THE FORMATION OF SAND-DUNES. 



359 



The dunes are built up by slow accretions, and at the top the sand- 

 grains are smaller than at the bottom. The process by which they 

 are formed is a continual rolling of sand-grains up-hill by wind-force, 

 and it is obvious that the lightest ones will attain the greatest eleva- 

 tion. These, too, are the ones that, on reaching the top of the hil- 

 lock, roll- over on the protected side of the dune, and there form a 

 mass of fine sand. But the winds are not uniform in force, and a con- 

 sequence is, the dunes are laminated in their structure, coarse and 

 fine layers alternating. The winds change in direction too, changing 

 the position of the sands, and thus the dunes are not only laminated, 

 but irregularly bedded in their structure, closely resembling in this 

 respect that of beaches formed by the plunge and flow of waves. 

 Both structures simply represent wave-motions, one of the water, the 

 other of the air. Fig. 1 represents a section of a large sand-dune, 

 and Fig. 2 a similar but coarser formation hardened into sandstone. 



Pig. 1. Section op a Sand-Hill, the STHtrcTURE or which mat have been peoduced bt 



THE Action of Waves ob Wind. 



Fig. 2. Section of Stkata of Sandstone. 



The exterior form of a dune undergoes continual change in dry 

 weather from gravity. The grains of sand roll down its sides until 

 the fine traces of wind-sculpture are obliterated, and a somewhat 

 uniform outline is obtained. It is found that in case of dry sand the 

 angle the side of the dune will finally assume is about 32. But the 

 winds rarely permit regularity in the form of dunes. A slight breeze 

 becomes a strong one when it rises to the top of an obstacle, or is 



