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nearly so compact a mass as vegetable mould； though each separate
particle was very compact。 Yet mould is far from being compact； as
is shown by the number of air…bubbles which rise up when the
surface is flooded with water。 It is moreover penetrated by many
fine roots。 To ascertain approximately by how much ordinary
vegetable mould would be increased in bulk by being broken up into
small particles and then dried； a thin oblong block of somewhat
argillaceous mould （with the turf pared off） was measured before
being broken up； was well dried and again measured。 The drying
caused it to shrink by 1/7 of its original bulk； judging from
exterior measurements alone。 It was then triturated and partly
reduced to powder； in the same manner as the castings had been
treated； and its bulk now exceeded （notwithstanding shrinkage from
drying） by 1/16 that of the original block of damp mould。
Therefore the above calculated thickness of the layer； formed by
the castings from the Terrace； after being damped and spread over a
square yard； would have to be reduced by 1/16； and this will reduce
the layer to 0。09 of an inch； so that a layer 0。9 inch in thickness
would be formed in the course of ten years。 On the same principle
the castings from the Common would make in the course of a single
year a layer 0。1429 inch； or in the course of 10 years 1。429 inch；
in thickness。 We may say in round numbers that the thickness in
the former case would amount to nearly 1 inch； and in the second
case to nearly 1。5 inch in 10 years。
In order to compare these results with those deduced from the rates
at which small objects left on the surfaces of grass…fields become
buried （as described in the early part of this chapter）； we will
give the following summary：…
SUMMARY OF THE THICKNESS OF THE MOULD ACCUMULATED OVER OBJECTS LEFT
STREWED ON THE SURFACE； IN THE COURSE OF TEN YEARS。
The accumulation of mould during 14。75 years on the surface of a
dry； sandy； grass…field near Maer Hall； amounted to 2。2 inches in
10 years。
The accumulation during 21。5 years on a swampy field near Maer
Hall； amounted to nearly 1。9 inch in 10 years。
The accumulation during 7 years on a very swampy field near Maer
Hall amounted to 2。1 inches in 10 years。
The accumulation during 29 years； on good； argillaceous pasture…
land over the Chalk at Down； amounted to 2。2 inches in 10 years。
The accumulation during 30 years on the side of a valley over the
Chalk at Down； the soil being argillaceous； very poor； and only
just converted into pasture （so that it was for some years
unfavourable for worms）； amounted to 0。83 inch in 10 years。
In these cases （excepting the last） it may be seen that the amount
of earth brought to the surface during 10 years is somewhat greater
than that calculated from the castings which were actually weighed。
This excess may be partly accounted for by the loss which the
weighed castings had previously undergone through being washed by
rain； by the adhesion of particles to the blades of the surrounding
grass； and by their crumbling when dry。 Nor must we overlook other
agencies which in all ordinary cases add to the amount of mould；
and which would not be included in the castings that were
collected； namely； the fine earth brought up to the surface by
burrowing larvae and insects； especially by ants。 The earth
brought up by moles generally has a somewhat different appearance
from vegetable mould； but after a time would not be distinguishable
from it。 In dry countries； moreover； the wind plays an important
part in carrying dust from one place to another； and even in
England it must add to the mould on fields near great roads。 But
in our country these latter several agencies appear to be of quite
subordinate importance in comparison with the action of worms。
We have no means of judging how great a weight of earth a single
full…sized worm ejects during a year。 Hensen estimates that 53；767
worms exist in an acre of land； but this is founded on the number
found in gardens； and he believes that only about half as many live
in corn…fields。 How many live in old pasture land is unknown； but
if we assume that half the above number； or 26；886 worms live on
such land； then taking from the previous summary 15 tons as the
weight of the castings annually thrown up on an acre of land； each
worm must annually eject 20 ounces。 A full…sized casting at the
mouth of a single burrow often exceeds； as we have seen； an ounce
in weight； and it is probable that worms eject more than 20 full…
sized castings during a year。 If they eject annually more than 20
ounces； we may infer that the worms which live in an acre of
pasture land must be less than 26；886 in number。
Worms live chiefly in the superficial mould； which is usually from
4 or 5 to 10 and even 12 inches in thickness； and it is this mould
which passes over and over again through their bodies and is
brought to the surface。 But worms occasionally burrow into the
subsoil to a much greater depth； and on such occasions they bring
up earth from this greater depth； and this process has gone on for
countless ages。 Therefore the superficial layer of mould would
ultimately attain； though at a slower and slower rate； a thickness
equal to the depth to which worms ever burrow； were there not other
opposing agencies at work which carry away to a lower level some of
the finest earth which is continually being brought to the surface
by worms。 How great a thickness vegetable mould ever attains； I
have not had good opportunities for observing； but in the next
chapter； when we consider the burial of ancient buildings； some
facts will be given on this head。 In the two last chapters we
shall see that the soil is actually increased； though only to a
small degree； through the agency of worms； but their chief work is
to sift the finer from the coarser particles； to mingle the whole
with vegetable debris； and to saturate it with their intestinal
secretions。
Finally； no one who considers the facts given in this chapteron
the burying of small objects and on the sinking of great stones
left on the surfaceon the vast number of worms which live within
a moderate extent of ground on the weight of the castings ejected
from the mouth of the same burrowon the weight of all the
castings ejected within a known time on a measured spacewill
hereafter； as I believe； doubt that worms play an important part in
nature。
CHAPTER IVTHE PART WHICH WORMS HAVE PLAYED IN THE BURIAL OF
ANCIENT BUILDINGS。
The accumulation of rubbish on the sites of great cities
independent of the action of wormsThe burial of a Roman villa at
AbingerThe floors and walls penetrated by wormsSubsidence of a
modern pavementThe buried pavement at Beaulieu AbbeyRoman
villas at Chedworth and BradingThe remains of the Roman town at
SilchesterThe nature of the debris by which the remains are
coveredThe penetration of the tesselated floors and walls by
wormsSubsidence of the floorsThickness of the mouldThe old
Roman city of WroxeterThickness of the mouldDepth