SAND DUNES AND PLAINS. 537 



Bliifting hillock upon the coast, the other that of barren plain in 

 the interior. The coast-dunes are composed of sand washed up 

 from the depths of the sea by the waves, and heaped in more or 

 less rounded knolls and undulating ridges by the winds. The 

 sand with which many plains are covered appears sometimes to 

 have been deposited upon them while they were yet submerged 

 beneath the sea, sometimes to have been drifted from the sea-coast, 

 and scattered over them by wind-currents, sometimes to have been 

 washed upon them by running water. In these latter cases, the 

 deposit, though in itself considerable, is comparatively narrow in 

 extent and irregular in distribution, while, in the former, it is 

 often evenly spread over a very wide surface. In all great bodies 

 of either sort, the silicious grains are the principal constituent, 

 though, when not resulting from the disintegration of siHcious 

 rock and still remaining in place, they are generally accompanied 

 with a greater or less admixture of other mineral particles, and 

 of animal and vegetable remains,* and they are, also, usually some- 

 what changed in consistence by the ever-varying conditions of 

 temperature and moisture to which they have been exposed since 

 their deposit. Unless the proportion of these latter ingredients 

 is so large as to create a considerable adhesiveness in the mass — 

 in which case it can no longer properly be called sand — it is in- 

 fertile, and, if not charged with water partially agglutinated by 

 iron, lime or other cement, or confined by alluvion resting upon 

 it, it is much incHned to drift whenever, by any chance, the 



* Organic constituents, such as comminuted shells, and silicious and cal- 

 careous exuviae of infusorial animals and plants, are sometimes found mingled 

 in considerable quantities with mineral sands. These are usually the remains 

 of aquatic vegetables or animals, but not uniformly so, for the microscopic 

 organisms, whose flinty cases enter so largely into the sand-beds of the Mark 

 of Brandenburg, are still living and prolific in the dry earth. See Wittwek, 

 PhysikaliscTie Oeographie, p. 142. 



The desert on both sides of the Nile is inhabited by a land-snail — of which 

 I have counted eighty, in estivation, on a single shrub barely a foot high — and 

 thousands of its shells are swept along and finally buried in the drifts by every 

 wind. Every handful of the sand contains fragments of them. Forchham- 

 MER, in Leonhard und Bronn's Jahrbuch, 1841, p. 8, says of the sand-hills 

 of the Danish coast : " It is not rare to find, high in the knolls, marine shells, 

 and especially those of the oyster. They are due to the oyster-eater {^Hamialopus 

 ostralegus], which carries his prey to the top of the dunes to devour it." See 

 also Staking, Be Bodem van Nederland, i., p. 321. 

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