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 31 2 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
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