54 On Deneholes.
can hardly have been so abnormally perverse as to prefer—
if wanting flint or chalk—to concentrate their excavations
just where they got the least return for their labour. My
maps (Plate I.) are taken from that of the Geological Survey
showing the environs of London, and their object is simply
to show the surface occupied by bare chalk as compared with
that covered by the overlying beds. I have accordingly
omitted all the subdivisions of the latter, the alluvial flats
being alone separated on account of their value in illustrating
the Physical Geography of these districts.
On Plate II. of Mr. Spurrell's paper are figured nine
varieties of Deneholes, and it seems to me that Nos. 2, 3,
and 4, which are all pits at Stankey, may be compared on
terms of equality with the three we have already descended
in Hangman's Wood. The three simple forms, Nos. 7, 8,
and 9, are all shallow pits in the chalk only. So far, indeed,
we have seen no pillars in the pits at Grays, while each of
the Stankey pits figured has one or more. But this marks
simply a later stage of development in the Stankey pits, not
a different plan of excavation. No. 1 Pit, Hangman's Wood
(Plate II.), may indeed have had at one time a general
resemblance to No. 3 Stankey, and may owe the downfall of
Thanet sand, which occupies so much of its space, partly to
the thinness of its chalk roof (from one foot six inches to
two feet) and partly to the removal of a pillar such as that
still existing in No. 3 Stankey. But Nos. 2 and 3, Hangman's
Wood, have evidently not reached the pillar stage. The
greatest length in each pit is seventy feet, and is along a line
passing through the centre of the shaft and of the openings
opposite each other at its base, by means of which admission
is given to the cavern. In No. 2, on one side of the shaft,
are two chambers with a direction at right angles to the line
of greatest length. On the opposite side of the shaft, how-
ever, there is a chamber on one side only, the sixth being
entirely wanting. In No. 3 the sixth chamber is added, and
the symmetry is complete. It would seem that there had
been more demand for space in No. 3 Pit than in No. 2.
For in No. 2 the chambers are very much narrower than in