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thickness。 Why worms should penetrate and bring up chalk in some
places and not in others I do not know。
In many districts where the land is nearly level； a bed several
feet in thickness of red clay full of unworn flints overlies the
Upper Chalk。 This overlying matter； the surface of which has been
converted into mould； consists of the undissolved residue from the
chalk。 It may be well here to recall the case of the fragments of
chalk buried beneath worm…castings on one of my fields； the angles
of which were so completely rounded in the course of 29 years that
the fragments now resembled water…worn pebbles。 This must have
been effected by the carbonic acid in the rain and in the ground；
by the humus…acids； and by the corroding power of living roots。
Why a thick mass of residue has not been left on the Chalk；
wherever the land is nearly level； may perhaps be accounted for by
the percolation of the fine particles into the fissures； which are
often present in the chalk and are either open or are filled up
with impure chalk； or into the solid chalk itself。 That such
percolation occurs can hardly be doubted。 My son collected some
powdered and fragmentary chalk beneath the turf near Winchester；
the former was found by Colonel Parsons； R。 E。； to contain 10 per
cent。； and the fragments 8 per cent。 of earthy matter。 On the
flanks of the escarpment near Abinger in Surrey； some chalk close
beneath a layer of flints； 2 inches in thickness and covered by 8
inches of mould； yielded a residue of 3。7 per cent。 of earthy
matter。 On the other hand the Upper Chalk properly contains； as I
was informed by the late David Forbes who had made many analyses；
only from 1 to 2 per cent。 of earthy matter； and 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 here be less pure than elsewhere。 The cause of the residue
accumulating more in some places than in others； may be attributed
to a layer of argillaceous matter having been left at an early
period on the chalk； and this would check the subsequent
percolation of earthy matter into it。
From the facts now given we may conclude that castings ejected on
our Chalk Downs suffer some loss by the percolation of their finer
matter into the chalk。 But such impure superficial chalk； when
dissolved； would leave a larger supply of earthy matter to be added
to the mould than in the case of pure chalk。 Besides the loss
caused by percolation； some fine earth is certainly washed 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 or would become scanty。
The following cases 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
degrees； 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 degrees and 20 degrees； 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 degrees and 3 degrees； it is
probable that most of the 8。3…inch layer of mould had been washed
down from the flanks of the valley； and not from the upper part。
But as a shepherd said that he had seen water flowing in this
valley after the sudden thawing of snow； it is possible 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。 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 0。25 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 2。5 to 3。5
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 9。5 inches。 On the embankment itself the mould was only 1 to
1。5 inch in thickness； and within the ditch at the bottom it varied
from 2。5 to 3。5； 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 about
24 degrees； the mould was very thin； namely； from 1。5 to 2。5
inches； whilst near the base； where the slope was only 3 degrees to
4 degrees； 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； a
supply of fresh earthy matter being afforded by the dissolution of
the chalk through atmospheric and other agencies。
CHAPTER VIICONCLUSION。
Summary of the part which worms have played in the history of the
worldTheir aid in the disintegration of rocksIn the denudation
of the landIn the preservation of ancient remainsIn the
preparation of the soil for the growth of plantsMental powers of
wormsConclusion。
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 muscu