MOVING SANDS DOWNS. 235 



cultivation of sandy soils if they are not watered, they reniain nearly 

 barren ; tlie only mode of making them productive is to lay them 

 out in plantations of timber. 



Those moving sandy plains of great extent, which are found in the 

 interior of many continents, seem at first sight stricken with eternal 

 barrenness. Nevertheless, the mobility of the sand of the desert, 

 which permits it to be swept hither and thither, and to be tossed 

 about like a liquid mass, depends less upon the total absence of ar- 

 gillaceous particles than upon the want of the moisture necessary to 

 agglutinate or to fix its grains. The burning steppes of Africa and 

 America have their oases here and there, the surface of which, 

 moistened by a spring, is green with vegetation ; and whenever 

 sandy plains are bathed by a river, it is possible to render them fit for 

 cultivation. In Spain, for instance, in the neighborhood of San Lu- 

 car de Baromeda, a powdery soil of extreme dryness has been fer- 

 tilized by the hand of man. The mammillated downs of San Lucar 

 are covered on the surface by a layer of quartzy sand, so loose that 

 it is blown about by the wind ; but by a happy disposition of things, 

 a lower stratum of these downs is kept constantly moist by the wa- 

 ters of the Guadalquiver, and it is only necessary to remove the su- 

 perficial sand, and to level the surface, in order to have a loose soil 

 which unites in the highest degree two essential conditions of fer- 

 tility, viz : openness, and a constant supply of moisture, which pene- 

 trates the soil in virtue of its permeability ; under the influence of a 

 fine climate and manure, the market gardens established in the midst 

 of this desert are remarkable for the rapidity and the vigor of their 

 vegetation. To avoid great expense, the labor of removing the sand 

 is only undertaken in places where the layer is least thick ; and what 

 is removed being heaped up as a mound around the soil which is clear- 

 ed, a kind of boundary wall is formed, which is not without its use 

 in affording shelter, and which becomes productive itself by the 

 plantations of vines and fig-trees that are made upon it with a view 

 mainly to its consolidation. In the same way in Alsace, in the plains 

 of Haguenau, the soil which was a moving desert of sand, has, in 

 the course of less than forty years, become one of the most fertile 

 under the influence of incessant cultivation ; in the same way also it 

 is that in Holland, mountains of sand, which had been accumulated 

 by the winds, have been fixed. This sand, which rests upon a wet 

 bottom, draws up the moisture by capillary attraction, and so be- 

 comes fit to support certain vegetables. These downs, which may 

 be said to have come out of the sea, have a constant tendency in 

 many places to encroach upon the cultivated lands. To oppose their 

 progress, the Dutch sow them with the arundo arenaria. the long 

 and creeping roots of which bind together the movmg mass and 

 imprison the particles of sand within a kind of net-work. These 

 masses of sand become fixed in this way ; but they remain nearly 

 or altogether unproductive. 



It is therefore a problem of the highest importance in many in- 

 stances to fix permanently masses of sand blown up from the sea, 

 by covering them with productive plantations. This problem was 



