Reaction and Succession in Relation to the Plant Covering . 395 
favour accretion of sand, so that, with the exception of the depression in 
its centre, the soil is more mobile than might be expected, whilst the 
character of some of the samples is scarcely commensurate with 
its age. 
In some dune areas all transitions up to and including heath are met 
with, though not present on Blakeney Point. Samples from the neighbour- 
ing system of Holkham indicate, however, that the edaphic sequence which 
accompanies increase of age on our area are still further accentuated in the 
final phrases of the succession. 
The carbonate content of the dune soils (cf. Table I, Appendix). 
Examination of the drift line reveals at once the large proportion of shell 
fragments which frequently accompany the vegetable remains. After 
removal of the loose drift, the underlying sand is found to contain a high 
proportion of carbonates, sometimes over four per cent, by weight (cf. 
Sample X. 37, Table XI). The drift line is the especial home of Salsola kali 
and Cakile maritima , and the following estimations show (Table II) that the 
sand is here well supplied both with organic material and carbonates. 
Table II. Drift Line. 
Sample. 
Location . 
Transect 
across 
D. line 
a 
a' 
b 
d 
e 
New spit with S. kali 
% • 
Just above actual D. line 
Just below „ 
Sea front drift line with 
Cakile maritima 
Sea front drift line 
Depth. 
p.H. 
in. 
0-2 
7.2 
4-8 
7.2 
0-4 
0-4 
7.2 
0-4 
7*2-7*3 
0-4 
7*° 
Organic 
Total 
content. 
carbonates. 
% 
0.38 
% 
°*37 
0.44 
0.65 
o-55 
°’95 
o-6o 
1.06 
o-57 
o-93 
For the carbonate total, both here and on the dune soils generally, 
calcium carbonate is almost entirely responsible, and the figures given for 
the carbonate content have been calculated as CaC 0 3 . 
Various marine mollusca are the main source of supply on the sea face, 
whilst on the landward side Paludestrina stagnalis is probably the most 
important. This species makes up in numbers what it lacks in size, and the 
shells of dead individuals often accumulate along the drift line near the 
Hood to a depth of as much as nine inches and in a zone more than a foot 
broad. Since 900 shells of P. stagnalis occupy only 5 cubic centimetres, 
a square foot of such a drift line would represent some two million shells , 
which gives one a glimpse of the stupendous prolificacy of these molluscs 
and the vast numbers involved in their mortality. The shells as found in 
the drift contain over 97 per cent, of calcium carbonate, and hence constitute 
a most important source of supply. 
The embryo dunes . Passing landwards from the drift line one comes 
to the small dunes which have arisen on the sea face and near the Life- 
