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Chapter V.

The action of worms in the Denudation of the Land.

[in pencil in another hand:] Hill

Evidence of the amount of denudation which the land has undergone — Sub-aerial denudation — The deposition of dust — Vegetable mould, its dark colour & fine texture largely due to the action of worms — The disintegration of rocks by the humus-acids —Similar acids apparently generated for decayed (?) within the bodies of worms — The action of the acids facilitated by the continued movement of the particles of earth — A thick bed of mould checks the disintegration of the underlying soil & rocks. Particles of stone worn or triturated in the gizzards of worms — Stones swallowed to serve as mill-stones — Levigated state of the castings — Fragments of brick & mortar in the castings over ancient buildings well rounded — The triturating power of worms into quite insignificant under a geological point of view. —

 

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Chapter IV V.

 

The action of worms in the Denudation of the Land.

 

No one doubts that our world at one time consisted of crystalline rocks, and that it is to their disintegration through the action of the air, water, changes of temperature, rivers, waves of the sea, earthquakes and volcanic outbursts, have given rise to that we owe our sedimentary depositions formations. These after being consolidated and sometimes recrystallised, have often been again disintegrated. Denudation means to the removal to a lower level of such disintegrated matter. Of the many striking results due to modern progress of geology there can are hardly any more striking than those which relate to denudation. It was long ago seen that there must been an immense amount of denudation; but until the successive formations were carefully mapped and measured, no one fully realised how great was the amount. One of the first and most remarkable memories ever published on this subject was that by Ramsay*,(1) who in 1846 showed that in Wales from

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* (1) On the denudation of South Wales &c. Memoirs of the Geological Survey of Great Britian. Vol I. p 297. 1846

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9000 to 11000 feet in thickness of solid rock had been stripped off large tracks 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 few 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 with not a remnant left.

Until the last 20 or 30 years, most geologists thought that the waves of the sea were the chief agents in the work of denudation; but we may now feel sure that air and rain, aided by streams and rivers, are much more powerful agents, — that is if we consider the whole area of the land. The long lines of escarpement which stretch across several parts of England were formerly viewed without a doubt as ancient coast-lines; but we now know that these project above the general surface merely from resisting air, rain and frost

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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 subject point by a single memoir; but Mr Whitaker of the Geological Survey of England was so fortunate when in 1867 he published to his paper "on sub-aerial Denudation and on Cliffs and Escarpements of the Chalk." *(2) Before this paper appeared, Mr A. Taylor had adduced important evidence on sub-aerial denudation, by showing that the amount of matter brought down by rivers must infallibly have lowered 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.*(3) 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

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*(2) Geological Magazine vol iv October & November 1867. Vol IV. p. 447 & 483. Copious references given the remarkable memoirs.

*(3) A. Taylor, On changes of the sea-level &c. Philosophical Mag. (Ser. 4h) Vol. 5 1853 p. 258. Archibald Geikie Transactions Geolog. Soc. Of Glasgow Vol III. p. 153 1868 (read March 1868). Croll 'On Geological Time' Philosophical Mag. May 1868 August & November 1868..— See also Croll 'Climate & Time' 1875 Chap. XX

In some recent estimates on information on the amount of sediment brought down by rivers. See Nature Sept 23rd 1880. Mr. T. Mellard Reade has published some another interesting articles on the astonishing amount of matte brought down in the solution by rivers: see address. Geolog. Soc. Liverpool 1876-77.—

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Government. The result is as Mr. Croll shows, that the mean level of its enormous area of drainage must have been lowered 1/4566 of a foot annually, or 1 foot in 4566 years. Consequently, taking the best estimate of the mean height of the N. American continent, viz; 748ft, 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 much more sediment annually into the sea and some a much less amount, relatively to their size, in comparison with the [illeg] Mississippi.

During volcanic outbursts and in arid countries much more is triturated & widely dispersed by the wind; & the in arid countries the wind plays an important part in the removal of already disintegrated matter, and wind-driven sand mechanically corrodes can also wear down the hardest rocks. I have shown*(4) that during four months of the year a larger quantity of dust is blown from the N. Western shores of Africa, and falls on the Atlantic over a space of 1600 miles in

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*(4) 'An account of the fine Dust which often falls on Vessels in the Atlantic Ocean: Proc. Geolog. Soc. Of London June 4th 1845

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latitude, and in longitude 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 coast shores of Africa. During a stay of three weeks at St. Jago in the Cape Verde Archipelago, the atmosphere was almost always hazy, and extremely fine dust, coming from Africa was continually falling. In some dust which fell at a distance of between 330 and 380 miles from the coast, there were many particles of stone, about 1/1000 of an inch square. Nearer to the coast the water has been seen to be so much discoloured by the falling dust, that a sailing vessel left a track behind her. In arid tropical countries, like the Cape Verde Archipelago, where it seldom rains and there are no frosts the solid rocks nevertheless disintegrate; and I have long thought that the in conformity with the views lately advanced by a distinguished Belgian geologist, De Koninck such disintegration may be attributed in chief part to the actions of the carbonic and nitric acids, together with the nitrates and nitrites of ammonia, dissolved in the dew.) were probably the chief disintegrating agents; and a distinguished Belgian geologist, de Koninck, has recently advanced an analogous view.

