diameter. Perhaps, then, we may conclude that a grain 1/10 of an inch in
diameter would be worn as much or more in drifting a mile as a grain
1/1000 of an inch in being drifted 100 miles. On the same principle a
pebble one inch in diameter would be worn relatively more by being
drifted only a few hundred yards. {236} Nor should we forget, in
considering the power which worms exert in triturating particles of rock,
that there is good evidence that on each acre of land, which is
sufficiently damp and not too sandy, gravelly or rocky for worms to
inhabit, a weight of more than ten tons of earth annually passes through
their bodies and is brought to the surface. The result for a country of
the size of Great Britain, within a period not very long in a geological
sense, such as a million years, cannot be insignificant; for the ten tons
of earth has to be multiplied first by the above number of years, and
then by the number of acres fully stocked with worms; and in England,
together with Scotland, the land which is cultivated and is well fitted
for these animals, has been estimated at above 32 million acres. The
product is 320 million million tons of earth.
CHAPTER VI.
THE DENUDATION OF THE LAND _continued_.
Denudation aided by recently ejected castings flowing down inclined
grass-covered surfaces The amount of earth which annually flows
downwards The effect of tropical rain on worm castings The finest
particles of earth washed completely away from castings The
disintegration of dried castings into pellets, and their rolling down
inclined surfaces The formation of little ledges on hill-sides, in part
due to the accumulation of disintegrated castings Castings blown to
leeward over level land An attempt to estimate the amount thus blown The
degradation of ancient encampments and tumuli The preservation of the
crowns and furrows on land anciently ploughed The formation and amount of
mould over the Chalk formation.
WE are now prepared to consider the more direct part which worms take in
the denudation of the land. When reflecting on sub-aerial denudation, it
formerly appeared to me, as it has to others, that a nearly level or very
gently inclined surface, covered with turf, could suffer no loss during
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