The Formation of Vegetable Mould through the Action of Worms

13. In another part of same field, trench 93 inches deep, stopped by concrete .. as ase Sa 14. In another part of same field, trench 9 inches deep, stopped by concrete .. ss 9 15. In another part of the same field, trench oA inches deep, when sand was reached « 6 16. In another part of same field, trench 30 inches deep, when stones were reached ; at one end of the trench mould 12 inches, at the other end 14 inches thick .. o oe aot ap Small field between “Old Works” and “Shop Leasows,” I believe nearly as high as the upper part of the latter field.

Thickness of mould in inches. 17. Trench 26 inches deep 3 - 24 18. », 10 inches deep, and then came ‘apen 2 causeway = oe oe oe Pro) 19. Trench 34 inches deep a = ee 10 was reached .. ae 16 22. Trench 29 inches deep, when qadienated eva was reached .. ee say) eke 23. Trench 14 inches deep, and then came upon @ Dr. Johnson distinguished as mould the earth which differed, more or less abruptly, in its dark colour and in its texture from the underlying sand or rubble. In the specimens sent to me, the mould resembled that which hes immediately beneath the turf in old pasture-land, excepting that it often contained small stones, too large to have passed through the bodies of worms. But the trenches above described were dug in fields, none of which were in pasture, and all had been long cultivated. Bearing in mind the remarks made in reference to Silchester on the effects of long-continued culture, combined with the action of worms in bringing up the finer particles to the surface, the mould, as so designated by Dr. Johnson, seems fairly well to deserve its name. Its thickness, where there was no causeway, floor or walls beneath, was greater than has been elsewhere ob- served, namely in many places above 2 ft., and in one spot above 3 ft. The mould was thickest on and close to the nearly level sum- mit of the field called “Shop Leasows,” and in a small adjoining field, which, as I believe, is of nearly the same height. One side of the former field slopes at an angle of rather above 2°, and I should have expected that

the mould, from being washed down during heavy rain, would have been thicker in the lower than in the upper part; but this was not the case in two out of the three trenches here dug. In many places, where streets ran beneatk the surface, or where old buildings stood, the mould was only 8 inches in thickness; and Dr. Johnson was surprised that in ploughing the land, the ruins had never been struck by the plough as far as he had heard. He thinks that when the land was first cultivated the old walls were perhaps intentionally pulled down, and that hollow places were filled up. This may have been the case; but if after the desertion of the city the land was left for many centuries uncultivated, worms would have brought up enough fine earth to have covered the ruins completely ; that is it they had subsided from having been under- mined. The foundations of some of the walls, for instance those of the portion still stand- ing about 20 feet above the ground, and those of the market-place, lie at the extra- ordinary depth of 14 feet; but it is highly improbable that the foundations were gener-

ally so deep. The mortar employed in the buildings must have been excellent, for it is still in parts extremely hard. Where- ever walls of any height have been exposed to view, they are, as Dr. Johnson believes, still perpendicular. The walls with such deep foundations cannot have been under- mined by worms, and therefore cannot have subsided, as appears to have occurred at Abinger and Silchester. Hence it is very difficult to account for their being now com- pletely covered with earth; but how much of this covering consists of vegetable mould and how much of rubble I do not know. The market-place, with the foundations at a depth of 14 feet, was covered up, as Dr. Johnson believes, by between 6 and 24 inches of earth. The tops of the broken-down walls of a caldarium or bath, 9 feet in depth, were likewise covered up with nearly 2 feet of earth. The summit of an arch, leading into an ash-pit 7 feet in depth, was covered up with not more than 8 inches of earth. When- ever a building which has not subsided is covered with earth, we must suppose, either that the upper layers of stone have been at

some time carried away by man, or that earth has since been washed down during heavy rain, or blown down during storms, from the adjoining land; and this would be especially apt to occur where the land has long been | cultivated. In the above cases the adjoining land is somewhat higher than the three speci- fied sites, as far as I can judge by maps and from information given me by Dr. Johnson. If, however, a great pile of broken stones mortar, plaster, timber and ashes fell over the remains of any building, their disintegration in the course of time, and the sifting action of worms, would ultimately conceal the whole beneath fine earth.

