m 61/2 to 81/2 inches; beneath which a few small
fragments of brick were found in one place. From these several cases,
it would appear, that during the last 29 years mould has been heaped
on the surface at an average annual rate of from .2 to .22 of an
inch. But in this district when a ploughed field is first laid down in
grass, the mould accumulates at a much slower rate. The rate, also,
must become very much slower after a bed of mould, several inches in
thickness, has been formed; for the worms then live chiefly near the
surface, and burrow down to a greater depth so as to bring up fresh
earth from below, only during the winter, when the weather is very
cold (at which time worms were found in this field at a depth of
26 inches), and during summer, when the weather is very dry.

A field which adjoins the one just described, slopes in one part
rather steeply (viz., at from 10 deg. to 15 deg.); this part was last ploughed
in 1841, was then harrowed and left to become pasture-land. For
several years it was clothed with an extremely scant vegetation, and
was so thickly covered with small and large flints (some of them half
as large as a child's head) that the field was always called by my
sons "the stony field." When they ran down the slope the stones
clattered together. I remember doubting whether I should live to see
these larger flints covered with vegetable mould and turf. But the
smaller stones disappeared before many years had elapsed, as did every
one of the larger ones after a time; so that after thirty years (1871)
a horse could gallop over the compact turf from one end of the field
to the other, and not strike a single stone with his shoes. To anyone
who remembered the appearance of the field in 1842, the transformation
was wonderful. This was certainly the work of the worms, for though
castings were not frequent for several years, yet some were thrown up
month after month, and these gradually increased in numbers as the
pasture improved. In the year 1871 a trench was dug on the above
slope, and the blades of grass were cut off close to the roots, so
that the thickness of the turf and of the vegetable mould could be
measured accurately. The turf was rather less than half an inch, and
the mould, which did not contain any stones, 21/2 inches in thickness.
Beneath this lay coarse, clayey earth full of flints, like that in any
of the neighboring ploughed fields. This coarse earth easily fell
apart from the overlying