OHAEACTEE OF DUNE SAND. 645 



is very similar when brought to bear upon loose particles of sohd 

 matter. It would, indeed, seem that the slow and comparatively 

 regular movements of the heavy, inelastic water ought to affect 

 such particles very differently from the sudden and fitful impulses 

 of the light and elastic air. But the velocity of the wind cur- 

 rents gives them a mechanical force approximating to that of the 

 slower waves, and, however difficult it may be to explain all the 

 phenomena that characterize the structure of the dunes, observa- 

 tion has proved that it is nearly identical with that of submerged 

 sand-banks.* The differences of form are generally ascribable to 

 the greater number and variety of surface accidents of the ground 

 on which the sand-hills of the land are built up, and to the more 

 frequent changes, and wider variety of direction, in the courses 

 of the wind. 



Character of Dwne Sand. 



" Dune sand," says Staring, " consists of well-rounded grains 

 of quartz, more or less colored by iron, and often mingled with 

 fragments of shells, small indeed, but still visible to the naked 

 eye.f These fragments are not constant constituents of dune 



* Forcliliammer ascribes the resemblance between the furrowing of the 

 dune sands and the beach ripples, not to the similarity of the effect of wind 

 and water upon sand, but wholly to the action of the wind ; in the first in- 

 stance, directly ; in the latter, through the water. " The wind-ripples on the 

 surface of the dunes precisely resemble the water-ripples of sand-flats occa- 

 sionally overflowed by the sea ; and with the closest scrutiny, I have never 

 been able to detect the slightest difference between them. This is easily ex- 

 plained by the fact, that the water-ripples are produced by the action of light 

 wind on the water which only transmits the air- waves to the sand." — Leon- 

 hard und Bronn, 1841, pp. 7, 8. 



f According to French authorities, the dunes of France are not always 

 composed of quartzose sand. "The dune sands" of different characters, 

 says Bremontier, " partake of the nature of the different materials which com- 

 pose them. At certain points on the coast of Normandy they are found to 

 be purely calcareous ; they are of mixed composition on the shores of Brittany 

 and Saintonge, and generally quartzose between the mouth of the Gironde 

 and that of the Adour." — Memoire stir les Dunes, Annates des Fonts et Chaus- 

 eees, t. vii., 1833, ler semestre, p. 146. 



In the dunes of Long Island and of Jutland, there are considerable veins 

 composed almost wholly of garnet. For a very full examination of the me- 

 chanical and chemical composition of the dune sands of Jutland, see Andre- 

 BEN, Om EXitformationen, p. 110. 



Fraas informs us, Aiis dem Orient, pp. 176, 177, that the dune sands of the 



