86
THE LOCAL EXTINCTION OF MOLLUSCS.
of salt in the field is a very different matter, since during dry
weather the evaporation of water from the surface causes an
upward movement of salt. Consequently, a much heavier rain-
fall than 13/4in. would be required to effect the desired result.
But there is no better method than thorough drainage and trust-
ing to the rainfall, and results indicate that in about two years'
time the salt may be reduced to harmless proportions, if proper
advantage is taken of the natural remedy. For the eighteen
months from December, 1897, to May, 1899, the total rainfall
was 263/4in., and a second analysis of the soil showed that during
this period the salt had been reduced in some cases to one-tenth,
and on the average to one-quarter of the amount present in the
beginning of 1898.
After showing the impoverishing effect which the salt has
had upon the soil, attention is directed to the consideration of the
crops best adapted for flooded land. Wheat, barley, oats, and
mangolds have all given a fair yield in some cases, and in others
have completely failed. Ryegrass, of which much was expected,
has proved disappointing, in some cases even failing to germi-
nate. The only class of crop or weed that was uniformly pro-
ductive was the Cruciferous order, kale, rape, mustard, turnips,
and the hated charlock, all prospering where other crops failed.
Very interesting details are given throughout the paper,
derived from chemical and cultural experiments, and in their
extended form, Messrs. Dymond and Hughes' researches are of
considerable importance from a scientific as well as an agricul-
tural point of view.
ON THE LOCAL EXTINCTION AND DIFFUSION
OF MOLLUSCS IN ESSEX.
By J. FRENCH.
[Read December 12th, 1896.]
THE object of these remarks is to show that local observation
in our districts may be rewarded by obtaining cases of
diminution and increase of species of Mollusca in quite recent
times, and to show how small are the factors that effect these
changes.1
1 [Mr. French began his paper by some general observations of a more or less theoretical
character, on the geological history of the Mollusca, and the agencies which may have caused
their variation and distribution. These observations are omitted as being hardly suitable for
our pages. The Editor has to thank Mr. T. V. Holmes, F.G.S., and Mr. W. Whitaker, F.R.S.,
for very kind advice and assistance in preparing the paper for the press A few notes by Mr.
Holmes are indicated by the initials T. V. H.—Ed.]