116 THE ESSEX NATURALIST.
Sections in the sands are visible higher up the hill behind the
brickpit, and also in two pits on the north side of Hambro' Hill.
In the higher of the two latter pits a small thickness of sandy-
gravel is seen resting with an erosive junction on the sands.
This gravel contains Tertiary flint pebbles and larger subangular
flints as well as Hythe chert, carstone, small quartz and lydite
pebbles, the last three constituents probably derived from the
Folkestone Sands. The sandy matrix is coarse and highly
ferruginous, and yields a suite of heavy minerals comparable
with that of the Kentish Greensand. From the constitution and
elevation of this gravel it is clear that we are dealing with a
fluviatile accumulation of ancient date derived from the south.
The material was apparently carried through the Medway Gap
in late Pliocene times before the incision of the present lower
Thames valley. It forms part of an extensive dissected sheet
of gravel, ranging from 200-250 ft. O.D., which can be traced
not only in the Rayleigh area, but all round the estuary of the
Thames.
THE STRATIGRAPHICAL BEARING OF THE LONDON
CLAY FAUNA.
It is proposed to offer a few remarks as to the bearing of the
London Clay fauna here recorded on some general questions of
Eocene geology. It must be remembered that the list is certainly
incomplete ; further collecting should add considerably to it.
Nevertheless Mr. Wrigley's study of the available material lends.
a measure of confirmation to the idea entertained for some time
past by the present writers, that the fauna of the topmost beds
of the London Clay is essentially constant throughout the
London Basin. It is true that the forms Paracyathus brevis and
Lamna obliqua, which appear in the Rayleigh list, are recorded
from the London Clay of Sheppey, while rare or absent further
west. However, the majority of the forms listed below are
known to occur west and south-west of London. It is possible
indeed that there is some change in the relative abundance of
species, as one proceeds eastward, but this is at present difficult
to verify ; collecting chances form too large a factor in the
problem. If then the fauna is to be regarded as essentially
uniform over this wide area, what conclusion does the fact
justify ?
It must be remembered that the London Clay thins westward