558 PEHR OLSSON-SEFFER 



had been pushed forward or rolled on the surface. But the opening 

 between the floors is high enough to allow a considerable part of the 

 grains to be lifted above ground. It seems most likely to me that the 

 sand is moving in short jumps. 



Different opinions have been expressed on this question. Bre- 

 montier considered that the sand is not lifted to any considerable 

 height as he says: 



Chacun des grains de sable dont elles (les dunes) sont composees n'est pas 

 assez gros pour resister aux vents d'une certaine force; ni assez petit pour etre 

 enleve comme de la poussiere, ils ne font que rouler sur la surface dont ils sont 

 arraches, s'elevent rarement a plus de 3 a 4 pouces d'hauteur.^ 



Andersen^ also maintains that the grains are mostly rolled on the 

 surface. Berendt, Hagen, and Sokoloff, among others, admit that it 

 is lifted quite high ; the latter^ correctly assumes that the many differ- 

 ent theories on this question most likely depend on the fact that the 

 observations refer to different places on the dunes sometimes to 

 the front slope, in other cases to the summit or to the leeward 

 side. 



Udden'* discusses this question in following words: 



Materials finer than dune sand are v/holly lifted up into sv/ifter currents 

 which promptly move them. The dune sand itself, on the other hand, is partly 

 lifted and also partly rolled just as the grains of the nearest larger sizes. Working 

 in this last manner the transporting power of the v^ind varies more nearly in approxi- 

 mation to its erosive force than to its lifting force. With changes in velocities the 

 latter varies as the sixth power v/hile the erosive force varies as the square. It is 

 therefore much easier for the coarser ingredients to be rolled along v.^ith the dune 

 sand than it is for the dune sand to be picked up and carried av/ay with the finer 

 ingredients. 



This holds true and the cause of the greater resistance of the finer 

 material is simply the greater coherence of the finer soil particles. 

 In drawing any inferences with regard to these matters we must not 

 forget, however, the influence which is exerted by the slope. On a 

 horizontal surface the effect of wind is not so great as on a slope. 



1 "Memoire sur les dunes," Ann. des ponts et chaussees (i), 1883, p. 148. 



2 Qm Klitformationen, 1861, p. 57. 



3 Sokoloff, Die Dilnen, 1894, p. 79. 



4 The Mechanical Composition of Wind Deposits, 1888, pp. 24 ff. 