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In all humid, even moderately humid, countries, worms aid in the work of denudation in several ways. The vegetable mould which covers, as with a mantle, the whole surface of the land, has all passed many times through their bodies. Mould differs in appearance from the   113)  subsoil only in its dark colour on blackness, 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 process of the soil is aided, as has already been remarked, by burrowing animals of many kinds, especially by ants. The In countries where the summer is long and dry, the mould (in protected places) must be largely increased in protected places 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 unenclosed land was so greatly completely changed that the inhabitants could not recognise the limits of their own estates; and this gave

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rise to endless lawsuits. Immense quantities of dust are likewise often blown about in Egypt and in the South of France. In China, as Richthofen mountains, beds appearing like fine sediment, several hundred feet in thickness and extending over an enormous area, owe their origin to dust blown from the high arid lands of central Asia*.(5) In humid countries like Great Britain, as long as the land remains in its natural state clothed with vegetation, the mould in any one place can hardly be much increased by dust; but in its present condition, the fields near high roads, where there is much traffic, must receive a considerable amount of dust, and when fields are harrowed during dry and windy weather, clouds of dust may be seen to be blown away. In But in all these cases the surface-soil is merely transported from one place to another. The dust which falls so thickly within our houses consists largely of organic matter, and its spread over the land would in time decay and disappear. It appears, however, from recent observations on the snow-fields of the Arctic regions that little meteoric dust

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*(5) For La Plata, see my Journal of Researches &c during the Voyage of the Beagle 1845. P. 133. Elie de Beaumont has given (Lecons de Geolog. Pratique Tom. I 1845 p. 183) an excellent account of the enormous quantity of dust deposited which is transported in some countries. I cannot but think that Mr. Procter has somewhat exaggerated (Pleasant Ways in Science 1879 p. 379) the agency of dust in a humid countries like Great Britain. James Geikie has given (Prehistoric Europe 1880 p.165) With respect to Richthofens conclusion The a full abstract of Richthofen's views, which he disputes. see also to a certain extent Richthofen views. They are supported to a certain extent by the facts given advanced by Humboldt- :(in his Asie Centrale Tom 2. p. 254.) quoted by E. de Beaumont

 

 

It is interesting that Darwin here cited Humboldt's Asie centrale (1843), a work nowhere else mentioned by Darwin. Darwin was referring to a reference to Humboldt in Élie de Beaumont, which may be why this was omitted from the published text of Earthworms.

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[in pencil in another hand:] Andrews

of extra-mundane origin is continually falling.

The dark colour of ordinary mould is obviously due to the presence of decaying organic matter, which, however, is present in but small quantities. The loss of weight which mould suffers when heated to redness seems to be in large part due to water in combination being dispelled. In one sample of fertile mould the amount of organic matter was ascertained to be only 1.76 per cent.; in some artificially prepared soil it was as much as 5.5 per cent., and in the famous black soil of Russia from 5 to even 12 per cent.*(7) In leaf-mould formed exclusively by the decay of leaves the amount is much greater, and in peat the carbon alone sometimes amounts to 64 per cent.; but with these latter cases we are not here concerned. The carbon in the soil tends gradually to oxidise and to disappear, except where water accumulates and the climate is cool;* so that in the oldest pasture-land there is no great excess of organic matter, notwithstanding the continued decay of the roots and the underground stems of plants, with the

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*(6) These statements are taken from Von Hensen in Zeitschrift für wissenschaft: Zoologie. Bd. XXVIII. 1877. p. 360. Those with respect to peat, are taken from Mr. A. A. Julien in Proc. American Assoc. Science 1879, p. 314.

*(7) I have given some facts on the climate necessary or favourable for the formation of peat, in my Journal of Researches 1845 p. 287.

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occasional addition of manure. The disappearance of the organic matter from mould is probably much aided by its being brought again and again to the surface in the castings of worms.

(Worms, on the other hand, add largely to the organic matter in the soil by the astonishing number of half-decayed leaves which they draw into their burrows to a depth of 2 or 3 inches. They do this chiefly for obtaining food, but partly for closing the mouths of their burrows and for lining the upper part. The leaves which they consume are moistened, torn into small shreds, partially digested, and intimately commingled with earth; and it is this process which gives to vegetable mould its uniform dark tint.

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It is known that various kinds of acids are generated by the decay of vegetable matter; & from the contents of the intestines of worms & from their castings being acid, it seems probable that the process of digestion aids the decay of induces an analogous chemical change in the swallowed, triturated, & half decayed leaves. The large quantity of calcereous carbonate of lime secreted by the calciferous glands apparently serves to neutralise the acids thus generated; for the digestive fluid of worms will not act unless it be alkaline. As the contents of the upper

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part of their intestines are acid, the acidity can hardly be due to the presence of uric acid. We may therefore infer conclude that the acids in the alimentary canal of worms are formed during the digestive process; & probably that they are nearly of the

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same nature as those in ordinary humus. The latter are well known to have the power of de-oxidising or dissolving peroxide 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 Pharmacopœia, produced less effect than did the acids in the intestines of the worms.)

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nitric and sulphuric acids a less effect than the acid in the castings.

Mr. A. A. Julien has lately collected all the extant information about the acids generated in humus, which, according to some chemists, amount to more than a dozen different kinds. These acids, as well as their acid salts (i.e., in combination with potash, soda, and ammonia), act energetically on carbonate of lime and on the oxides of iron. It is, also, known that some of these acids, which were called long ago by Thénard azohumic, are enabled to dissolve colloid silica in proportion to the nitrogen which they contain.* 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

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*(8) A. A. Julien "on the Geological action of the Humus Acids," Proc. American Assoc. Science vol XXVIII 1879 p. 311. Also on "Chemical erosion on Mountain Summits"; New York Academy of Sciences as Oct 14 1878, as quoted in the American Naturalist.

See, also, on this subject, J. W. Johnson, "How Crops Feed" 1870 p. 138.

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highly important part, according to the recent observations of Mr. Julien, in the disintegration of various kinds of rocks. It has long been known that the carbonic acid, and no doubt nitric and nitrous acids, which are present in rain-water, act in like manner. There

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is, also, a great excess of carbonic acid in all soils, especially in rich soils, & this is dissolved by the water in the ground. The living roots of plants, moreover, as Sachs & others have shown, quickly corrode & leave their impressions on polished slabs of marble, dolomite & phosphate of lime. They will attack even basalt & sandstone.*(9) But we are not here concerned with agencies which are wholly independent of the action of worms.)

