RHAXELLA-CHERT IN EPPING FOREST GRAVELS. 257
little (if any) that can be labelled as from the Lower Greensand.
Eocene rocks are represented by small sarsens, by siliceous
sandstones containing flint-splinters, and by rounded flint
pebbles derived from Lower Tertiary beds. A small decayed
Belemnite (possibly from the Chalk), and some fragments of
shells of Inoceramus (certainly from the Chalk), are practically
the only derived fossils noted, other than the Crinoids and
sponge-spicules preserved in some of the chert fragments ; and
of contemporaneous fossils, or of unquestioned human imple-
ments, no trace has been found. The workmen state that
neither bones nor shells have ever been seen by them in this
gravel.
The above assemblage of rock-fragments points to deriva-
tion approximately from the north-west, but as this Monk
Wood Gravel is certainly Pre-Chalky Boulder Clay (which by
no means implies Pre-Glacial) the agency by which these far-
travelled rocks reached the gravel, in common with some high-
level gravels of the Thames Valley (as, for instance, that at
Dartford Heath, in Kent, where a very similar set of rock-
constituents is met with), remains open to debate.
No signs of contortions or "trail" have been observed in
the Monk Wood gravel, and the overlying brick-earth rests with
perfect conformity upon it, but, in September 1907, I found
one waterworn piece of a crystalline felspathic rock which
showed faint but certain striae and polish.
One remarkable rock which occurred not infrequently in the
Monk Wood gravel interested me greatly, both from its beauty
when viewed with a pocket-lens, and also because one or two-
fragments contained casts of fossil shells. It occurred usually
as angular or cuboidal pieces, the angles rounded, but otherwise
little rolled, up to five inches by 31/2 inches by 31/2 inches, but
sometimes as fully rounded pebbles. Fractured surfaces show
an uniform buff colour, but weathered exterior faces may be
bleached white, or stained deeper yellow by iron. When
examined with a lens, a regular minutely vesicular structure
is apparent, resembling delicate lacework.
Not until the present year (1913) was the identity of this
interesting rock made known to me. Last spring I attended
an excursion of the Geologists' Association to Dartford Heath
in North Kent, and there, in high-level gravel at 130 feet above