PELOPHILOUS FORMATION OF LEFT BANK OF SEVERN ESTUARY. 19 
In this diagram the zones are numbered and three points dis- 
tinguished upon each, which require some little discussion. 
The point A on any zone will always tend to be a little below 
the normal level of the zone, because of the tidal scour caused 
by the fall of water from the zone above as the tide recedes. 
This means that the water will tend to stand about here as the 
tide falls, while from its position the drainage in the case of a 
wide zone will be slow. A, therefore, will always represent a 
point of bad drainage, where the salt content is likely to be high. 
B. should be a place of poor drainage in a level and wide zone, 
and should represent what might be called the typical drainage 
and salt condition for that particular association. 
C, on the other hand, should always be well drained, more 
porous and much freer from salt. 
These conditions might explain the occasional appearance of a 
relatively lower zone at the point A on one zone, and of a 
relatively higher zone at the point C (see p. 17). 
It seemed advisable to see whether these theoretical conditions 
were borne out in practice, and for this purpose a number of 
soil samples were taken and investigated. The soil is particularly 
suitable for this purpose owing to its uniform structure. Being 
a soft clay, suitable samples could be obtained by means of a 
geotome, 1 of diameter 1 \ inch, the samples in a series being always 
taken to the same depth, pressed out from the tube direct into 
a well corked bottle and then taken back to the laboratory for 
investigation. Some idea as to the drainage capacities of the soil 
can be obtained from the water content, taken in conjunction 
with previous rainfall, so that the water content was determined 
with most of the samples. 
The total chlorine present was estimated in most series by a 
gravimetric method ; in one or two of the later series, titration 
with standardized solutions of silver nitrate was resorted to, to save 
time. The gravimetric method used was a modification of Wolffs' 
method, 2 the dry soil being treated with excess nitric acid. A fairly 
strong solution of nitric acid was needed for some soils as the 
carbonate present was in many cases considerable. In one series 
of soils the CO ;5 present varied from about 4*5 % to 0/3 % of the 
dry soil. The same strength of nitric acid was always used in any 
series of determination, the dissolved chloride being filtered off, 
precipitated with excess silver nitrate, and the precipitate collected 
on asbestos in a Gooch crucible dried at red heat and weighed 
as silver chloride. 
1 Clements, loc. cit. p. 25. 
2 Wiley, loc. cit. p. 474. 
