DRIFTING SANDS. 353 



There are also other terms to designate the rocky 

 (warr) or bushy (ghaba) districts, while " Ghoud" is 

 the region of sand dunes. * 



These drifting sands constitute quite a history in 

 themselves. They testify in the first place to the pre- 

 valence of strong gales of wind blowing from the same 

 quarter, whose force has been sufficient to raise up these 

 great systems of sand dunes from fifty to sixty, and 

 sometimes one hundred feet in height, and more. 

 Wherever the sand is deep these dunes are likely to 

 be formed, and where it is shallow, as upon the wide 

 expanse of flat plains where the wind passes over the 

 surface without being able to get much hold of the 

 sand, there are simply ripples, such as are seen on the 

 sea shore when the tide is low; but generally here 

 and there through these plains regular undulations may 

 be observed, evidently formed by the heavier drifts 

 which almost exactly represent the rollers which follow 

 each other in rhythmical sequence across the ocean 

 after the subsidence of a severe storm. 



Sand in fact, notwithstanding its great specific gravity, 

 acts in many respects very much like water, under the 

 influence of the winds; and where it meets with an 

 obstacle, it mounts up in spray, exactly like broken 

 water hurled by the violence of the waves upon a rock- 

 bound coast ; but of course its non-fluid character prevents 

 any reflux, as in the case of water, and so these dunes 

 are gradually heaped up and are always kept moving 

 on. The forward movement of the sand waves pro- 

 ceeds under a regular system, which has been described 

 by Count D'Escayrac de Lauture with great minuteness. 



* Le Desert et Le Soudan, par M. Le Comte D'Escayrac de Lauture, 

 1853, p. 18. 



VOL. I. 23 



