3
All that remains of the deposit in England is a mass extend-
ing from Sizewell Bay, by Aldeburgh, to Boyton, opposite the
end of Orford Beach, with small separated patches at Ramsholt,
Sutton, and Waldringfield, on the river Deben below Wood-
bridge ; Trimley, near Felixstow ; and Tattingstone, between
Ipswich and Manningtree. We have no traces of it left in Essex,
but there is reason to suppose that it originally extended well
across the county.
The outline of the main mass suggests a bank like those off
the existing coast-line, accumulated by tidal currents, and this
suggestion is borne out by the internal structure of the deposit,
which is strongly current-bedded in places. Yet we have
evidence, in some specimens from Aldeburgh, of absolutely still
water, at least on the floor of the shallow sea. These are casts
of the elegant Voluta lamberti, the shell of which is composed of
the rather soluble form of carbonate of lime known as aragonite,
and has been dissolved away, leaving the cast of the interior
only. In the specimens referred to, whilst the dead shell was
lying at the bottom of the Crag sea, the fine calcareous mud that
entered as the body decomposed could only rise, in each whorl,
to the level of the entrance from the next preceding one,
necessarily lower than the bulge of the shell, and, owing to the
taper of the spire, lower in each whorl than in that before it.
Consequently, the cast is cut off by a series of planes,
successively nearer to the axis of the spiral as the apex is
approached. These surfaces are not absolutely planes, being
somewhat curved, as follows. Inside of a slight rise on the
margin, due to the adhesion of the fine impalpable mud to the
side of the shell, there is a ring of depression, and then an
upward bulging of the centre. This curvature points to the
presence of an elastic cushion of gas, evolved by the decomposi-
tion of the animal, acted upon by varying pressure (probably
tidal), communicated through the mud from the mouth of the
shell. But the chief point of interest lies in the proof that the
dead shell was never once rolled over by a bottom current, or the
gas would have escaped by the spiral.
A further testimony to the general tranquillity prevalent on
the sea-floor of the Coralline Crag is to be found in the numbers
of bivalve shells of Lamellibranchiata and Brachiopoda occurring
with both valves united as in life. Their interior, filled with
shell-detritus, shews that they were not buried alive, but any
disturbance of the banks would have separated the dead valves.