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 29 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 of 0.22 inch per year.
Beneath the line of chalk nodules there was in parts hardly any fine
earth free of flints, while in other parts there was a layer, 21 4 inches
in thickness. In this latter case the mould was altogether 91 4 inches
thick; and in one such spot a nodule of chalk and a smooth flint pebble,
both of which must have been left at some former time on the surface,
were found at this depth. At from 11 to 12 inches beneath the surface,
the undisturbed reddish clay, full of flints, extended. The appearance
of the above nodules of chalk surprised me, much at first, as they
closely resembled water-worn pebbles, whereas the freshly-broken
fragments had been angular. But on examining the nodules with a lens,
they no longer appeared water-worn, for their surfaces were pitted
through unequal corrosion, and minute, sharp points, formed of broken
fossil shells, projected from them. It was evident that the corners of
the original fragments of chalk had been wholly dissolved, from
presenting a large surface to the carbonic acid dissolved in the
rain-water and to that generated in soil containing vegetable matter, as
well as to the humus-acids. {131} The projecting corners would also,
relatively to the other parts, have been embraced by a larger number of
living rootlets; and these have the power of even attacking marble, as
Sachs has shown. Thus, in the course of 29 years, buried angular
fragments of chalk had been converted into well-rounded nodules.
Another part of this same field was mossy, and as it was thought that
sifted coal-cinders would improve the pasture, a thick layer was spread
over this part either in 1842 or 1843, and another layer some years
afterwards. In 1871 a trench was here dug, and many cinders lay in a
line at a depth of 7 inches beneath the surface, with another line at a
depth of 51 2 inches parallel to the one beneath. In another part of this
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