16 ON THE MOLLUSCA OF THE SHELL-MARL.
depth. They are much more modern than the Copford and Clacton
deposits, and yet far older than the Felstead alluvium (which term I
use chronologically to imply lowest terrace). The beds described by
Mr. French are very interesting from the variation they indicate in
molluscan fauna from that now found in Essex ; but I do not think
they demonstrate any general change of conditions. Springs with
calcareous water, such as are frequent in the Boulder-clay district,
whether in the bottoms of valleys, or (as two sites mentioned in the
paper) well up on the slopes or plateaux, will always, unless artificially
led off, tend to develop bogs, and form peat and tufa side by side.
Land-shells and slugs frequent such spots for their moisture and
supply of calcareous matter, and get drowned if excess of water comes
suddenly; and their shells cannot be destroyed in water surcharged
with bicarbonate of lime.
In referring to migration of land mollusca from the Continent, it
must be borne in mind that the North Sea was probably non-existent,
and land-shells can easily cross a river, even of brackish water, not of
course voluntarily, but carried down by winter floods and stranded
on the opposite shore (otherwise how comes it that some twenty or
thirty purely terrestrial species occur on islands like Foulness ?).
We must also remember that so far from the deposits under consi-
deration ranging back to the submergence that isolated Britain, they
are much posterior to at least three other sets of beds probably
subsequent to that event.
The Copford is older than the Clacton, this than the Witham and
Lexden beds, and these than the Felstead and Cann valley alluvia.
The changes in the molluscan fauna are probably correlated with
some (that we cannot trace for want of evidence) in the flora, and
both are dependent on some climatal variations.
No deepening of valleys will extinguish or introduce species, though
it may alter the hygrometric state of considerable areas (as measured
in yards). But the same conditions will always obtain somewhere in
each valley, and changes made by gradual erosion merely mean
migration of exceeding slowness on the part of the evicted molluscs.
Of course, faunal change means lapse of time, and here lies the
value of such facts as Mr. Christy and Mr. French have so assi-
duously collected. They demonstrate the slowness with which
erosion is going on, by the presence of many locally-extinct species
in what is correctly called modern alluvium, though not modern in
years or allutum in origin, for it grew on the spot a very long time ago.