201 THE OCEAN. 



ments of shells, then the large arenaceous debris, then the fine 

 sands.* 



If the inclined plane which the dune turns towards the sea re- 

 mained perfectly even, the zone of the shore would only present, in 

 all its extent, a single rampart of sand gradually encroaching on the 

 lands. But at length the slope of each dune cannot fail to offer 

 some inequalities caused by foreign bodies, or by plants that take 

 their origin in the sand. All the salient points strong enough to 

 resist the wind serve as supports to new dunes, grafted, so to say, on 

 the sides of the ancient one. These new dunes themselves bristle 

 with irregularities, which other sand-hillocks soon cover, and it is 

 thus that all those ranges of moving hills arise, which are separated 

 by long and narrow valleys, called lettes or Ihdes by the peasants of 

 the French Landes. In certain places, especially between Biscarosse 

 and La Teste, these " lettes," for a length of several leagues, resemble 

 the dried-up beds of large rivers, surrounding large islets of verdure 

 with their sandy waves. 



Notwithstanding the apparent disorder of these hillocks, in the 

 midst of which an inexperienced traveller might easily lose his way, 

 the general disposition of the sands can always be referred to a uni- 

 form type, which local geographical facts variously modify, such as 

 the contours of the marine shore, the nature of the soil, the force and 

 direction of the winds, the presence or absence of vegetation. The 

 dune nearest to the sea, and, in consequence, the most recent, is 

 less elevated than the older hillock situated immediately beyond ; 

 and this in the same way attains a less considerable height than the 

 following hill. In a system of dunes, generally each range which is 

 developed further inland exceeds the preceding ones in elevation, 

 and forms, as it were, a new step on the slope of the great primitive 

 dune which serves as an a rant garde to the army of sands. This last 

 dune, the true crest of the entire system, enlarges itself, little by 

 little, with all the materials which have served for the formation of 

 the inferior dunes situated on the side nearest to the sea. The grain 

 of sand which the air carries to the summit of the first hillock, and 

 which falls afterwards into a ravine, may remain during centuries 

 under the superincumbent masses ; but owing to the constant pro- 

 gress of the dune, the superficial layers of which are swept by the wind 

 and then let fall by it further down the talus, this grain of sand at 

 last reappears, is carried anew to a summit, it descends again, and 

 thus does not cease to travel from dune to dune, to the last. 

 * Marcel de Serres, Bulletin dela Soctete Geologique de France, 1859. 



