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of the ground-water in the dunes. This not only is a consequence 
of this, that water enclosed in sand must rise three times as much 
as in an open basin, the supply being equal; but also of that 
circumstance, that the sand in question is particularly loose and 
composed of grains with equal dimensions, thus readily absorbing 
and giving way to the downfalling water. Further favourable conditions 
are that the ground is very uneven, not admitting of superficial 
flowing off, and only thinly covered with mosses, grasses, shrubs of 
Hippophaé rhamnoides, Salie repens and Ligustrum vulgare and 
with loose-crowned trees, especially birches. Really Mr. pr Bruyn 
found that, even during the dry years 1895 till 1902, at least half 
the rainfall served as a supply to the ground-water 5. Moreover 
the geological constitution of the dune region, where eolian sand 
reposes upon the very little permeable fine clayey marine sands 
(Old Sea-clay and Sea-sand of Staring), favours very much the accu- 
mulation of the excess of rainfall. 
The circumstance that so many low plains amidst the dunes, 
having subsisted during centuries, have undergone simultaneously 
quite the same up and down variations in the height of the ground- 
water, proves, as it seems to me, that we should not, generally, 
impute the becoming dry of the dunes to a successively filling up of 
those low places with eolian sand. 
Another proof of no less strength I find in the phenomenon, already 
observed and rightly explained by Gevers, that, on the whole, the 
surface line of the plains in the dunes runs parallel to the line of 
the ground-water, descending toward the sea and toward the polderland. 
It is indeed unconceivable that those remarkably flat and pretty 
well horizontal, often very extensive low grounds amidst the dunes, 
commonly called in North-Holland “vlakken” and “velden”, have 
had another origin than the sand being blown off — before the time 
that such blowing off was prevented by the planting of sand-binding 
grasses — till the level was reached where it was moistened by the 
ground-water, raised by capillarity to about thirty centimeters above 
its free level. Really we observe, as far as natural influences prepon- 
derate, that generally only where the character of the underground 
changes, making the water sink down accumulate in such places, in 
other places, these modifications in the geological structure modify 
the line of the ground-water, but at the same time, in consequence 
thereof, that of the surface of the dune plains. 
1) Handelingen van het 9de Natuur- en Geneeskundig Congres, ‘s-Gravenhage, 
1903, p. 148. 
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