568 CHARACTER OF DUNE SAND. 
sand. They are sometimes found at the very summits of the 
hillocks, as at Overveen ; in the King’s Dune, near Egmond, 
they form a coarse, calcareous gravel very largely distributed 
through the sand, while the interior dunes between Haarlem 
and Warmond exhibit no trace of them. It is yet undecided 
whether the presence or absence of these fragments is deter- 
mined by the period of the formation of the dunes, or whether it 
depends on a difference in the process by which different dunes 
have been accumulated. Land shells, such as snails, for exam- 
ple, are found on the surface of the dunes in abundance, and 
iany of the shelly fragments in the interior of the hillocks may 
be derived from the same source.” * 
Sand concretions form within the dunes and especially in the 
depressions between them. These are sometimes so extensive 
and impervious as to retain a sufficient supply of water to feed 
perennial springs, and to form small permanent ponds, and 
they are a great impediment to the penetration of roots, and 
consequently to the growth of trees planted, or germinating 
from self-sown seeds, upon the dunes.t 
ted by the action of the waves and by meteoric influences—a process still going 
ou—and it is now again subsiding with the coast it rests on. 
The calcareous sand arising from the comminution of corals forms dunes 
on some of the West India Islands.—AGaAssiz, Bulletin of the Museum of Com- 
parative Zodlugy, vol. i. 
* De Bodem van Nederland, i., p. 3238. 
+Srarmne, De Bodem van Nederland, i., p. 317. See also BERGSOR, 
Reventiov’s Virksomhed, ii., p. 11. 
‘*TIn the sand-hill ponds mentioned in the text, there is vigorous growth of 
bog plants accompanied with the formation of peat, which goes on regularly 
as long as the dune sand does not drift. Butif the surface of the dunes is 
broken, the sand blows into the ponds, covers the peat, and puts an end to its 
formation. When, in the course of time, marine currents cut away the coast, 
the dunes move landwards and fill up the ponds, and thus are formed the 
remarkable strata of fossile peat called Martérv, which appears to be un- 
known to the geologists of other parts of Europe.”—ForRCHMAMMER, in 
LEONHARD und Bronn, 1841, p. 13. Martérvy has a specific gravity thrice 
as great as that of ordinary peat in consequence of the pressure of the sand. 
—ASBJORNSEN, Tory og Torvdrift, p. 26. 
