WANSTEAD AND WALTHAMSTOW, ESSEX. 119
and near South Ockenden, and in gravels south of the Thames at
Tilehurst near Reading, on the hills above Bisham and Cookham,
and on Dartford Heath.
The second is a fragment of a large pebble which was found by
Mr. James Baker during the excursion of the Geologists' Association
to Walthamstow on May 7th, 1892. It was in the gravel of the cut-
ting on the railway at Stoneydown. On being broken up it was
found to be composed of a very hard brownish-red quartzite, with
scattered particles of a glistening mineral, probably mica, and to con-
tain a number of casts and impressions of a small Brachiopod shell,
Orthis budleighensis (Dav.).6 This shell is of Lower Silurian age. It
is said to fill whole beds at May and other places in Normandy and
to occur in Brittany. It has also been recorded from the quartzites
near Gorran Haven, in Cornwall. It has been found in pebbles in
the Triassic pebble bed of Budleigh Salterton on the south coast of
Devonshire. The origin of these pebbles is doubtful, and the ques-
tion has given rise to great discussion, but the best opinion seems to
be that they were to a large extent derived from Cambrian and
Silurian rocks which were destroyed during the formation of the
English Channel.
Pebbles containing this shell have been found in the Bunter
Pebble Beds of Staffordshire, and are recorded from the Drift near
Birmingham, Warwick, Leicester, and Nottingham. Professor Bonney
showed me one which he found in Staffordshire—a whitish quartzite,
full of casts of the shell in question. The place from which these
pebbles were derived is uncertain, but there can, I think, be little
doubt that the Walthamstow pebble was derived from a northern
source.7
Besides these pebbles of pink quartzite and brownish-red quartzite
with fossils, which very probably are derived from the Bunter con-
glomerate, many of the other quartzite and quartz pebbles found
in the Thames Gravels have probably come from the same source.
As to the sandstone pebbles, it was suggested by Mr. J. W. Davis
that they may very likely be Coal Measure sandstone.
The Lower Greensand chert, I believe, came originally from the
south. The proportion varies a good deal in the different gravels.
6 See the report of the excursion by J. W. Gregory, F.G.S., Proc. Geol. Assoc, 1892, vol. xii.,
p. 338; and also Essex Nat., vol. vi., p. 97.
7 My authorities for the above are Davidson's "Brit. Foss. Brachiopoda," vol. iv., p. 317 ;
Bonney, "Geol. Mag.," 1880, vol. vii., p. 404 ; H. B. Woodward, "Geol. of Eng. and Wales,"
2nd edit., 1887, pp. 75, 225, etc.