Conclusion.— The cases given in this chapter show that worms have played a considerable part in the burial and concealment of several Roman and other old buildings in England ; but no doubt the washing down of soil from the neighbouring higher lands, and the de- position of dust, have together aided largely in the work of concealment. Dust would be apt to accumulate wherever old broken-down walls projected a little above the then exist- ing surface and thus afforded some shelter. The floors of the old rooms, halls and passages have generally sunk, partly from the settling of the ground, but chiefly from having been undermined by worms; and the sinking has commonly been greater in the middle than near the walls. The walls themselves, when- ever their foundations do not lie at a great depth, have been penetrated and undermined by worms, and have consequently subsided. The unequal subsidence thus caused, probably explains the great cracks which may be seen in many ancient walls, as well as their inclination from the perpendicular.

Evidence of the amount of denudation which the land has undergone—Subaerial 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 hLumus-acids—Similar acids apparently generated within the bodies of worms—The action of these acids facilitated by the continued movement of the particies 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 levi- gated 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 dis- integrated. Denudation means the removal of such disintegrated matter te 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 forma- tions were carefully mapped and measured, no one fully realised how great was the amount. One of the first and most remark- able memoirs ever published on this subject was that by Ramsay,* who in 1846 showed that in Wales from 9000 to 11,000 feet in thic ness 0 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 denuda- tion; 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 for- tunate when, in 1867, he published his paper “On sub-aerial Denudation, and on Cliffs and Escarpments of the Chalk.”* Before this

* “Geological Magazine,’ October and November, 1867, vol. iv. pp. 447 and 483. Copious references on the subject are given in this remarkable memoir. 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.* 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 Govern- ment. The result is, as Mr. Croll shows, that the mean level of its enormous area of

* A. Tylor “On changes of the sea-level,” &c., ‘ Philosophical Mag.’ (Ser. 4th) vol. v., 1853, p. 258, Archibald Geikie, Transactions Geolog. Soc. of Glasgow, vol. ili., p. 153 (read March, 1868). Croll “On Geological Time,” ‘Philosophical Mazg.’, May, August, and November, 1868. See also Croll, ‘ Climate and ‘lime,’ 1875, Chap. XX. For some recent information on the amount of sediment brought down by rivers, see ‘ Nature,’ Sept. 23rd, 1880. Mr. T. Mellard Reade has published some interesting articles on the astonishing amount of matter brought down in solution by rivers. See Address, Ccolog. Soe, Liverpool, 1876-77.

drainage must be lowered gs4, of a foot annually, or 1 foot in 4566 years. Con- sequently, 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 re- latively 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 im- portant part in the removal of such matter. Wind-driven sand also wears down the hardest rocks. I have shown* 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

* “ An account of the fine dust which often falls on Vessels in the Atlantic Ocean,” Proc. Geclog. Soc. of London, June 4th, 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 ex- tremely fine dust coming from Africa was con- tinually 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 +255 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 never- theless disintegrates ; and in conformity with the views lately advanced by a distinguished Belgian geologist, De Koninck, such disin- tegration 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.

countries, worms aid in the work of denuda- tion 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 ap- pearance 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 law- suits 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 extend- ing over an enormous area, owe their origin to dust blown from the high lands of central Asia.* 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 con- dition, 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 trans- ported from one place to another. The dust which falls so thickly within our houses con-

* For La Plata, see my ‘ Journal of Researches,’ during the voyage of the Beagle, 1845, p. 183, Elie de Beaumont has given (‘ Lecons de Géolog. pratique,’ tom. I. 1845, p. 183) an excellent account of the enormous quantity of dust which is transported in some countries. I cannot but think that Mr, Proctor has somewhat exaggerated (‘ Pleasant Ways in Science,’ 1879, p. 3879) the agency of dust in a humid country like Great Britain, James Geikie has given (‘ Prehistoric Europe,’ 1880, p- 165) a full abstract of Richthofen’s views, which, however, he disputes.

