FORM OF DUNES. 561 
the sands will soon be carried out of the reach of the highest 
waves, and transported continually farther and farther into the 
interior of the land, unless obstructed by high grounds, vegeta- 
tion, or other obstacles. 
The laws which govern the formation of dunes are substan- 
tially these. We have seen that, under certain conditions, sand 
is accumulated above high-water mark on low sea and lake 
shores. So long as the sand is kept wet by the spray or by 
capillary attraction, it is not disturbed by air-currents, but as 
soon as the waves retire sufficiently to allow it to dry, it becomes 
the sport of the wind, and is driven up the gently sloping 
beach until it is arrested by stones, vegetables, or other obstruc- 
tions, and thus an accumulation is formed which constitutes the 
foundation of a dune. However slight the elevation thus cre- 
ated, it serves to stop or retard the progress of the sand-grains 
which are driven against its shoreward face, and to protect 
from the further influence of the wind the particles which are 
borne beyond it, or rolled over its crest, and fall down behind 
it. Jf the shore above the beach line were perfectly level and 
straight, the grass or bushes upon it of equal height, the sand 
thrown up by the waves uniform in size and weight of particles 
as well as in distribution, and if the action of the wind were 
steady and regular, a continnous bank would be formed, every- 
where alike in height and cross section. But no such constant: 
conditions anywhere exist. The banks are curved, broken, un- 
equal in elevation ; they are sometimes bare, sometimes clothed 
with vegetables of different structure and dimensions ; the sand 
thrown up is variable in quantity and character; and the winds 
are shifting, gusty, vortical, and often blowing in very narrow 
currents. Irom all these causes, instead of uniform hills, there 
rise irregular rows of sand-heaps, and these, as would naturally 
be expected, are of a pyramidal, or rather conical shape, and 
connected at bottom by more or less continuous ridges of the 
same material. 
Elisée Reclus, in describing the coast dunes of Gascony, ob- 
serves that when, as sometimes happens, the sands are not heaped 
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