OBSTACLES TO DUNES. 215 



CHAPTER XXV, 



OBSTACLES OPPOSED BY NATURE TO THE PROGRESS OP DUNES. — FIXATION OF 

 THE SANDS BY SEEDS. 



The work of nature is, however, double ; and if on one side she 

 hastens the advance of the sands, on the other she attempts to arrest 

 them. She herself points out the means of prevention, or else pre- 

 vents spontaneously the disasters of which she is the cause. In certain 

 places, and especially on a part of the coasts of the Landes, she 

 exercises a physical and chemical action by employing the oxide of 

 iron which the water contains to consolidate the sands and transform 

 them gradually into actual rocks. Elsewhere organic cements, com- 

 posed of broken shells and remains of silicious and calcareous infusoria, 

 agglutinate the arenaceous particles, and give them the necessary sta- 

 bility to resist the winds. But these means of consolidating the 

 sands are exceptional. It is principally vegetation which fixes the 

 moving hills on the sea-shore. On almost all coasts the sandy and 

 calcareous debris of the soil contain enough fertilizing principles to 

 nourish a certain number of hardy plants, which do not fear the salt 

 air of the sea, and which send down their roots to a great depths so 

 as to absorb the necessary moisture. Among these hardy veget- 

 ables the commonest, and most useful at the same time, is Marram- 

 grass {Arundo arenaria), whose slender and flexible stems can hardly 

 arrest the wind, but whose strong roots, sometimes 12 or 15 yards 

 long, develop all the better the less consistence the sand has. 

 Various species of convolvuli creep over the ground, and fixing 

 their vigorous cordage from place to place, sometimes envelop an 

 entire dune in their network of leaves and flowers. Other plants 

 rise more proudly, but if their stem is buried in the sands they trans- 

 form it into a root, and give birth to a new shoot, which may be in- 

 terred in its turn, without the plant being in danger of perishing. 

 Thus such a seed germinating at the base of the dune often pro- 

 duces a plant which ends by spreading to the summit of the moun- 

 tain, and fastens by a cable of roots the arenaceous strata which the 



