1876.] of the Great Indian Desert. 99 



falls over the crest of the ridge.* Sand-dunes along the coast are 

 rendered irregular in shape by accidents of the surface on which they 

 have accumulated, but the laws of their formation are precisely similar 

 to those of the ripples, and the same principles govern the formation of 

 inland sand-hills. The latter are often even more irregular in form than 

 the sand-dunes of the coast, because they are not formed along one general 

 line, but depend on the accidental accumulation of sand wherever the charac- 

 ter of the surface is favoru'able. In every case, however, the direction of 

 the wind to which the drifting of the sand is due is marked by the two 

 slopes in opposite directions, the long slope to windward, the steep slope to 

 leeward. The sand-hills near Balmir are evidently due to the transport 

 of sand by a south-west wind. 



I made many enquiries in the desert country as to the prevailing wind. 

 From all whom I asked I received one answer, that during the hot season, 

 May, June, and July, a strong wind blows steadily from the south-west. 

 Even in March, on two occasions, a violent wind sprang up in the afternoon 

 from that quarter, and the ah" was so thick with sand that at times it was 

 impossible to see more than a dozen yards. There is no meteorological sta- 

 tion fairly within the desert region, but the registers of wind-direction at 

 Karachi and Disa shew a great prevalence of south-westerly winds in the 

 hotter months of the year, the general direction being more westerly at 

 Disa than at Karachi ; up to April the general direction at Disa is north of 

 west. At other periods of the year the winds are light, and during the 

 months of January, February, and March, when I was in the desert, light 

 breezes from the north or south prevailed alternately, but with the excep- 

 tion of the south-west winds already mentioned, they were quite insuffi- 

 cient to move the sands. 



I do not think that further evidence is needed to prove that the forma- 

 tion of sand-hills throughout the eastern part of the desert is due to the 

 south-west winds of the hot season, but there is a much greater difficulty 

 as regards the long north-east to south-west ridges of the Thar. That they 

 are also due to the prevailing winds is apparent from the circumstance 

 (already mentioned) of their frequently terminating in a high bluff with a 

 steep slope to the north-east, but still their general direction, identical with 

 that of the prevailing wind, is rather difficult of explanation, because ridges 



* The formation of sand-dunes will be found discussed in any elementary treatise 

 on Physical Geography or Geology. The following works contain excellent descrip- 

 tions of the phenomena exhibited by blown sand: — Lyell, Principles, Vol. I, p. 516; 

 De la Beche, Geological Observer, p. 59 ; Jukes, Manual, p. 154 ; Nauraann, Geognosie, 

 II, p. 1170 ; Ansted, Physical Geography, p. 467 ; and especially Marsh, ' Man and Na- 

 ture,' pp. 471-483, and Rectus, 'The Ocean' (English translation), I, pp. 19S-214. I 

 am indebted to my brother Mr. H. F. Blanford for the latter references. 



