COMPOSITION OF SAND. 91 



The size of the grains of sand is very different in different places. 

 The largest grains measure about a cubic millimetre, but the averages 

 in diffei-ent drift sands vary from 0'02 millimetres to 0-47. 



The coarsest sands in Europe are those of the Northern Binnen 

 Sands; those of the strand are disproportionately finer; in sand 

 basins the particles are finest in the direction in which the diluvial 

 waters found their exit ; the sand is finer in dunes than in the blown 

 out hollows between them; and in an extensive sand-waste the 

 particles are finer at the extremity towards which the wind blows 

 than at that by which it comes. The size of the particles has an 

 influence, we have seen, not only on the degree to which it is liable to 

 drift, but in the degree in which it may manifest many properties 

 important to soil, amongst others, those of retaining moisture, one 

 which is absolutely necessary to vegetation. 



In accordance with what has been advanced is the testimony of 

 Clav6 : 



"Composed of pure sand resting on an impermeable stratum 

 called alios, the soil of the Landes was for centuries," writes Clav6,* 

 " considered incapable of cultivation. Parched in summer, drowned 

 in winter, it produced only ferns, rushes, and heath, and scarcely 

 furnished pasturage for a few half starved flocks. To crown its 

 miseries, this plain was continually threatened by the encroachments 

 of the dunes, vast ridges of sand thrown up by the waves, for a 

 distance of more than fifty leagues along the coast, and continually 

 renewed, were driven inland by the west wind ; and as they rolled 

 over the plain they buried the soil and the hamlets, overcame all 

 resistance, and advanced with fearful regularity. The whole 

 province seemed doomed to certain destruction when Bremontier 

 invented his method of fixing the dunes by plantations of the 

 maritime pine." 



The mobility of the sand is also most effectually arrested and 

 prevented by moisture, and it is only renewed as desiccation takes 

 place. 



It may have been remarked that the sand on the sea-shore does 

 not begin to drift so long as it is moist — that it only does so when it 

 has become thoroughly dry ; and that it ceases so soon as it is again 

 moistened, whether by rain or by the rising tide. But what is 

 mainly contemplated here is simply to show how it comes to pass 



* Etudes Forestieres, p. 250, See also Reclus. La Terre, I,, 105-106. 



