ON THE GRAYS AND UPMINSTER RAILWAY. 149
POSTSCRIPTS.
Since the above paper was read I have twice traversed this line
of railway, my latest visit having been on Oct. 4th (1890). On the
first occasion two, and, on the second, three natural cavities were
visible in the gravel on the southern side of Back Lane. The
largest was about 3 ft. in length, and 9 or 10 inches high, its axis
being nearly horizontal. Cavities of this kind are not uncommon
in certain localities where gravel and sand lie directly above the
Chalk. Pipes are formed in the latter, but the gravel and sand are
here and there much more coherent than usual, owing their greater
firmness to the presence of a ferruginous cement. Consequently,
while the less coherent material descends to fill the pipes in the
Chalk, that which lies above a hard band is prevented from doing so,
and a hollow is formed beneath the hard band. For accounts of
similar cavities, which are often of much greater size, I may refer to my
"Notes on the Oldhaven Pebble-Beds at Caterham" (Proc. Geol.
Assoc, vol. ix., p. 105, Aug., 1885).
I may add that my most recent visits have tended to confirm my
doubts as to the need of any form of glacial action to produce the
extremely irregular junction between this gravel and the Chalk.
Ordinary pipe-making action seems to me at least to be quite com-
petent to have produced anything 1 have seen, whether it actually
was the sole agent or not.
October 22nd, 1890. Mr. Whitaker and I drove to South Ocken-
don and walked back along the course of the new railway to West
Thurrock. I found that little, if any, progress had been made since
July between South Ockendon and the Mardyke. At Back Lane, the
completion of the bridge, and the consequent removal of heaps of
bricks, timber, &c., which had been lying close to it, allowed us to
see the junction between the Chalk and the Thanet Sand. It
appeared close to the bridge on its northern side. The cutting
in Chalk between a and c (see sketch map) had been much
deepened, and we noticed that the undulating flint band, with a
general northerly dip, visible at c, was traceable for a considerable
distance southward, the same undulating course and northerly
inclination being maintained.
Micronympha elegans v. rufescens, Leach. — Mr. T. D. A Cockerell
mentions ("Entom. Record," i., p. 173) having taken this dragon-fly near Leigh,
Essex.