ON A SECTION AT RAYLEIGH.
117
along the axial plane of the synclinal trough, as well as towards
its sides. It therefore follows that either the top or the base
of the formation, or conceivably both, must transgress the time
planes, i.e., vary in age from east to west. On general grounds
it seems probable that marine conditions lingered in Essex during
and after the encroachment of the Bagshot delta in the west. In
such case the top of the London Clay in Essex would be younger
than the corresponding beds in Surrey and Berkshire, and we
should be forced to conclude that the contained fauna was a
"facies fauna," reflecting the eastward passage of a wave of
shallowing.3 On the other hand, it might be urged that under
these conditions the incoming of Bracklesham forms might be
expected in the eastern localities, and that since such forms are
unknown in Essex, we must take the faunal evidence at its face
value and suppose the top of the London Clay to be contem-
poraneous throughout the area. This would imply that the
varying thickness of the London Clay reflected the original
form of the basin and that the irregularities of the latter had
been obliterated by sedimentation by the end of the London
Clay episode, so that conditions became uniform throughout the
area. It is at present impossible to choose between these alterna-
tives without begging the question. More facts are wanted,
and these can only be obtained by a close scrutiny of all new
sections, large and small, which may be opened in the London
Clay of Essex.
LIST OF FOSSILS FROM THE TOP BEDS OF THE
LONDON CLAY AT RAYLEIGH BRICKWORKS,
RAYLEIGH.
BY ARTHUR WRIGLEY.
c = common.
Pseudamusium corneum (J. Sow.).
c. Pteria media (J. Sow.).
P. papyracea (J. de C. Sow.).
Pinna affinis J. Sow.
Modiola sp. indet.
Nucula sp. indet.
c. Venericardia trinobantium Wrigley.4
3. The fact that some of the fossils of the upper sandy London Clay :first appear in the purer
clays below does not preclude us from regarding them as "facies fossils" if other evidence de-
mands it, for a fauna responds to depth of water as well as to the character of the sea bottom,
and no doubt the London Clay sea was shallowing long before the sandy regime supervened.
4. Proc. Malcolog. Soc., xvi. (1925, p. 233.)