IV.
weathering at the surface to a rich chocolate brown.
In this neighbourhood it is especially noted for the
clayey limestone or cement-stone, which sometimes
occurs in the form of continuous layers and sometimes
as isolated nodular masses or Septaria, It also con-
tains a great many small nodules of iron-pyrites, on
decomposition causing an efflorescence' of persulphate
of iron, and ultimately mouldering away to an offen-
sive mass of sulphur, oxide of iron and earthy matter,
to the detriment of the cabinet in which such speci-
mens are deposited. The mineral is still collected on
the beach at Walton and Frinton for the manufacture
of copperas to the amount of about 150 tons annually.
The cement-stone was formerly worked on the coast
for the manufacture of Roman cement, and was taken
from the foreshore, or dredged from the sea-bed to such
an extent as greatly to aid the sea in its encroach-
ment on the land.
The following analysis of a Septarian nodule from
Walton was made by Mr. E. Phillips :*—
Carbonate of lime ............................ 68.5
Silica........................................ 14.4
Alumina................................... 3.2
Oxide of iron, with a little manganese ........ 7.4
Water........................................ 4.0
Organic matter and loss...................... 2.5
100.0
The London Clay is shown by its fossils to have
been of marine origin, though the frequency of plant-
remains in it in many places points to the nearness of
land. In this district the plants are usually fossilised
by iron-pyrites, but sometimes they occur as lignite.
When the cement-stone was worked many remains of
turtles were found in it, and indeed it is for these
reptiles that the London Clay of Harwich was par-
ticularly noted. The number of species found in this
district is, however very small, the only safely recorded
species (from Harwich) being Chelone crassicostatus
and C. planimentum. With these occurred the Mam-
mals, Coryphodon Eocaenus and Pliolophus vulpiceps.
Fine specimens of Nautili, the interior filled with
cement stone, are occasionally found in place in ex-
cavations, or dredged up off the coast.
All the upper part of the London Clay has been re-
moved by denudation, the greatest thickness known
being about 120 feet, instead of 450 feet where the
also has been removed, for the London Clay is here
whole of the formation occurs, at Sheppey, on the
south. If any part of the next overlying series,
the Bagshot Beds, were deposited in this district, it
* Lond. Geol. Journ. 1846, p. 18. For analysis of Harwich
cement-stone, see Geological Survey Memoirs, vol. iv., p. 278.