File:The Quarterly journal of the Geological Society of London (12711537483).jpg

PRESTWICH SAND- AND GRAVEL-PIPES.
69
as from his long-continued study of the superficial deposits of the
south of England his opinions are necessarily entitled to consider-
able weight. In this examination also, as the main features of these
sand-pipes have been brought forward, much further description will
be obviated. With the general view advanced by Cuvier and
Brongniart and by Dr. Buckland, and more critically laid down by
Sir Charles Lyell, I fully agree ; and my object now will merely be to
adduce some fresh proofs in its favour, and to suggest a general
cause for the formation of these peculiar excavations.
§ 2. Special Phcenomena.
As the name implies, the pipes are common both under sand and
gravel beds. They occur in fact wherever a loose and non-calcareous
permeable stratum of any extent overlies the Chalk or some calca-
reous rock. They present an infinite variety, but I will confine my-
self to the few essential points.
Some years since I met with an instance of one of these pipes in
a chalk-pit near Lower Elmsden, a few miles south-west of Canter-
bury, which seemed to me conclusive of their formation by the slow
and gradual action of water after the deposition of the superincumbent
strata. The following is a section of this sand-pipe.
Fig. 1. — Section of a Sand-pipe in a Chalk-pit near Lower Elmsden.
Cl.
a Thin stony band in the Thanet Sands.
b Seam of clay and sand with green-coated flints.
c Chalk.
It will be observed that this pipe, which is about 1 2 feet deep, is
filled with the Thanet Sands, underlaid by the seam of clay and
sand (6) with the angular green-coated flints which always occurs at
the base of this deposit. It is not often that these sands are solidified,
but in this case a thin band (a) is semi-indurated — ^just hard enough
to hold together in blocks when broken, but not hard enough to allow
of any wear or exposure. This layer of soft stone runs horizontally
about 2 feet above the surface of the chalk. When, however, it reaches