I 



198 THE OCEAN. 



BOOK v.— THE DUNES. 



CHAPTER XXIII. 



DUNES RESULTING FROM THE DECOMPOSITION OP ROCKS. —FORMATION OF MOVINa 

 DUNES ON THE SEA-SUORE. — SYilMETRICAL DISPOSITION OF RIDGES OP SAND. 



It is principally upon the sandy beaches of the ocean that those 

 changing hillocks known under the name of dunes rise in long rows. 

 Nevertheless, the phenomenon of the elevation of the sand in moving 

 hills may also occur at a great distance from the present sea-shore. 

 Dunes are formed on all points of the globe where the wind finds and 

 drives before it light sandy materials ; but we must remark that 

 these substances only exist in considerable quantities on the shores 

 of the sea and large lacustrine basins, at the bottom of ancient gulfs 

 and straits transformed into deserts, on the banks of rivers, which roll 

 sand along their beds, and which are exposed to frequent changes of 

 level by the alternation of droughts and inundations. It is the waters 

 which, by their destructive action on the clifis, prepare the sandy par- 

 ticles necessary for the construction of dunes ; and this origin allows 

 us to consider the shifting ridges of sand, whatever be their distance 

 from the shore, as products of the ocean. 



In all the great deserts of Asia and Africa, we see some of these 

 terrestrial waves caused by aerial currents.* Some exist also on the 

 banks of the Nile and other great rivers. Even in France very fine 

 dunes about 30 feet high rise on the banks of the Garden imme- 

 diately below the celebrated Roman bridge ; it is the mistral which 

 has raised them. In leaving the gorge which encloses it, this wind 

 seizes the particles of fine sand left on the shores and dried by the 

 sun, and deposits them at the entrance of the plain, where it spreads 

 over a wider extent, and loses In Intensity what it gains in surface. 



A certain number of dunes have been formed on the spot during 

 * See in Vol. I. the section entitled Plains. 