[in pencil in another hand:] Lanham

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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

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[in pencil in another hand:] Miller

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*(9) See for references on this subject S. W. Johnson 'How Crops Feed' 1870 p. 326.

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worms, during the digestive process; and it should be remembered that the entire mass of the mould over every field, passes, in the course of a few years, through their alimentary canals. Moreover as the old burrows slowly collapse, and as fresh castings are continually brought to the surface, the whole superficial layer of mould slowly revolves or circulates; and the friction of the particles one with another will rub off the finest films of disintegrated matter as soon as they are formed. Through these several means, minute fragments of rocks of many kinds and mere particles in the soil will be continually exposed to chemical decomposition; and thus the amount of soil will tend to increase.

As worms line their burrows with their castings, and as the burrows penetrate to a depth of 5 or 6, or even more feet, some small amount of the humus-acids will be carried far down, and will there act on the underlying rocks and fragments of rock. Thus the thickness of the soil, if none be removed from the surface, will steadily though slowly tend to increase; but the accumulation will after a time

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to delay the the process of disintegration of the underlying rocks and of the more deeply seated particles. For the humus-acids which are generated chiefly in the upper layer of vegetable mould, are extremely unstable compounds, and are liable to decomposition before they reach any considerable depth.*(10) 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.*(10A) 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.*(11) It is therefore, necessary to

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*(10) This statement is taken from Mr. Julien: Proc. American Assoc. Science vol. XXVIII 1879, p. 330.

*10A

*(11) Many geologists have sometimes expressed 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 removal by atmospheric agencies of the outer modified layer of a surfaces of freely exposed flints, though no doubt excessively slow, together with the modification travelling inwards, will, as I believe as may be suspected, ultimately lead to the th their their complete disintegration, although apparently so durable  a substance. notwithstanding that they appear to be so extremely durable.)

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* (10A) The preservative power of a layer of mould and turf is often shown by the perfect state of the glacial scratches on rocks when first uncovered. Mr. J. Geikie maintains, in his last very interesting work (Prehistoric Europe 1881), that the more perfect scratches are probably due to the last access of cold and increase of ice, during the long-continued, intermittent glacial period.

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obtain flints for building purposes from the bed of red clay overlying 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,*(12) as "une véritable armature." The gizzard is surrounded by powerful transverse muscles, which, according to Claparède, are about ten times as thick as the longitudinal ones; and Perrier saw them contracting energetically. Worms belonging to one genus, Digaster, have two distinct but quite similar gizzards; and in another genus, Moniligaster, the second gizzard consists of four pouches, one succeeding the other, so that it may almost be said to have five gizzards.*(B) In the same manner as Gallinaceous and Struthious birds swallow stones

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* (12) Archives de Zoolog, expér. tom. III. 1874, p. 409.

* (13)  Nouvelles Archives du Muséum tom. VIII. 1872, p. 95, 131.

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to aid in the trituration of their food, so it appears to be with terricolous worms. The gizzards of thirty-eight of our common worms were opened, and in twenty-five of them small stones or grains of sand, sometimes together with the hard calcareous concretions formed within the anterior calciferous glands, were found, and in two others concretions alone. In the gizzards of the remaining worms there were no stones; but some of these were not real exceptions, as the gizzards were opened late in the autumn, when the worms had ceased to feed and their gizzards were quite empty.*(14)

[in pencil in another hand:] Barker

When worms make their burrows through earth abounding with little stones, no doubt many will be unavoidably swallowed; but it must not be supposed that this fact accounts for the frequency with which stones and sand are found in their gizzards. For beads of glass and fragments of brick and of hard tiles were scattered over the surface of the earth, in pots in which worms were kept and had already made their burrows; and very many of these beads and fragments were picked up and swallowed by the worms, for they were found in their castings,

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*(14) Morren, in speaking of the earth in the alimentary canals of worms, says, "præsepè cum lapillis commixtam vidi:" De Lumbrici terrestris, 1829, p. 16.

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intestines, and gizzards. They even swallowed the coarse red dust, formed by the pounding of the tiles. Nor can it be supposed that they mistook the beads and fragments for food; for we have seen that their taste is delicate enough to distinguish between different kinds of leaves. It is therefore manifest that they swallow hard objects, such as bits of stone, beads of glass and angular fragments of bricks or tiles for some special purpose; and it can hardly be doubted that this is to aid their gizzards in crushing and grinding the earth, which they so largely consume. That such hard objects are not necessary for crushing leaves, may be inferred from the fact that certain species, which live in mud or water and feed on dead or living vegetable matter, but which do not swallow earth, are not provided with gizzards,*(15) and therefore cannot have the power of utilising stones.

During the grinding process, the particles of earth must be rubbed against one another, and between the stones and the tough lining membrane of the gizzard. The softer particles will thus suffer some attrition, and will perhaps even be crushed,

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* (15) Perrier, Archives de Zoolog. expér. tom. III. 1874, p. 419.

 

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and those of the harder rocks would be a very little worn. This conclusion is supported by the appearance of freshly ejected castings, for these often reminded me of the appearance of paint which has just been ground by a workman between two flat stones. Morren remarks that the intestinal canal is "impleta tenuissimâ terrâ, veluti in pulverem redactâ."*(16) Perrier also speaks of "l'état de pâte excessivement fine à laquelle est réduite la terre qu'ils rejettent," &c.*(17)

As the amount of trituration which the particles of earth undergo in the gizzards of worms possesses some interest (as we shall hereafter see), I endeavoured to obtain evidence on this head by carefully examining many of the fragments which had passed through their alimentary canals. With worms living in a state of nature, it is of course impossible to know how much the fragments may have been worn before they were swallowed. It is, however, clear that worms do not habitually select already rounded particles. This latter supposition is,

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*(16) Morren, De Lumbrici terrestris p. 16.

