the formation of vegetable mould-第40章
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the same angle to the south…west； the furrows were scarcely
perceptible in the lower part； although these same furrows when
followed on to some adjoining level ground were from 2。5 to 3。5
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 2。5 inches； and in the lower part 4。5 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 degrees to 10 degrees； 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 tend to accumulate； and at the base it almost
disappeared。 The thickness of the mould in this furrow in the
upper part was 2。5 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。
Formation and amount of mould over the Chalk Formation。Worm…
castings are often ejected in extraordinary numbers on steep；
grass…covered slopes； where the Chalk comes close to the surface；
as my son William observed near Winchester and elsewhere。 If such
castings are largely washed away during heavy rains； it is
difficult to understand at first how any mould can still remain on
our Downs； as there does not appear any evident means for supplying
the loss。 There is； moreover； another cause of loss； namely； in
the percolation of the finer particles of earth into the fissures
in the chalk and into the chalk itself。 These considerations led
me to doubt for a time whether I had not exaggerated the amount of
fine earth which flows or rolls down grass…covered slopes under the
form of castings； and I sought for additional information。 In some
places； the castings on Chalk Downs consist largely of calcareous
matter； and here the supply is of course unlimited。 But in other
places； for instance on a part of Teg Down near Winchester； the
castings were all black and did not effervesce with acids。 The
mould over the chalk was here only from 3 to 4 inches in thickness。
So again on the plain near Stonehenge； the mould； apparently free
from calcareous matter； averaged rather less than 3。5 inches in
thickness。 Why worms should penetrate and bring up chalk in some
places and not in others I do not know。
In many districts where the land is nearly level； a bed several
feet in thickness of red clay full of unworn flints overlies the
Upper Chalk。 This overlying matter； the surface of which has been
converted into mould； 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 th
