Page:Popular Science Monthly Volume 8.djvu/373

Rh 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 elevation. These, too, are the ones that, on reaching the top of the hillock, 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 consequence 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.





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