200 THE OCEAN. 



the grains of sand soon become dry, and cease to adhere to each 

 other, and thus allow themselves to be carried towards the land by 

 the wind from the open sea. These are the materials of dunes. 

 If the shore rises towards the interior of the continent in a per- 

 fectly even manner, this sand, cast up by the waves above the 

 sea level, and carried far by successive gusts of wind, would ex- 

 tend over the ground in layers of uniform thickness ; but the 

 inequalities of the surface prevent this. Pebbles, branches and 

 trunks of trees covered with shells, plants and bushes with tough 

 roots, project above the beach, and oppose the advance of the 

 wind, which glides over the ground, carrying the grains of sand 

 that have remained on dry land. These slight obstacles suffice to 

 determine the origin of dunes by obliging the breeze to let fall the 

 little cloud of arenaceous or calcareous dust with which it is charged. 

 The horizontality of the shore is thus interrupted; rows of sandy 

 knolls, which are subsequently to rise to real hills, commence to be 

 traced upon the ground. 



When the wind from the open sea blows with sufficient force, we 

 can not only witness the growth of the dunes, but we can also aid in 

 their formation, and verify by direct experiment the assertions of 

 theory. If we deposit some object on the ground, or, better still, 

 thrust a row of stakes into the sand, perpendicularly to the direction 



Fig. 89.— Formation of a Dune. 



of the wind, the current of air which strikes against the obstacle will 

 instantly rebound to form an eddy or whirlpool, the diameter of which 

 is always proportioned to the height of the stake. Arrested by this 

 eddy, the grains of sand carried by the wind are gradually deposited 

 on the near side of the barrier, till the summit of the miniatiire 

 dune is on a level with the imaginary line leading from the shore to 

 the upper end of the obstacle. Then the sand driven by the breeze 

 from the sea, which ascends the inclined plane presented by the 

 front of the hillock, no longer allows itself to be carried in the eddy 

 and brought back. It crosses the little ravine which the gyration of 

 the air has produced in front of the palisade, and falls beyond it to 

 accumulate gradually on the other side of the obstacle, taking the 



