work down, and at the same rate, through the matted roots of a
grass-covered surface, is a question which has probably never occurred to
them. {138b}
_The Sinking of great Stones through the Action of Worms_. When a stone
of large size and of irregular shape is left on the surface of the
ground, it rests, of course, on the more protuberant parts; but worms
soon fill up with their castings all the hollow spaces on the lower side;
for, as Hensen remarks, they like the shelter of stones. As soon as the
hollows are filled up, the worms eject the earth which they have
swallowed beyond the circumference of the stones; and thus the surface of
the ground is raised all round the stone. As the burrows excavated
directly beneath the stone after a time collapse, the stone sinks a
little. {139} Hence it is, that boulders which at some ancient period
have rolled down from a rocky mountain or cliff on to a meadow at its
base, are always somewhat imbedded in the soil; and, when removed, leave
an exact impression of their lower surfaces in the underlying fine mould.
If, however, a boulder is of such huge dimensions, that the earth beneath
is kept dry, such earth will not be inhabited by worms, and the boulder
will not sink into the ground.
A lime-kiln formerly stood in a grass-field near Leith Hill Place in
Surrey, and was pulled down 35 years before my visit; all the loose
rubbish had been carted away, excepting three large stones of quartzose
sandstone, which it was thought might hereafter be of some use. An old
workman remembered that they had been left on a bare surface of broken
bricks and mortar, close to the foundations of the kiln; but the whole
surrounding surface is now covered with turf and mould. The two largest
of these stones had never since been moved; nor could this easily have
been done, as, when I had them removed, it was the work of two men with
levers. One of these stones, and not the largest, was 64 inches long, 17
inches broad, and from 9 to 10 inches in thickness. Its lower surface
was somewhat protuberant in the middle; and this part still rested on
broken bricks and mortar, showing the truth of the old workman s account.
Beneath the brick rubbish the natural sandy soil, full of fragments of
sandstone was found; and this could have yielded very little, if at all,
to the weight of the stone, as might have been expected if the sub-soil
had been clay. The surface of the field, for a distance of about 9
inches round the stone, gradually sloped up to it, and close to the stone
stood in most places about 4 inches above the surrounding ground. The
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