OCCURRING AT FELSTEAD AND IN OTHER PARTS OF ESSEX. 15
and was certainly eminently suited to their preservation, for it must
be recollected that the land-shells owe their present existence to sub-
mersion in water receiving and depositing mineral matter.
It will be urged that nothing can be built on negative evidence,
but that is not quite true. I have myself examined many thousands
of specimens from all localities hereabout, but have failed to come
across any freshwater forms except those mentioned. The individuals
were common enough, and one of the species appears in all the
published lists.
The only inference then that seems possible, is that no other fresh-
water shells were then in the locality.
The local extinction of species, when read in the light of a theory
of migratory-population, furnishes a conclusive argument for the
antiquity of the shell-marls. Let us suppose the case (which we
know to have really occurred), that is, the populating of England with
molluscs from the Continent. We should then reason that a record
of this migration would be best sought for in those counties in nearest
contiguity to the Continent, and we should also say that such a record
might contain some or all of the British species, irrespective of recent
forms. The Felstead shell-marl, I submit, is a fragment of such a
record, and of pretty early immigrants too, else how do we account
for the hiatus which exists in the distribution of some three forms
above adduced (see paragraph immediately following the table of past
and present distribution). Their presence in the marl is fully
accounted for under this theory, and even the present distribution of
the mollusca in Essex receives something like an explanation when
regarded as a continuation of such a record.
In conclusion, as it may be advanced that I have theorized on in-
sufficient grounds, I would beg to point out that the best refutation
or confirmation, as the case may be, lies in a more extended examina-
tion of the bed. I have done my best for my own neighbourhood,
and, at least, I think that I have made out a case proving the
necessity for more extended study.
Note on the above, by W. H. DALTON, F.G.S., late of the Geological Survey.
Conditions of deposit and even sequence of beds may recur at wide
intervals, so that other physical evidence than that of examination of
sections must be got. The Witham beds, for instance, whilst
extending to the level of the bed of the river, rise high above it, the
valley having not been eroded since their deposition to its previous