1885.] SAND-PLAINS OF BELGIUM. 441 



masses to a distance, and deposited on fertile land, is altogether 

 erroneous. A storm or a hurricane may blow fine particles of sand 

 as it does dust, but this is not its usual mode of advance ; and it is 

 in this it resembles the drift of dry snow, subject to the difference 

 resulting from the specific gravity of the latter being 09, and that 

 of the sand being from 2 6 to 2'9. 



The undulating or wave-like advance of the sand resembling that 

 of the snow-drift, and the water blown upon by the wind, it is best 

 seen in its effect, producing the elegantly rounded outline of the 

 sand-hills, which forcibly remind one of that of the waves of the sea, 

 and the snow-drift on the dry land ; and the resemblance goes so far 

 as to show the wavelets with which the larger wave is ribbed, and 

 by the deviations which they have taken, the prevailing deviations 

 of the wind are more permanently indicated ; while the effects of the 

 rainfall may be seen in the wavelets and furrows crossing the larger 

 undulations at angles more or less acute, and that thus : If the 

 prevalent wind had brought rain, the sand would become immobile 

 till dried ; and the drying being effected by a wind coming from 

 another quarter, this wind would forthwith produce undulations ; but 

 ere these had acquired like magnitude with those produced by the 

 prevalent wind, this would again be in force. In other cases 

 the furrows may have been produced by eddies, or variations of the 

 prevalent winds themselves. And the greater specific gravity of 

 the constituents of the sand may in a like subordinate manner give 

 form to the undulations by resisting the lighter winds, and only ad- 

 vancing before the stronger, and even then advancing but a little way. 



Besides irregularities in the level of the sand-drift, occasioned by 

 the rebound of the wind, by variations in its power, and by differ- 

 ences in the size and weiglit of sand-grains, there are differences 

 resulting from the whole suiierficies of a sand-field not being alike 

 mobile. On most places and plains covered with vegetation or other 

 covering, the sand is comparatively immobile. These contribute little 

 or nothing to the drifting sand. There a portion of the drift rests, 

 and to that extent breaks the wind and shelters the sand to leeward. 

 It is a sand-dune in miniature, and may be called a sand-dune in 

 embryo : the higher it is, the more sand does it arrest ; and the 

 more sand it arrests, the higher it becomes. It may be, being so 

 slightly elevated and so expanded, tliat it requires a practised eye 

 and close observation to perceive its elevation above the sand-plain ; 

 yet even then there may be traced blown-out hollows, and outstanding 

 higher levels, and troughs and dunes in course of formation, both 

 of them taking the direction of the prevalent wind, and being drawn 

 out in that direction. At first and for a long time the inclination 

 of the sides of the rising projections is slight, but as the sand 



