646 CHARACTER OF DUNE SAND. 



sand. They axe sometimes fomid at tlie very summits of the 

 hillocks, as at Overveen ; in the King's Dune, near Egmond, 

 they form a coarse, calcareous gravel very largely distributed 

 throusrh 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 determined 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 ac- 

 cumulated. Land shells, such as snails, for example, are found 

 on the surface of the dunes in abundance, and many 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 conse- 

 quently to the growth of trees planted, or germinating from seK- 

 sown seeds, upon the dunes.f 



Egyptian coast arise from the disintegration of the calcareous sandstone of 

 the same region. This sandstone, composed in a large proportion of detritus 

 of both land and sea shells mingled with quartz sand, appears to have been 

 consolidated under water during an ancient period of subsidence. A later 

 upheaval brought it to or near the surface, when it was more or less disin- 

 tegrated by the action of the waves and by meteoric influences — a process 

 still going on — 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. — Agassiz, Bulletin of the Museum of Com- 

 parartive Zoology, vol. i. 



* De Bodem van Nederland, i., p. 323. 



f Staring, De Bodem van Nederland, i., p. 317. See also BergsOe, BH' 

 ventlov's Virksomhed, ii., p. 11. 



" In the sand-hill ponds mentioned in the text, there is a vigorous growth 

 of bog plants accompanied with the formation of peat, which goes on regu- 

 larly as long as the dune sand does not drift. But if 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 fossil peat called MartOrv, which appears to be un- 

 known to the geologists of other parts of Europe," — Forchhammer, in Leon- 

 hard imd Bronn, 1841, p. 13, MartOrv has a specific gravity thrice as great 

 as that of ordinary peat in consequence of the pressure of the sand. — 

 AsbjObnsen, Torv og Torcdrift, p. 26. 



