e 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 sub-stratum 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 pasture 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. 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 pasture, there is a layer a few inches in thickness of
dark-coloured vegetable mould.

A quantity of broken chalk was spread, on December 20, 1842, over a
part of a field near my house, which had existed as pasture certainly
for 30, probably for twice or thrice as many, years. The chalk was
laid on the land for the sake of observing at some future period to
what depth it would become buried. At the end of November, 1871, that
is, after an interval of twenty-nine years, a trench was dug across
this part of the field; and a line of white nodules could be traced on
both sides of the trench, at a depth of 7 inches from the surface. The
mould, therefore (excluding the turf), had here been thrown up at an
average rate