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258 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .Jan. 23,
Fig. 3.— Section at Pebble Hill.
Ochreous gravel, composed chiefly of round flint pebbles.
.London clay; blackish sandy clay, passing downwards into
brown clay; round flint pebbles and very friable shells
dispersed irregularly throughout.
. .Coarse ferruginous sand, fuU of round flint pebbles 1 to 12
inches in diameter ; some chalk pebbles ; many of the flint
pebbles decomposed throughout into a white friable struc-
ture. This bed frequently passes into an iron sandstone
conglomerate.
d. Blottled clays, chiefly of a light greenish colour, overlying an
irregular bed of sand, below which succeed other irregular
beds of mottled clays.
The chalk crops out about 6o feet below "c."
Stratum "c" here contains no organic remains, except the teeth
of the same species of Lamnce which occur at Clarendon Hill, and
which we shall find to accompany this hed very constantly in the
London district. This point forms the apex of a long and roughly
triangular area, occupied by the tertiary eocene strata, and stretching
eastward to the German Ocean. The southern side of this triangle
extends from Pebble Hill to the cliff near the Reculvers in Kent, a
distance of about 1 00 miles, and the northern side from Pebble Hill
to Woodbridge in Suffolk, nearly 140 miles. Owing to the thickness
of the London clay in the tract between these two lines, it is only
by well-sections that we can learn anything of its basement bed. If
however we follow the outcrop of the beds, we shall find this stratum
coming to the surface with much regularity along the southern edge
of the tertiary area, whilst along its northern edge it forms a more
broken and irregular line. This arises from the tertiary deposits being,
on the south from Inkpen to Croydon, tilted up at a considerable
angle against the ridge of chalk hills, which throws them out sud-
denly and sharply, whereas towards the north they rise gradually,
and form with the chalk a tolerably regularly inclined plane from their
outcrop from below the London clay to the edge of the chalk escarp-
ment, disappearing only gradually according as the chalk attains
a higher level, and adapting themselves to all the irregularities and
variations of the surface.
On this latter side, therefore, the tertiary strata often form hills
overlooking the chalk district, whilst on the south side the chalk
hills almost constantly command fine and extensive views over the
tertiary area.
In following the basement bed of the London clay eastward from
Pebble Hill, it will be convenient to take these two sides of the
triangle separately. It happens that many of the beds between the
chalk and the London clay are of considerable economical value for
their sands, and tile and pottery clays, and they are consequently
worked to a great extent. A zone of brick and tile fields in fact
marks their outcrop from Marlborough to Ewell on the one side, and
to Woodbridge on the other. We are thus furnished with a series