*(17) Archives de Zoolog. Expér. Tom. III. 1874 p. 418.

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however, contradicted by sharply angular bits of flint and of other hard rocks were often found in their gizzards or intestines. On three occasions sharp spines from the stems of rose-bushes were thus found. Worms kept in confinement repeatedly swallowed angular fragments of hard tile, coal, cinders, and even the sharpest fragments of glass. Gallinaceous and struthious birds retain the same stones in their gizzards for a long time, which thus become well rounded; but this does not appear to be the case with worms, judging from the large number of the fragments of tiles, glass beads, stones, &c., commonly found in their castings and intestines. So that unless the same fragments were to pass repeatedly through their gizzards, visible signs of attrition in the fragments could hardly be expected, except perhaps in the case of very soft stones.

I will now give such evidence of attrition as I have been able to collect. In the gizzards of some worms dug out of a thin bed of mould over the chalk, there were many well-rounded small fragments of chalk,

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and two fragments of the shells of a land-mollusc (as ascertained by their microscopical structure), which latter were not only rounded but somewhat polished. The calcareous concretions formed in the calciferous glands, which are often found in their gizzards, intestines, and occasionally in their castings, when of large size, sometimes appeared to have been rounded; but with all calcareous bodies the rounded appearance may be partly or wholly due to their corrosion by carbonic acid and the humus-acids. In the gizzards of several worms collected in my kitchen garden near a hothouse, eight little fragments of cinders were found, and of these, six appeared more or less rounded, as were two bits of brick; but some other bits were not at all rounded. A farm-road near Abinger Hall had been covered seven years before with brick-rubbish to the depth of about 6 inches; turf had grown over this rubbish on both sides of the road for a width of 18 inches, and on this turf there were innumerable castings. Some of them were coloured of a uniform red owing to the presence of much brick-dust, and they contained many

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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

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the action of carbonic acid.

[in pencil in another hand:] Graves

The best 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.(a)text)  Now in the castings ejected between the tesseræ on one of the buried floors of the Roman villa at Abinger, there were many particles (from ½ to 2 mm. in diameter) of tiles and concrete, which it was impossible to look at with the naked eye or through a strong lens, and doubt for a moment that they had almost all undergone much attrition. Some resembled little I speak thus after having

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examined small water-worn pebbles, formed from Roman bricks, which M. Henri de Saussure had the kindness to send me, & which he had extracted from sand & 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.)

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(a) text

It should be premised that in the several following cases, the finer matter was first washed away from the castings, & then all the particles of bricks, tiles & concrete were collected without any selection, & afterwards examined.

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castings, and then all the particles of brick, tiles &c were indifferently collected without any selection and afterwards examined. (In Four castings found on the recently uncovered, tesselated floor of the great room in the Roman villa at Brading, contained many particles of tile or brick, of mortar, and of hard white cement; and the majority of these appeared plainly worn. The particles of mortar, however, seemed to have suffered more corrosion than attrition, for grains of silex often projected from their surfaces. Castings from within the nave of Beaulieu Abbey, which was destroyed by Henry VIII., were collected from a level expanse of turf, overlying the buried tesselated pavement, through which worm-burrows passed; and these castings contained innumerable particles of tiles and bricks, of concrete and cement, the majority of which had manifestly undergone some or much attrition. There were also many minute flakes of a micaceous slate, the points of which were rounded. If the above supposition, that in all these cases the same minute fragments have in

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these cases passed several times through the gizzards of worms, be rejected, notwithstanding its inherent probability, we must then assume that in all the above cases the many rounded fragments found in the castings had all accidentally undergone much attrition before they were swallowed; and this is highly improbable.

On the other hand it must be stated that fragments of ornamental tiles, somewhat harder than common tiles or bricks, which had been swallowed only once by worms kept in confinement, were with the doubtful exception of one or two of the smallest grains, not at all rounded. Nevertheless some of them appeared a little worn, though not rounded. Notwithstanding these cases, if we consider the evidence above given, there can be little doubt that the fragments, which serve as millstones in the gizzards of worms, suffer, when of a not very hard texture, some amount of attrition; and that the smaller particles in the earth, which is habitually swallowed in such astonishingly large quantities by worms, are ground together and are thus levigated. If this be the case, the "terra tenuissima,"—the "pâte excessivement

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fine,"—of which the castings largely consist, is in part due to the mechanical action of the gizzard;*(18) and this fine matter, as we shall see in the next chapter, is that which is chiefly washed away from the innumerable castings on every field during each heavy shower of rain. If the softer stones yield at all, the harder ones will suffer some slight amount of wear and tear. (The trituration of small particles of stone in the gizzards of worms is of more importance under a geological point of view than may at first appear to be the case; for Mr. Sorby has clearly shown that the ordinary means of disintegration, namely running water and the waves of the sea, act with less and less power on fragments of rock the smaller they are. "Hence," as he remarks, "even making no allowance for the extra buoying up of very minute particles by a current of water, depending on surface cohesion, the effects of wearing on the form of the grains must vary directly as their diameter or thereabouts. If so, a grain 1/10 of an inch in diameter would be worn ten times as much as one 1/100 of an inch in diameter, and at least a hundred times as much as one 1/1000 of an inch in diameter. Perhaps, then, we may

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*(18) 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 tranquil and waves cannot triturate the blocks of coral. This mud must, as I believe (Coral-Reefs 2d edit. 1874, p. 19), be attributed to the innumerable annelids and other animals which burrow into the dead coral, and to the fishes, Holothurians, &c., which browse on the living corals.