sists largely of organic matter, and if spread over the land would in time decay and dis- appear 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 com- bination 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 1t was as much as 5°5 per cent., and in the famous black soil of Russia from 5 to even 12 per cent.* In leaf- mould formed exclusively by the decay of leaves the amount is much greater, and in peat the carbon alone sometimes amounts to

* These statements are taken from Von Hensen in ‘ Zeitschrift fiir wissenschalt. Zoologie, Bd. xxviii., 1877, p. 860. ‘Those with respect to peat are taken from Mr. A. A. Julien in ‘ Prog, American Assoc. Science, 1879, p. 314. 64 per cent.; but with these latter cases we are not here concerned. The carbon in the soil tends gradually to oxidise and to dis- appear, except where water accumulates and the climate is cool;* so that in the oldest pasture-land there is no great excess of organic matter, notwithstanding the con- tinued 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 astonish- ing number of half-decayed leaves which they draw into their burrows to a depth of 2 or 3 inches. They do this chiefly for obtain- ing 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 * T have given some facts on the climate necessary or favour- able for the formation of peat, in my ‘Journal of Researches, 1845, p. 287.

earth ; and it is this process which gives to vegetable mould its uniform dark tint. It is known that various kinds of acids are gen- erated by the decay of vegetable matter ;. and from the contents of the intestines of worms and from their castings being acid, it seems pro- bable 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 neutra- lise 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 in- testines are acid, the acidity can hardly be due to the presence of uric acid. We may there- fore conclude that the acids in the alimentary canal of worms are formed during the diges- tive process; and that probably they are nearly of the same nature as those in ordinary 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 this sand; and even hydrochloric, nitric and sulphuric acids, diluted as in the Pharmacopceia, 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 gen- erated 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 azo- humic, are enabled to dissolve colloid silica in proportion to the nitrogen which they contain.* 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.

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 nitrie 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 correde and leave their impressions on polished slabs of marble, dolomite and phosphate of lime. They will attack even basalt and sandstone.* 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 pro: cess; 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 re- volves 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 decomposi- tion; 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 re- moved 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 com- pounds, and are liable to decomposition before they reach any considerable depth.* 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.* 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.f It is

* The preservative power of a layer of mould and turf is often skown by the perfect state of the glacial scratches on rocks when first uncovered. Mr. J. Geikie maintains, in his last very inter- esting work (‘ Prehistoric Europe,’ 1881), that the more pertect scratches are probably due to the last access of cold and increase of ice, during the long-continued, intermittent glacial period. ¢ Many geologists have felt much surprise at the complete disappearance of flints over wide and nearly level areas, from which the chalk has been removed by subaerial denudation. But the surface of every flint is coated by an opaque modified layer, which will just yield to a steel point, whilst the freshly- fractured, translucent surface will not thus yield. The re- moval by atmospheric agencies of the outer modified surfaces of freely exposed flints, though no doubt excessively slow, to- gether with the modification travelling inwards, will, as may be suspected, ultimately lead to their complete disintegration, note withstanding that they appear to be so extremely durable.

therefore necessary to obtain flints for build- ing purposes from the bed of red clay over- lying the chalk (the residue of its dissolution by rainwater) or from the chalk itself. Not only do worms aid indirectly 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,* as “une véritable armature.” The gizzard is sur- rounded by powerful transverse muscles, which, according to Claparede, are about ten times as thick as the longitudinal ones; and Perrier saw them contracting energetically. Worms belonging to one genus, Digaster, have two distinct but quite similar gizzards; and in another genus, Moniligaster, the second gizzard consists of four pouches, one succeeding the other, so that it may almost be said to have five gizzards.— 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 cal- ciferous 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.*