WITH A LIST OF THE MOLLUSCA OCCURRING THEREIN.
9
the species of Vertigo may occur in the neighbourhood. I am sur-
prised at the abundance of Vertigo angustior and Helix arbustorum ;
while, on the other hand, the scarcity of H. caperata and Clausilia
rugosa is also noteworthy. It is not likely that more than a very few,
if any, of the species ever lived on the spot in their present abundance.
The following species, among others, live commonly in the district at
the present time, but I have not detected a trace of them in the
deposit:— Vitrina pellucida, Helix aspersa, H. cantiana, H. ericetorum,
and Bulimus obscurus, I have not as yet detected any species which
do not now inhabit Britain.
Mr. Clark asks, "Can we, from any of the species of shells, learn
anything as to the temperature of the times in which they lived ?"
and, without wishing to say that we can, I may remark that Helix
arbustorum var. alpestris is a form which frequents elevations of 6,000
or 7,000 feet; whilst H. fulva and H. lamellata, both of which
occur, are now species inhabiting the north, rather than the south, of
England; and several other species, which may be regarded as more
truly southern types, such as Helix aspersa, H. cantiana, H. cartu-
siana, H. pomatia, and H. rufescens, are either absent or extremely
rare. Further than this I do not think we can say anything. It
would be rash to attempt to assign any precise age to these beds,
and the shells which they contain, but there can be no doubt that
they are considerably more recent than those at Copford.
Remarks on the above Paper, by W. H. DALTON, F.G.S, late of the Geological Survey.
The period which has elapsed since the recession of Arctic condi-
tions from the south of England is much longer than is generally
supposed. Judged indeed by the criterion of thickness of deposits,
it is insignificant; but it is a closely-written chapter of the geological
record, and records more events than some of the vastly larger
volumes of the older rocks.
It is, therefore, the duty of the Post-Glacial geologist to determine,
where possible, not merely the relative ages of the isolated deposits,
but the sequence of events and their relative periods, whether repre-
sented or not.
The Copford beds, for instance, represent a lake of earlier date
than the formation of the valley of the Roman River, now flowing
across its site some twenty feet lower. The Clacton deposit shows
that (1) the land stood at least ten feet above its present level, then
(2) sunk fully thirty feet, rising again (3) about twenty feet.