258 RHAXELLA-CHERT IN EPPING FOREST. GRAVELS.
O.D. (the highest terrace of the Thames Valley Drift), I found a
typical pebble of my Monk Wood rock, containing casts of an
Ammonite (probably a juvenile specimen of Cardioceras corda-
tus ?) and of a bivalve shell (? Lucina) ; and Mr. A. L. Leach,
F.G.S., who conducted the party, was able at once to identify
the specimen as being Rhaxella-Chert, derived from Jurassic
rocks of Corallian age.
This chert occurs in situ over a limited area at Arngrove,
near Brill, in Buckinghamshire ; and has also been described
from the neighbourhood of Scarborough, Yorks. In both
localities, the geological horizon is the lower portion of the Coral-
Rag, i.e., the Lower Calcareous Grit, immediately above the
Oxford Clay. The minute coral-like structure is due to myriads
of globate spicules of a tetractinellid siliceous sponge to which
Dr. G. J. Hinde gave the name of Rhaxella perforata ; alike in
the parent rock and in my Valley Drift pebbles many of the
spicules have disappeared by solution, leaving minute spherical
cavities surrounded by a delicate meshwork of the cherty rock.
A full account of the rock and its mode of occurrence in Bucks
is given by Dr. A. M. Davies, F.G.S. (in Quarterly Journal of
the Geological Society, 1907), who describes it as a thinly bedded
stone broken up in situ into roughly rectangular blocks, some-
times as much as four inches square and 11/2 inches thick, but
often smaller : and Mr. E. T. Newton, F.R.S., contributed a
Note on the few Drift specimens of this chert in Proc. Geologists'
Association, xx., 1907, p. 127.
Dr. A. E. Salter found fossiliferous pebbles of Rhaxella-chert
in the Cromer Drift, and Dr. A. Irving writes me that he has it
rather frequently in the Upper Rubble Drift of the Stort Valley
at Hockerill, near Bishops Stortford, at about 200 feet above
O.D.
I have myself met with this Chert, in addition to the places
already mentioned, at Coopersale Common, one mile east of
Epping, in high level gravel (at about 340 feet above O.D.)
included by Prestwich in his "Westleton Beds," but which is
not improbably, in my opinion, a continuation of the highest
terrace of the Roding Valley Gravel referred to above. Recently
(September to November 1913), a small temporary gravel-pit
on the Uplands Estate at Loughton, opened for road-ballast,
together with the adjoining Loughton Cemetery, has yielded