HEIGHT OF DUNES. 541 



elevation of six hundred feet. This is one of the very few points 

 known to geographers where desert sands are advancing sea- 

 wards,* and here tliej rise to the greatest altitude to which sand- 

 grains can be carried by the wind. 



The hillocks, once deposited, are held together and kept in 

 shape, partly by mere gravity, and partly by the slight cohesion 

 of the lime, clay and organic matter mixed with the sand ; and 

 it is observed that, from capillary attraction, evaporation from 

 lower strata and retention of rain-water, they are always moist 

 a httle below the surface.f By successive accumulations, they 



* " On the west coast of Africa the dunes are drifting seawards, and always 

 receiving new accessions from the Sahara. They are constantly advancing 

 out into the sea." — Naumank, Oeognosie, ii., p. 1172. 



f "Dunes are always full of water, from the action of capillary attraction. 

 Upon the summits, one seldom needs to dig more than a foot to find the sand 

 moist, and in the depressions, fresh water is met with near the surface." — 

 FoRCHHAanrEU, in Leonhard und Bronn, for 1841, p. 5, note. 



On the other hand, Andresen, who has very carefully investigated this as 

 well as all other dune phenomena, maintains that the humidity of the sand 

 ridges can not be derived from capillary attraction. He foimd by experiment 

 that a heap of drift-sand was not moistened to a greater height than eight and 

 a half inches, after standing with its base a whole night in water. He states 

 the minimum of water contained by the sand of the dunes, one foot below 

 the surface, after a long drought, at two per cent., the maximum, after a rainy 

 month, at four per cent. At greater depths the quantity is larger. The hy- 

 groscopicity of the sand of the coast of Jutland he found to be thirty-three 

 per cent, by measure, or 21.5 by weight. The annual precipitation on that 

 coast is twenty-seven inches, and, as the evaporation is about the same, he 

 argues that rain-water does not penetrate far beneath the surface of the dunes, 

 and concludes that their humidity can be explained only by evaporation from 

 below. — Om KUtformationen, pp. 106-110. 



In the dunes of Algeria, water is so abundant that wells are constantly dug 

 in them at high points on their surface. They are sunk to the depth of three 



or four mt:tres only, and the water rises to the height of a m^tre in them. 



Laurent, Memoire sur le Sahara, pp. 11, 12, 13. The dunes of the Sahara, 

 in some places, supply pasturage for the caravans. See interesting notice of 

 Pomel's observations in L'Annee Geographi'que, January, 1873, p. 258, seq. 



Laurent also observes (p. 14) that the hollows in the dunes are planted with 

 palms which find moisture enough a little below the surface. It would hence 

 seem that the proposal to fix the dunes which are supposed to threaten the 

 Suez Canal, by planting the maritime pine and other trees upon them, is not 

 altogether so absurd as it has been thought to be by some of those disinter- 

 ested philanthropists of other nations who were distressed with fears that 