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"conclude that a grain 1/10 of an inch in diameter would be worn as much or more in drifting a mile as a grain 1/1000; of an inch in being drifted 100 miles. On the same principle a pebble one inch in diameter would be worn relatively more by being drifted only a few hundred yards."*(19) 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,   [remainder of page in Darwin's hand:]   which is sufficiently damp and not too sandy, gravelly or rocky for worms to inhabit, a weight of more than ten tons of earth annually passes through their bodies and is brought to the surface. The result for a country of the size of Great Britain, within a period not very long in a geological sense, such as a million years, cannot be insignificant; for the ten tons of earth has to be multiplied first by the above number of years, and then by the number of acres fully stocked with worms; and in England, together with Scotland, the land which is cultivated and is well fitted for these animals, has been estimated at above 32 million acres.)

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*(19) Anniversary Address: The Quarterly Journal of the Geological Soc. May 1880, p. 59

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Chapter VI.

The Denudation of the Land, continued.—

Denudation aided by recently ejected castings flowing down inclined 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 completely away from castings — The disintegration of dried castings into pellets, & 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 & tumuli — The preservation of the crowns & furrows on land anciently ploughed — The formation & amount of mould over the Chalk formation.—

Left
Denudation of the Land

Right
Aided by worms

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Denudation of the Land, continued.

We are now prepared to consider the more direct part which worms take in the denudation of the land. When reflecting on subaerial 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 examining 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 inclined surface, covered with vegetation and closely matted with

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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 levigated 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.

[in pencil in another hand:] Hill

The observations on which these several statements are founded must now be given in some detail. Castings when first ejected are viscid and soft; and during

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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 seen by me after heavy rain, in many places on land of all kinds.

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

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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 considerably 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 some 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 between 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

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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 7½ 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, and 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 2/3 two-thirds of the ejected

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earth lay below the mouths of the burrows.

(These observations led me to make others with more care. Eight 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 1/8 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 cut with a knife into two parts in a line passing through the mouth of the burrow (which was discovered by slicing off the turf) at right angles to the line of the greatest slope, and all the ejected earth was separately

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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 than the angle of the slope, in determining the quantity of the earth which flows down a slope than its angle. Thus with 4 castings on my lawn (included in the above 11) where the mean slope was 7° 19′, the difference in the amount of earth above and below the burrows was greater than with 3 other castings on the

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[in pencil in another hand:] Salhead[?]

same lawn where the mean slope was 12° 5′.

We may, however, take the above eleven cases, 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-third above and the upper half of the two-thirds below, there is no flow of earth down the hill-side. The earth constituting 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-

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-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 1/36 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 1/3 of the earth brought up can be considered to flow downwards; hence 1/3 of 1/36 or 1/108 of 7.453 lbs. will cross the lower side of our square yard in a year. Now 1/108 of 7.453 lbs. is 1.1 oz. Therefore 1.1 oz. of dry earth will annually cross each linear yard, marked out transversely perpendicular

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along a slope;  per annum. From this it is clear that very nearly 7lbs of dry earth cross every horizontal 100 yards of the hill-side  having the above inclination; or very nearly 7 lbs. 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 castings annually brought to the surface on a square yard, if uniformly spread out would form a layer .2 of an inch in thickness: it therefore follows by a calculation similar to the one already given, that 1/3 of .2×36, 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 slope.(a)(text)

(In the above calculations it has been assumed that the castings flow a short distance downwards 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

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(a)(text) (lead on)

 

This bulk of damp castings was found to weigh 1.85 oz. Therefore 11.56 lb of damp earth, instead of 7 lb. of dry earth as by the former calculation, would annually cross a line, 100 yards in length, on our inclined surface.)

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finest earth is washed to a considerable distance from the castings, even where the slope is an extremely 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,    129)    are apt to disintegrate into little pellets, which often roll or are blown down any inclined surface. Therefore 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 intersect 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

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Bottom of the valley. Here a thick bed of alluvium will accumulate, ready to be washed away in the course of centuries,*(20) as the stream in the middle meanders from side to side.

In countries where the rain is very heavy, 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 1½ 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

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*(20)

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 collapsed from the weight of the superincumbent soil, the collapsing would inevitably cause the whole bed of vegetable mould to sink or slide slowly down the inclined surface. But to ascertain the direction of many burrows was found too difficult and trouble some. A straight piece of wire was, however, 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.

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slight slope of 5.83 inches. As the earth had spread very little 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 composed 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."

[in pencil in another hand:] Lauer

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 precipitated 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

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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 surface of the lawn, which was here inclined at an angle of 16° 20′; nevertheless many particles of the chalk were found three inches below the casting. The experiment was repeated 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 casting; 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 the two other cases the surface of the

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ground was rendered somewhat white for a distance of one inch from the casting; and some soil collected at a distance of 2½ inches, where the slope was 7°, effervesced slightly when placed in 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 presented a subsided appearance. The worms here eject their

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castings only during the rainy season; and at the time of Dr. Kings 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 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 secretions. Frost seems to be less effective in their

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disintegration than might have been expected. Nevertheless they readily disintegrate into small pellets, after being alternately 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 sides, inclined at an angle of 27°.

