1876.] of the Great Indian Desert. 101 



consist of the characteristic blown sand ; the Allah Bnnd, to which Sir Bartle 

 compares them, is only 20 feet high and of great breadth, and consists of 

 the silt which forms the Ran*, whilst even the fact of the elevation being 

 due to the earthquake appears not clearly established. The only alter- 

 native conclusion as to the origin of the Thar sand-ridges is that they 

 were due to the wind blowing in the same direction as that in which they lie. 

 Sir B. Frere objects to their origin by the wind that tbey are higher than 

 any known ridges of blown sand, but I find it recorded that in the Landes 

 of Gascony many dunes exceed the elevation of 225 feet and one attains the 

 height of 391 feet, whilst on the west coast of Africa hills of blown sand 

 are said to be found in the neighbourhood of Cape Verde no less than 600 

 feet high.f 



This view of the sand-ridges having been produced by winds blowing 

 in the same direction is supported by the frequent occurrence (already men- 

 tioned) of abrupt terminations of the ridges at their north-eastern extremi- 

 ties. It should be borne in mind that the ridges, although extending for 

 considerable distances, often for some miles, do end or coalesce every here 

 and there, and that there is not any regularity in the size of the valleys 

 that intervene ; some of these valleys being of considerable breadth, others 

 narrow. As a rule, the intervening valleys do not exceed half a mile in 

 breadth where the ridges are tolerably regular, and in many places the hol- 

 lows are, as a rule, much narrower. Not unfrequently a tract is found where 

 ridge and valley succeed each other with the greatest regularity for a few 

 miles, the valleys being from twice to three times as broad as the ridges. 



I am not able to explain the mode of formation of these parallel ridges 

 satisfactorily to myself. I can suggest three modes in which they may 

 have been formed, aud I think it possible that all may have acted at times. 



The first is the mode of formation from a ridge transverse to the direc- 

 tion of the wind. When such a ridge is driven forward, the ends advance 

 more rapidly than the centre, and a crescent is formed, the convex side to 

 windward. This on a small scale is a common and familiar phenomenon, 

 and is mentioned and explained in all text-books. I can conceive it proba- 

 ble tbat, with constant supplies of sand, tbe ends of the crescent may con- 

 tinue to be produced until they form parallel ridges. But I must say I 

 have not seen this change in progress in the sand-hills of the desert. 



The second suggestion is that the sand is carried along in lines by the 

 wind. I once came across a tract in which a sand-ridge appeared to be in 

 process of formation. This was about 50 miles W. N. W. of Jaysalmir, at 

 the spot where the Thar or sand-hill covmtry was entered from the undula- 



* See Wynne, Geology of Kutch, Memoirs Geological Survey of India, IX, p. 40. 

 f Naumann, Geognosie ; Reclus, L 'Ocean, 11. c. &c. The latter miter gives Bitter 

 as his authority. 



