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 21 2 to 31 2 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 21 2 inches, and in the lower part 41 2 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 tend to accumulate, and at the base it
almost disappeared. The thickness of the mould in this furrow in the
upper part was 21 2 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,
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