Dr. King informs me that Near Nice, in places where the great cylindrical castings, previously described, abound, the soil consists of very fine arenaceo-calcareous loam; and Dr. King informs me that 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 distinguishable from the surrounding soil. He sent me a mass of such disintegrated castings, collected on the top of a bank, where none could have been rolled

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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 ¾ 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.) (When the castings near Nice had been ejected on an inclined surface, the disintegrated 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 2½ feet wide and 9 inches deep, had been made to catch the surface drainage from the adjoining hill-side. The bottom of this drain was covered for a distance of several hundred yards, to a depth of from 1½ to 3 inches, by a layer of broken castings, still retaining their characteristic shape. Nearly all these innumerable fragments had rolled down from above, for extremely few castings had been ejected in the drain itself. The hill-side was steep, but varied

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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 embankments, 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 collected 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. It is almost certain from what I have seen elsewhere that 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 remarks adds: "I can have no doubt that worms have

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help greatly in the process of denudation." Mr. Scott states also remarks on the crumbling of the castings near Calcutta (and on the mountains of Sikkim) during the hot and dry season, is very evident.

[in pencil in another hand:] Graves[?]

Ledges on mountain-sides and hill slopes.— Little horizontal ledges, one above another, have been observed on steep grassy slopes in many parts of the world. Their formation has 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 Henslow (a most careful observer) told Sir J. Hooker that he was convinced that this was not the sole cause of their formation. Sir J. Hooker 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

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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.) I have also heard

[pasted in slip in Emma Darwin's hand:]

(My son Francis examined a Chalk escarpment near Lewes; and here on a part which was very steep, sloping at 40° with the horizon, about 30 flat ledges extended horizontally 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

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7 inches in thickness. Above the ledges, the thickness of the earth over the chalk was in 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 and bare parts, consisted of disintegrated 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 bank 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

(21B

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less steep, and here the ledges ceased rather abruptly; but little embankments only a foot or two in length were still present. The slope became steeper lower down the hill, and the regular ledges then reappeared. 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 embankments showed no signs of having been trampled on by sheep, but in the lower parts such signs were fairly plain. No long continuous ledges had here been formed.)

(21 C

Ch 6.

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      If the little embankments above the Corniche road, which Dr. King saw in the act of formation by the accumulation of disintegrated and rolled worm-castings, were to become confluent along horizontal lines, ledges would be formed. Each 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

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projection 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 supposed, of these ledges, and that of the ripples of wind-drifted sand as described by Lyell.*(21)

But sheep & cattle often graze usually move in horizontal lines whilst grazing on a gentle slope, which are sometimes becomes thus faintly marked by them as may be seen, when viewed from a distance.

It is probable that animals move horizontally along a slope  on a sloping surface, instead of ascending it, in order that their whole weight may not be thrown on their hinder quarters; but why they should travel along the same lines, as they must do to make an impression on the ground, I do not know, unless it be that whatever cause influence some one animal to follow a certain line, afterwards influenced others. Ledges, however, will not be formed unless the slop is steep enough to favour the rolling and flowing down of an abundant

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*(21)Elements of Geology, 1865, p. 20.—

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Supply of castings, which are arrested along certain lines. It should be added that in some cases the little ledges which we have here considered do not run horizontally along the steep sloping sides of a valley, but are inclined parallel to its bed. This may perhaps be accounted for by horizontal liens on a steep but not lofty slope, being too short to be conveniently used by grazing animals.

[in pencil in another hand:] Lauer

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 accompanied by rain to leeward. This has been 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 resemble on a miniature scale glacier-ground hillocks of rock. They are often cavernous on the leeward side, from the

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upper part having curled over the lower part. During one unusually 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. On a 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 1° 49′) 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 to leeward and below the

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mouths of the burrows, to that on the windward side and above the mouths of the burrows was as 2¾ 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 disintegration 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 sloping bank was blown quite away by a strong south-west wind. Dr. King believes that the wind plays a great part in removing the old crumbling castings near

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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 consist of old worm-castings.

In all the many observed cases of soft castings 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 N. and N.E. direction. This fact

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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 castings, by our wet southern gales to the northeast, 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 contemporaneous with the Druidical stones. Castings

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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 at a 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 thickness 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

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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 measurements 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.

[in pencil in another hand:] Lenester

The amount of fine earth brought to the surface under the form of castings, and afterwards 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

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cannot fail to be considerable and deserves attention. É. de Beaumont looks at the vegetable mould which everywhere covers the land as a fixed line or zero, from which the amount of denudation may be measured.*(22) He ignores the continued formation of fresh mould by the disintegration of the underlying rocks and fragments of rock; and it is curious to find how much more philosophical were the views, maintained 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 continued destruction of the rocks."*(23)

Ancient encampments and tumuli.— E. 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, evidence that the slopes

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* (22) Leçons de Géologie pratique, 1845; cinquième Leçon. All Élie de Beaumont's arguments are admirably controverted controverted by Prof. A. Geikie in his paper essay in Transact. Geolog. Soc. of Glasgow vol III. p. 153, 1868.

* (23) Illustrations of the Huttonian Theory of the Earth. p. 107.

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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, 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 on a talus thus formed than elsewhere; and these would often

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be partially washed away by every heavy shower and be spread over the adjoining 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 least 2000 years old; and we should bear in mind that in many places about one inch of mould is brought to the surface 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,

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although their inclinations 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 afterwards 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 for from

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a very ancient period.*(24) 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 different 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 ½ to 2 inches thicker in the furrows than 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,

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* (24) 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-fielets 'high fields" or 'heathern-fields' (Hochäcker, & Heide heilennäcker) that they correspond much in their situation on hills & wastes 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 heathern-fields represent tillage by an ancient & barbaric population."

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It appears from several p

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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.

[in pencil in another hand:] Andrews

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 difference in the state of the furrows in the upper and lower parts of sloping fields, supposed 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 commenced in some places. Thus in a grass field in North Wales, known to have been ploughed about 65 years ago, and which sloping sloped

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at an angle of 15° to the north-east, the depth of the furrows (only 7 feet apart) was carefully measured, and was found to be about 4½ 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 southwest, the furrows were scarcely perceptible in the lower part; although these same furrows when followed on to some adjoining level ground were from 2½ to 3½ 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 2½ inches, and in the lower part 4½ inches in thickness.

