to the surface. Immediately beneath the layer of red sand, the original
substratum of black sandy peat extended.
A grass field, likewise not far from Maer Hall, had formerly been thickly
covered with marl, and was then left for several years as pasture; it was
afterwards ploughed. A friend had three trenches dug in this field 28
years after the application of the marl, {126} and a layer of the marl
fragments could be traced at a depth, carefully measured, of 12 inches in
some parts, and of 14 inches in other parts. This difference in depth
depended on the layer being horizontal, whilst the surface consisted of
ridges and furrows from the field having been ploughed. The tenant
assured me that it had never been turned up to a greater depth than from
6 to 8 inches; and as the fragments formed an unbroken horizontal layer
from 12 to 14 inches beneath the surface, these must have been buried by
the worms whilst the land was in pasture before it was ploughed, for
otherwise they would have been indiscriminately scattered by the plough
throughout the whole thickness of the soil. Four-and-a-half years
afterwards I had three holes dug in this field, in which potatoes had
been lately planted, and the layer of marl-fragments was now found 13
inches beneath the bottoms of the furrows, and therefore probably 15
inches beneath the general level of the field. It should, however, be
observed that the thickness of the blackish sandy soil, which had been
thrown up by the worms above the marl-fragments in the course of 321 2
years, would have measured less than 15 inches, if the field had always
remained as pasture, for the soil would in this case have been much more
compact. The fragments of marl almost rested on an undisturbed
substratum of white sand with quartz pebbles; and as this would be little
attractive to worms, the mould would hereafter be very slowly increased
by their action.
We will now give some cases of the action of worms, on land differing
widely from the dry sandy or the swampy pastures just described. The
chalk formation extends all round my house in Kent; and its surface, from
having been exposed during an immense period to the dissolving action of
rain-water, is extremely irregular, being abruptly festooned and
penetrated by many deep well-like cavities. {128} During the dissolution
of the chalk, the insoluble matter, including a vast number of unrolled
flints of all sizes, has been left on the surface and forms a bed of
stiff red clay, full of flints, and generally from 6 to 14 feet in
thickness. Over the red clay, wherever the land has long remained as
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