Reaction and Succession in Relation to the Plant Covering. 397 
that the carbonate content, though below the average for the smallest dunes, 
is higher in general than that of the main dune ridge. Incidentally these 
data show that any accretion which may occur on the older dune phases, 
i. e. those farthest from the sea, will be comparatively rich rather than 
deficient in calcium. So that any deficiency of carbonates in the older 
phases must be the outcome of leaching and not of selective wind 
action. 
In view of the foregoing considerations it is not surprising to find 
considerable variation in the carbonate content as between the surface 
and subsurface of these embryo dunes. In two instances (cf. samples on, 
012 and 030, 021) the top four inches showed a lower content, whilst in 
a third locality the reverse condition obtained (cf. 013, 016). Judging by 
the p. H. values this is true also for the area represented by 017 and 018. 
Passing to the main ridge, the average carbonate content is slightly 
lower than that in the youngest phases, viz. 0*341 per cent. This repre- 
sents about 4*9 grm. in a cubic decimetre, or a decrease of about 20 per cent, 
as compared with the ‘ embryo dunes ’. What period of leaching this 
represents is not accurately known, but probably not less than 60-80 years. 
The range noted varies between 0*15 per cent, and 0-65 percent., the 
lower limit being thus considerably less, about half that of the youngest 
dunes, whilst the maximum is slightly higher. 
Since these main ridge dunes are the highest of the whole series, they 
will only receive comparatively large fragments of shell during high winds. 
From the data already given, however (cf. Table III), it is clear that such 
accretion will consist of sand comparatively rich in calcium, and the 
occasional high values on the seaward face (E24) and in the hollow 
represented by E 9 are perhaps the outcome of selective wind action on 
shell fragments of relatively large surface. 
Comparison of the hydrogen-ion values for surface and subsurface for 
five locations show a higher value for the subsurface in three cases (E 14, 
E 15 ; E 18, E 19 ; E 20, E 21). In one case the same reaction was found (E 1, 
E 2), whilst in the fifth case (E 8, E 9) the surface was practically neutral and 
the subsurface slightly aci’d. Carbonates were only determined in one of 
these pairs, and this showed a higher content at the lower level. On the 
whole this then would appear to be the general tendency, and would 
indicate a preponderance of leaching action over accretion. 
All the remaining series of samples represent dunes which for the most 
part have ceased to accrete any appreciable amount of fresh sand, though 
‘blow-outs’, rabbit-burrows, &c., may effect a partial, if local, rejuve- 
nescence. 
The Laboratory ridge shows an average carbonate content of 0-155 P er 
cent., and a range from 0*64 per cent, to 0*03 per cent. The average is 
equivalent to about 2 grm. per cubic decimetre. 
F f 