On the Chalk Downs at about a mile distance from Stonehenge, my son William examined a grass-covered, furrowed surface, sloping at from 8° to 10°, which an old shepherd 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 accumulate;

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and at the base it almost disappeared. The thickness of the mould in this furrow in the upper part was 2½ 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 continued 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 furrows on land formerly ploughed, but now covered with grass, tend slowly to disappear when the surface is inclined; and this is probably 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.

The amount of mould on chalk-downs Formation and amount of mould over the Chalk Formation.— If many Worm-castings

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are often ejected in extraordinary numbers in many places on steep grass-covered slopes, where the chalk comes close to the surface, as my son William observed near Winchester & elsewhere. If such castings are largely washed away during heavy rains, it is  at first  difficult to understand at first how any mould can still remain on our Downs, as there does not appear at first sight any evident means for supplying the loss. There is Moreover there is reason to believe in another cause of loss, namely to percolation of the finer particles of earth into the fissures into in the chalk & into the chalk itself. These considerations led me to doubt for a time whether I had greatly not exaggerated the amount of fine earth under the form of castings flows or rolls down grass-covered slopes; & I sought for additional information. In some places, the castings on Chalk Downs consist largely of calcareous matter, & 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

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black & did not effervesce in the least 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 3½ inches in thickness. Why worms should penetrate & bring up chalk in some places & not in others I do not know.

(In many other places districts where the land is nearly level, a bed several feet in thickness of red clay full of unworn flints, with only the surface converted into mould, overlies the upper chalk. This overlying matter consists of the undissolved residue from the chalk left on the. It may be well here to recall the case of the angular fragments of chalk buried beneath worm-castings strewed on on one of my fields, which became so much corroded, as to resemble pebbles the angles of which were so completely rounded in the course of only 29 years, after their burial beneath worm-castings that the fragments now resembled cemented water-worn pebbles. This must have been effected by the carbonic acid in the rain & ground, & by the dissolving power of the humus-acids & by the corroding power of living roots. Why a thick mass of residue has not been left on the chalk, wherever wherever the land is

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nearly level, may probably perhaps be accounted for by the percolation of the finer particles into the fissures in the chalk, which may often be seen in the chalk either filled up with impure chalk or open, & into the solid chalk itself. That such percolation occurs on a large scale can hardly be doubted.

[pasted in slip in copyist hand:]

chalk. 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.

[remainder in Darwin's hand:]

On the flanks of the escarpment at near Abinger in Surrey, some chalk close beneath a layer of flints, 2 inches in thickness covered by 8 inches of mould, yielded a residue of 3.7 per cent. On the other hand the Upper Chalk when from pure 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; & two samples from pits near my house contained 1.3 and 0.6 per cent. I mention these latter cases, because from the thickness of the overlying bed of red clay with flints, I had imagined that the underlying chalk might be here be less

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pure than elsewhere. The cause of the residue accumulating more in some places than in others may perhaps be attributed to a layer of argillaceous matter having been left at an early period on the chalk, which would greatly check the subsequent percolation of earthy matter into it.)

[in pencil in another hand:] Dawes

From the facts now given we may conclude that castings ejected on our chalk Downs suffer some loss by the percolation of  some of their finer matter into the chalk. But such  the impure superficial chalk, when dissolved, will leave a larger supply of earthy matter to be added to the mould than with in the case of pure chalk. Besides the loss this caused by percolation, some fine earth, as we shall immediately see, is certainly washed

[slip pasted on in copyist hand:]

down the sloping grass-covered surfaces of our Downs. The washing-down process, however, will be checked in the course of time; for although I do not know how thin a layer of mould suffices to support worms, yet a limit must at last be reached; and then their castings would cease to be ejected and to be washed downwards or would become scanty. Moreover the amount which is washed down on an inclined chalk-down during heavy rain is perhaps less than elsewhere on other grassy slopes,  owing to the fineness and compactness of the turf. My son found that

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the castings on such places adhered after dry weather closely to the fine blades of grass, so that very few diintegrated particles appeared to have rolled downwards. (Nevertheless  The following cases apparently show that a considerable quantity of fine earth is washed down. The thickness of the mould was measured at points 12 yards apart across a small valley in the Chalk near Winchester. The sides sloped gently at first; then became inclined at about 20°; then more gently to near the bottom, which transversely was almost level and about 50 yards across. In the bottom, the mean thickness of the mould from five measurements was 8.3 inches; whilst on the sides of the valley, where the inclination varied between 14° and 20°, its mean thickness was rather less than 3.5 inches. As the turf-covered bottom of the valley sloped at an angle of only between 2° and 3°, it is probable that most of the 8.3-inch layer of mould had been washed down from the flanks of the valley, It is however possible that some may have & not from the upper part. But as a shepherd said that he had seen the bottom of water flowing in this valley after the sudden thawing of snow, it is possible possible not improbable

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that some earth may have been brought down  from the upper part; or, on the other hand, that some may have been carried further down the valley. washed away; for a shepherd said that he had seen the whole bottom of the valley filled with flowing water after the sudden thawing of the snow Closely similar results, with respect to the thickness of the mould, were obtained in a neighbouring valley.

