202 THE OCEAN. 



the descending side under successive beds ; tlien when this part is 

 entirely hidden, the front begins to be buried in its turn. The wind, 

 instead of developing itself according to a horizontal plane, as on the 

 surface of the ocean, is obliged to take an oblique direction to ascend 

 the slope of the dune. As soon as it is sufficiently elevated, the at- 

 mospheric current passes freely above the obstacle which arrested it 

 before, the little eddy which revolves in front ceases its gyrations, 

 and nothing then hinders the sand from gradually filling up the 

 ravine which the aerial current had maintained in front of the bar- 

 rier. Soon the summit of the dune coincides with that of the 

 obstacle : the latter disappears completely, and the hillock, growing 

 like a wave which approaches the shore, and constantly raising its 

 crest higher, which is incessantly displaced, continues to encroach 

 upon the land. The various strata of sand which the wind from 

 the open sea successively brings to the summits of the dunes, spread 

 in large sheets over tlie descending talus, and glide down to the 

 base. In the Landes of the Gironne the western slope of the dunes, 

 whose base is not worn away by the sea, is, on an average, from 7 to 

 12 degrees. The eastern slope, which is that of the descending 

 talus, is from 29 to 32 degrees ; that is to say, three times as great. 

 It would be 45 degrees if the rains did not make ravines in the 

 talus and thus prolong the inclination.* 



Thus the dunes incessantly gain, ow4ng to the new layers of sand 

 added to their changing talus. But the action of the prevailing 

 wind does not limit itself to increasing them; it ends by dis- 

 placing them entirely, and making them, so to say, travel over the 

 ground. The object at the base of which the eddy of air had de- 

 posited the first grains of sand is at length decomposed ; inclemencies 

 of the weather, insects, moisture, and chemical agencies destroy it, 

 and when it has disappeared the sand which it retained shifts again. 

 The wind, which only carried away the superficial beds of the dune 

 to replace them incessantly by new sheets of sand, can now carry 

 away all the anterior part of the hillock ; it lengthens the descending 

 talus at the expense of the shore side, and the base of the hill, worn 

 away by the wind, constantly retreats from the shore. The dune is 

 on the march ; it advances inland. Such is the mobility of the sands 

 that even when the waves erode the foot of the dune, and force it to 

 fall into the sea, the summit does not the less a'dvance towards the 

 continent. Destroyed on one side it invades on the other, like those 

 voracious insects which, even when cut in half, do not cease to eat. 

 * Raulin, Geographic Girondine. 



