

FOEMATION OF " ETANGS." 



211 



wliicli are clianged into ponds, have been gradually driven inland by 

 the parallel rows of dunes. Under the enormous pressure of the 

 sand they have climbed, so to say, the slope of the continent. At the 



rig. 94.— Formation of "Etangs." 



same time the rains and rivulets, arrested in their course, have in- 

 cessantly brought their contribution of fresh water to the new lakes, 

 while the salt water retreated gradually by natural channels between 

 the hillocks. Thus the grains of sand which the wind drives before it 

 have sufficed, in the course of centuries, to change gulfs of salt water 

 into ponds of fresh water, and carry them into the interior of the con- 

 tinent, to a height considerably above the Atlantic. 



The same phenomena occur also in the sandy islands which are 

 found in the middle of the sea. The greater part of these islands 

 have a perfectly regular form, due at the same time to the currents 

 which bathe them, and to the winds which form the dunes. In 



Fig. 95.— Isle Thelenji in the Caspian Sea. 



the centre of the triangular or crescent-shaped space which they sur- 

 round with their moving hillocks, they enclose one or several ponds, 



p 2 