St. Catherine's Hill, near Winchester, is 327 feet in height, and consists of a steep cone of chalk about ¼ of a mile in diameter. The upper part was converted by the Romans, or, as some think, by the ancient Britons, into an encampment, by the excavation of a deep and broad ditch all round it. Most of the chalk removed during the work was thrown upwards, by which a projecting bank was formed; and this effectually prevents worm-castings (which are numerous in parts), stones, and other objects from being washed or rolled into the ditch. The mould on the upper and fortified part of the hill was found to be in most places only from

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2½ to 3½ inches in thickness; whereas it had accumulated at the foot of the embankment above the ditch to a thickness in most places of from 8 to 9½ inches. On the embankment itself the mould was only 1 to 1½ inch in thickness; and within the ditch at the bottom it varied from 2½ to 3½, but was in one spot 6 inches in thickness. On the north-west side of the hill, either no embankment had ever been thrown up above the ditch, or it had subsequently been removed; so that here there was nothing to prevent worm-castings, earth and stones being washed into the ditch, at the bottom of which the mould formed a layer from 11 to 22 inches in thickness. It should however be stated that here and on other parts of the slope, the bed of mould often contained fragments of chalk and flint which had obviously rolled down at different times from above. The interstices in the underlying fragmentary chalk were also filled up with mould.

My son examined the surface of this hill to its base in a south-west direction. Beneath the great ditch, where the slope was

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about 24°, the mould was very thin, namely from 1½ to 2½ inches; whilst near the base where the slope was only 3° to 4°, it increased to between 8 and 9 inches in thickness. We may therefore conclude that on this artificially modified hill, as well as in the natural valleys of the neighbouring Chalk Downs, some fine earth, probably derived in large part from worm-castings, is washed down, and accumulates in the lower parts, notwithstanding the percolation of an unknown quantity into the underlying chalk; and The whole of this fine earth must be derived from the residual matter left during the dissolution of the chalk.

[in Darwin's hand:] A supply of fresh earthy matter being afforded by the dissolution of the chalk through atmospheric & other agencies.)

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rain and by the humus-acids will leave a larger supply of earthy matter to be added to the superficial mould, than in the case of pure chalk. The case formerly given of the fragments of chalk left on the surface of one of my fields, which was rounded and greatly corroded in the course of 29 years, after they had been buried by the worms, proving how quickly chalk is thus dissolved. Besides the loss which castings [bottom of page excised]

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[in pencil in another hand:] Lenester

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Chapter VII.

Conclusion.

Summary of the part which worms have played in the history of the world—Their aid in the disintegration of rocks—In the denudation of the land—In the preservation of ancient remains—In the preparation of the soil for the growth of plants—Mental powers of worms—Conclusion.—

[pasted on slip in copyist hand]

(Worms have played a more important part in the history of the world than most persons would at first suppose. In almost all humid countries they are extraordinarily numerous, and for their size possess great muscular power. In many parts of England a weight of more than ten tons (10,516 kilogrammes) of dry earth annually passes through their bodies and is brought to the surface on each acre of land; so that the whole superficial bed of vegetable mould thus passes in the

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course of every few years. From the collapsing of the old burrows the mould is in constant though slow movement, and the particles composing it are thus rubbed together. By these means fresh surfaces are continually exposed to the action of the carbonic acid in the soil, and of the humus-acids which appear to be still more efficient in the decomposition of rocks. The generation of the humus-acids is probably hastened during the digestion of the many half-decayed leaves which worms consume. Thus the particles of earth, forming the superficial mould, are subjected to conditions eminently favourable for their decomposition and disintegration. Moreover, the particles of the softer rocks suffer some amount of mechanical trituration in the muscular gizzards of worms, in which small stones serve as mill-stones.

The finely levigated castings, when brought to the surface in a moist condition, flow during rainy weather down any moderate slope; and the smaller particles are washed far down even a gently inclined surface. Castings when dry often crumble

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into small pellets and these are apt to roll down any sloping surface. Where the land is quite level and is covered with herbage, and where the climate is humid so that much dust cannot be blown away, it appears at first sight impossible that there should be any appreciable amount of subaerial denudation; but worm-castings are blown, especially whilst moist and viscid, in one uniform direction by the prevalent winds which are accompanied by rain. By these several means the superficial mould is prevented from accumulating to a great thickness; and a thick bed of mould checks in many ways the disintegration of the underlying rocks and fragments of rock.

The removal of worm castings by the above means leads to results which are far from insignificant. It has been shown that a layer of earth, .2 of an inch in thickness, is in many places annually brought to the surface per acre; and if a small part of this amount flows, or rolls, or is washed, even for a short distance down every inclined surface, or is repeatedly blown in

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one direction, a great effect will be produced in the course of ages. It was found by measurements and calculations that on a surface with a mean inclination of 9° 26′, 2.4 cubic inches of earth which had been ejected by worms crossed, in the course of a year, a horizontal line one yard in length; so that 240 cubic inches would cross a line 100 yards in length. This latter amount in a damp state would weigh 11½ pounds. Thus a considerable weight of earth is continually moving down each side of every valley, and will in time reach its bed. Finally this earth will be transported by the streams flowing in the valleys into the ocean, the great receptacle for all matter denuded from the land. It is known from the amount of sediment annually delivered into the sea by the Mississippi, that its enormous drainage-area must on an average be lowered .00263 of an inch each year; and this would suffice in four and half million years to lower the whole drainage-area to the level of the sea-shore. So that, if a small fraction of the layer of fine earth, .2

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of an inch in thickness, which is annually brought to the surface by worms, is carried away, a great result cannot fail to be produced within a period which no geologist considers extremely long.

(one line open)

Archæologists ought to be grateful to worms, as they protect and preserve for an indefinitely long period every object, not liable to decay, which is dropped on the surface of the land, by burying it beneath their castings. Thus, also, many elegant and curious tesselated pavements and other ancient remains have been preserved; though no doubt the worms have in these cases been largely aided by earth washed and blown from the adjoining land, especially when cultivated. The old tesselated pavements have, however, often suffered by having subsided unequally from being unequally undermined by the worms. Even old massive walls may be undermined and subside; and no building is in this respect safe, unless the foundations lie 6 or 7 feet beneath the surface, at a depth at which worms cannot work. It is probable that many monoliths and some old walls have fallen down from

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