THE FORMATION OF SAND-DUNES. 361 



It is easy to see that this more accurately describes a dust-storm 

 than it does the movement of sand on our beaches. It is the fine 

 material only which is thus swept through the air. The coarser sands 

 are driven along the surface, and constitute the hills of the desert, 

 and they are built up as similar ones are of the still coarser sands of 

 our coast. Grain by grain they rise at the touch of the invisible 

 architect. This is true not only of the great dunes, but of the smaller 

 ones, or ripple- marks, which cover the surface of the sands. These 

 beautifully cut and wavy furrows represent the undulatory movement 

 of the air. With a full breeze, they are all seen to be in motion. The 

 g'-ains hop and bound along as the air passes, and the form shown in 

 Fig. 3 is the one which the sands continually assume. But, even Avhile 



Fig. 3. 



we watch, each little ridge or mound has been transferred to the space 

 which was a furrow only a few moments before. 



These sand-ripples rise on the sandy floor, however level and 

 smooth they may be, as the wind in passing strikes it, in a series of 

 wave-like undulations. 



Ripple-marks thus formed are, sometimes, as we have witnessed, 

 covered by drifting sand, and are retained with wonderful distinct- 

 ness, when the material is hardened into sandstone. All the vast 

 beds of this material existing in the crust of the globe are but the 

 compacted ruins of rock still older, and their furrowed tablets re- 

 peat to our eyes the rhythmic beat of winds and waters in ages long 

 past. Fig. 4 represents a slab of sandstone covered with ripple-marks, 

 evidently produced by water, but which differs in form only from 

 those produced by wind. 



Sand-dunes are not only blown away piecemeal, but the winds 

 pour upon their flanks a ceaseless shower of sand, and, as the frail 

 masonry gives way, the falling grains are caught and carried on by 

 the gale. By this natural sand-blast rocks are sculptured on the 

 highlands of the Rocky Mountains, and the glass of windows on ex- 

 posed beaches is sometimes cut through. 



On the north side of Long Island, upon the banks along the Sound, 

 are a great number of sand-hills from twenty to eighty feet high. The 

 banks are of glacial drift, with bowlders of immense size, and eastward 

 of Port Jefferson Harbor, for upward of forty miles, are crowned in 

 many places by these broken, desolate hills. In some places they ad- 

 vance slowly inland. A farm, near the village of Baiting Hollow, in 

 Suffolk County, has lost from this cause thirty acres in half a century. 

 Other farms have lost valuable land in a similar way, and we are in- 

 formed that, during the time mentioned, 100 acres of arable and tim- 



