OEiaiN OE SAND-DUNES. 199 



the course of centuries by the disintegration of freestone rocks. 

 Fogs, rains, frosts, and other atmospheric agents, gradually wear 

 away the stone and transform it into sand, which falling leaves 

 fresh beds at the surface. These are subject in their turn to the 

 destructive meteoric influences, and it is thus that the rock, once 

 solid, is gradually changed, often to a considerable depth, into a mass 

 of crumbling sand. The grains chafed against each other during their 

 fall become finer and finer, and when the wind is high, it can carry 

 away these sandy particles, cause them to ascend tlie slope of the talus, 

 and sometimes even raise them in clouds like the smoke of a volcano. 

 Nevertheless, the dune, still enveloping a solid kernel and composed 

 in great part of grains heavier than those of the sea-coast, is not 

 entirely displaced by the action of storms ; it only takes another form 

 in consequence of the gradual change of its slopes. Several moun- 

 tains of this kind near Ghadames, which were formerly rocky hills, 

 rise to 150 and 600 feet high. One of them, which is not less than 

 510 feet, has an inclination of 37 degrees on the side exposed to 

 the wind ; nearly the greatest slope that a talus of sand can pre- 

 sent.* 



As to dunes properly so called, those which are found far in the 

 interior of continents cannot be 'compared in importance with those 

 which are developed in long ridges, parallel to the sandy shores of 

 the sea. On the strands of the ocean which are not rocky the ex- 

 istence of dunes is almost constant ; the only low shores which 

 are destitute of them are those which the waves have formed of 

 clayey substances, of compact mud, or sand much mixed with animal 

 and vegetable detritus. The sandy shores of the Mediterranean, of 

 the Baltic, and other inland seas, where the tides are hardly percep- 

 tible, also present very insignificant dunes, because the want of ebb and 

 flow does not allow the sand to acquire sufficient mobility. We see, 

 however, some more than 90 feet high between Yera-Cruz and 

 Tampico, on the shores of the Gulf of Mexico, where the tides are 

 very slight. On all oceanic coasts, the sand of which is loose enough 

 to allow itself to be raised by the wind, the formation of dunes is 

 accomplished with perfect regularity. 



These hillocks rising, so to say, beneatb the very eyes of the ob- 

 server, it is not difficult to follow their progress, nor to offer a theory 

 regarding them. The waves constantly agitating the shifting found- 

 ation of the shore, become charged with arenaceous matters, and 

 spread them in thin layers over the strand. Then, at low tide, 



* Vattone, Mission de Ghadames ; Earth, Zeitschrift fiir die Erdkunde, March, 1864. 



