CEESCENT-SHAPED DUNES. 205 



As the Innumerable arenaceous particles are moyed by virtue of 

 rigorous laws, we can consequently measure the force of the winds, 

 by the height of the mass, and the rapidity of the displacement of 

 the hillocks. Attentive observation permits us, in the same way, to 

 corapare with each other the various atmospheric currents Vhich 

 drive the sands onward, and to indicate exactly the one whose 

 action is the most energetic. Thus, in the Peninsula of Arvcrt or 

 La Tremblade, situated between the mouth of the Gironde and that of 

 the Seudre, the chain of dunes rises gradually in a northerly direc- 

 tion, and it is at the northern extremity that the highest hillock is 

 found. This phenomenon is explained by the frequency and intensity 

 of the south-west wind which blow^s in these parts; in virtue of ''the 

 parallelogram of forces," it carries the sand further and higher than 

 the winds from the west and north-west can. 



Every isolated dune assumes clearly-defined contours, resembling 

 those of a crescent. It is easily understood why the hill must ad- 

 vance in such a manner as to project a curved point on each side of 

 its principal mass. The grains of sand which the wind causes to 

 ascend the height of the central part of the dune have to describe 

 a longer path and to slide further down the counter-slope than the 

 particles of the two lateral extremities. They proceed consequently 

 with less speed ; the ends, exceeding in rapidity the rest of the dune, 

 bend forward, in the shape of advanced horns, and give the whole of 

 the moving hill the aspect of a volcano whose crater has fallen in. That 

 which contributes still more to cause these sandy hillocks to assume 

 this semi-circular form is, that the prevailing wind does not always 

 blow perpendicularly to the mass of the dune. Its direction is often 

 oblique ; now in one direction, and now in the other. It then makes 

 the wings of the dune, the crest of which it strikes at right angles, 

 advance more rapidly. 



In the desert of Atacama, the Pampas of Tamarugal, in the plains 

 of Texas, in the Sahara of Algiers, in the Nubian deserts, and in 

 almost all the regions traversed by shifting sands, the crescent-shaped 

 dunes present such a regularity of form that all travellers have been 

 struck by it.* The Landes of Gascony also offer remarkable ex- 

 amples of this semicircular arrangement of the crest of the dunes. 

 In the environs of Arcachon and La Teste all these hillocks have the 

 appearance of f alien-in volcanoes, and are distinguished by the rich 

 vegetation of broom and bushes which fill their craters or cronJiots. 

 In those parts of the coast of the Landes w^here the crater-shaped 

 * Poeppig, Meyen, BoUaert, Gillis, Laurent, Georges Pouchet. 



