappeared. The thickness of the mould in this furrow in the upper part was 23 inches, which increased to 5 inches a little above the steepest part of the slope; and at the base, in the middle of the narrow valley, at a point which the furrow if con- tinued would have struck, it amounted to 7 inches. On the opposite side of the valley, there were very faint, almost obliterated, traces of furrows. Another analogous but not so decided a case was observed at a few miles distance from Stonehenge. On the

whole it appears that the crowns and fur- rows on land formerly ploughed, but now covered with grass, tend slowly to disappear when the surface is inclined; and this is pro- bably in large part due to the action of worms; but that the crowns and furrows last for a very long time when the surface is nearly level. Formation and amount of mould over the Chalk Formation.—Worm-castings are often ejected in extraordinary numbers on steep, grass-covered slopes, where the Chalk comes close to the surface, as my son William observed near Winchester and elsewhere. If such castings are largely washed away during heavy rains, it is difficult to understand at first how any mould can still remain on our Downs, as there does not appear any evident means for supplymg the loss. There is, more- over, another cause of loss, namely in the per- colation of the finer particles of earth into the fissures in the chalk and into the chalk itself. These considerations led me to doubt for a time whether I had not exaggerated the amount of fine earth which flows or rolls down grass- covered slopes under the form of castings; and

I sought for additional information. In some places, the castings on Chalk Downs consist largely of calcareous matter, and here the supply is of course unlimited. But in other places, for instance on a part of Teg Down near Winchester, the castings were all black and did not effervesce with acids. The mould over the chalk was here only from 3 to 4 inches in thickness. So again on .the plain near Stonehenge, the mould, apparently free from calcareous matter, averaged rather less than 33 inches in thickness) Why worms should penetrate and bring up chalk in some places and not in others I do not know.

In many districts where the land is nearly level, a bed several feet in thickness of red clay full of unworn flints overlies the Upper Chalk. This overlying matter, the surface of which has been converted into mould, con sists of the undissolved residue from the chalk. It may be well here to recall the case of the fragments of chalk buried beneath worm- castings on one of my fields, the angles of which were so completely rounded in the course of 29 years that the fragments now resembled water-worn pebbles. This must

have been effected by the carbonic acid in the rain and in the ground, by the humus- acids, and by the corroding power of living roots. Why a thick mass of residue has not been left on the Chalk, wherever the land is nearly level, may perhaps be accounted for by the percolation of the fine particles into the fissures, which are often present in the chalk and are either open or are filled up with impure chalk, or into the solid chalk itself. That such percolation occurs can hardly be doubted. My son collected some powdered and fragmentary chalk beneath the turf near Winchester; the former was found by Colonel Parsons, R.E., to contain 10 per cent., and the fragments 8 per cent. of earthy matter. On the flanks of the escarpment near Abinger in Surrey, some chalk close beneath a layer of flints, 2 inches in thickness and covered by 8 inches of mould, yielded a re- sidue of 3°7 per cent. of earthy matter. On the other hand the Upper Chalk properly contains, as I was informed by the late David Forbes who had made many analyses, only from 1 to 2 per cent. of earthy matter; and