{108} Zeitschrift fur wissenschaft. Zoolog. B. xxviii. 1877, p. 356.
{113} Perrier, Archives de Zoolog. exper. tom. 3, p. 378, 1874.
{126} This case is given in a postscript to my paper in the Transact.
Geolog. Soc. (Vol. v. p. 505), and contains a serious error, as in the
account received I mistook the figure 30 for 80. The tenant, moreover,
formerly said that he had marled the field thirty years before, but was
now positive that this was done in 1809, that is twenty-eight years
before the first examination of the field by my friend. The error, as
far as the figure 80 is concerned, was corrected in an article by me, in
the Gardeners Chronicle, 1844, p. 218.
{128} These pits or pipes are still in process of formation. During the
last forty years I have seen or heard of five cases, in which a circular
space, several feet in diameter, suddenly fell in, leaving on the field
an open hole with perpendicular sides, some feet in depth. This occurred
in one of my own fields, whilst it was being rolled, and the hinder
quarters of the shaft horse fell in; two or three cart-loads of rubbish
were required to fill up the hole. The subsidence occurred where there
was a broad depression, as if the surface had fallen in at several former
periods. I heard of a hole which must have been suddenly formed at the
bottom of a small shallow pool, where sheep had been washed during many
years, and into which a man thus occupied fell to his great terror. The
rain-water over this whole district sinks perpendicularly into the
ground, but the chalk is more porous in certain places than in others.
Thus the drainage from the overlying clay is directed to certain points,
where a greater amount of calcareous matter is dissolved than elsewhere.
Even narrow open channels are sometimes formed in the solid chalk. As
the chalk is slowly dissolved over the whole country, but more in some
parts than in others, the undissolved residue that is the overlying mass
of red clay with flints, likewise sinks slowly down, and tends to fill up
the pipes or cavities. But the upper part of the red clay holds
together, aided probably by the roots of plants, for a longer time than
the lower parts, and thus forms a roof, which sooner or later falls in,
as in the above mentioned five cases. The downward movement of the clay
may be compared with that of a glacier, but is incomparably slower; and
this movement accounts for a singular fact, namely, that the much
elongated flints which are embedded in the chalk in a nearly horizontal
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