PELOPHILOUS FORMATION OF LEFT BANK OF SEVERN ESTUARY. 23 
and consequent removal of the salt by rainfall. All three zones 
are liable to high salt percentages during times of drought and 
high tides, but the Salicornia zone has practically no relief from 
these conditions, while the higher Festuca zone is often free from 
them, and the Sclerochloa zone at its margin may become so free 
from salt by drainage that its conditions and therefore its vegeta- 
tion become equivalent to that of the normal Festuca zone. The 
relation between salt content and drainage is shown clearly in 
some of the series taken after rain ; it for instance can be shown 
diagrammatically, as is done in Figure II., for the series taken near 
Portishead on December nth, 1909. 
A B 
Oatline Section t - 
of Zones J III. 
(not to scale) \ 
25 
Text Figure II. 
If the two series of points plotted in this diagram are compared, 
it is clear that high water content, that is to say defective 
drainage usually accompanies high salinity. 
The only marked exception in this series is the chloride per- 
centage for II. C, which is comparatively unusually high. Perhaps 
this is to some extent due to the fact that the fall from II. zone 
to I. zone is not very great at this part of the coast, being only 
some six inches to a foot. 
Except for one reading taken during the dry weather of the 
Spring of 1910, the few readings from zone IV. show low chlorine 
content coupled with high water content. The explanation is 
certainly, I think, the much greater amount of humus present in 
the soil, owing probably to grazing and cultivation without the 
scouring effect of any but the most occasional tide to remove 
the manure and decaying matter on the surface of the ground. 
This humus so improves the texture of the surface layers of the 
ground that the salt rapidly draws away after the occasional 
