28 



GEOLOGY, 



manently preserved if the proper relations between the supply of sai^d 

 and strength and direction of wind are preserved. In the dune region 

 of the Indian desert ^ the prevailing winds are alternately the southwest 

 and northeast monsoons, the former being the stronger. The supply 

 of sand comes from the southwest. Near the southwest coast the 

 dune ridges are parallel to the direction of the wind (Fig. 8); in the 

 interior, where the winds are less strong, the dunes are transverse to 

 it (Fig. 9) ; while between the districts where these two types prevail in- 



FiG. 10. — Crescentic dunes in ground-plan, the convexities facing the wind. 

 (Bokhara.) (Walther.) 



termediate forms occur. The transverse dune ridges (Fig. 9) are said to 

 be the result of the lateral growth and erosion of longitudinal dunes. ^ 

 In region3 of changeable winds the shape of the dunes is subject to 

 great variation. Dunes are sometimes crescentic, the convexity facing 

 the wind (Fig. 10). 



^ Blanford. Geology of India, 2d ed., p. 455 et seq. 

 2 Cornish, loc. cit 



