Reaction and Succession in Relation to the Plant Covering. 399 
of the differences observed we must take into consideration the water- 
content. Data respecting the natural water-content of the different dune 
phases over a sufficient period are not available, but assuming that these 
would be more or less proportional to the maximum water-contents ob- 
served under laboratory conditions, it will be realized from a perusal of 
Table IV that the differences in concentration must be very pronounced. 
Table IV. 
Summary of the Carbonate Content of the various Dune Types. 
Dune type. 
Av. carbonate 
content by 
weight. 
Av. carbonate 
content per 
cubic decimetre. 
Av. concentration 
of carbonates 
at maximum 
water-content. 
% 
grm. 
% 
‘ Yettow ’ j 
1 Embryo dunes J and G 
0.425 
6*077 
1.697 
dunes i 
[ Main ridge E 
°- 34 r 
4.8763 
1-343 
‘Grey’ ! 
dunes 1 
[ Lab. ridge D 
o-i 55 
2.139 
o-66o 
Long Hills B 
0*010 
o-i 344 
0.034 
[ Hood A 
0*010 
0-1281 
0-027 
The figures in the third column represent the average concentrations 
if all the carbonates present were in solution at once, so that they have little 
significance except in their relation to one another. 
If we take the embryo dunes as unity then the relative accessibility of 
carbonates, on the assumption that the solubility in carbonated water is 
proportional to that in dilute HC 1 , is approximately as follows: embryo 
dunes, 1 ; main ridge, o*8 ; Laboratory ridge, 0*33 ; Long Hills, 0-02 ; 
Hood, 0-016. As the amount of carbonate present is the limiting factor 
for the amountdissolved, the values realized in nature would show an even 
steeper gradient than these figures indicate. 
The organic content of the dune soils (cf. Table I, Appendix, and 
Table V). 
Embryo dunes. The outstanding feature of the early phases, apart from 
their mobility, is the low proportion of organic material, probably almost 
entirely derived from drift. 
In the fourteen localities for which the loss on ignition of ‘embryo’ 
dunes was determined, the range was from 0-16 per cent, to 0*52 per cent., 
and it is significant that the highest value was obtained from a very young 
dune near the drift line. The average value is 0-360 per cent., or approxi- 
mately 5 grm. per cubic decimetre. 
The main ridge. Here, associated with the denser vegetation and 
greater lapse of time, the organic content ranges from 0*24 per cent, to 0-69 
per cent., whilst the average value (12 loci) was found to be 0-501 per cent., 
or approximately 7-1 grm. per cubic decimetre. 
The ridges C and D. The vegetation here has become an almost con- 
F f 2 
