640 FOEM OF DUISTES. 



seaward face, and to protect from the furtlier influence of tlie 

 wind the particles which are borne beyond it, or rolled over its 

 crest, and fall down behind it. If the shore above the beach line 

 were perfectly level and straight, the grass or bushes upon it of 

 equal height, the sand thrown up by the waves uniform in size 

 and weight of particles as well as in distribution, and if the action 

 of the wind were steady and regular, a continuous bank would be 

 formed, everywhere ahke in height and cross section. But no 

 such constant conditions anywhere exist. The banks are curvejj, 

 broken, unequal in elevation ; they are sometimes bare, sometimes 

 clothed with vegetables of different structure and dimensions ; the 

 sand thrown up is variable in quantity and character ; and the 

 winds are shifting, gusty, vortical, and often blowing in very nar- 

 row currents. From all these causes, instead of uniform hills, 

 there rise irregular rows of sand heaps, and these, as would natu- 

 rally be expected, are of a pyramidal, or rather conical shape, and 

 connected at bottom by more or less continuous ridges of the same 

 material. 



EHsee Reclus, in describing the coast-dunes of Gascony, ob- 

 serves that when, as sometimes happens, the sands are not heaped 

 in a continuous, irregular bulwark, but deposited in isolated hill- 

 ocks, they have a tendency to assume a crescent shape, the convex- 

 ity being turned seawards, or towards the direction from which 

 the prevailing winds proceed. This fact, the geological bearing 

 of which is obvious, is not noticed by previous French writers or 

 even by Andresen, though a semi-lunar outline has been long 

 generally ascribed to inland dunes. It is, however, evident that 

 such a form would naturally be produced by the action of a wind 

 blowing long in a given direction upon a mass of loose sand with 

 a fixed centre — such as is constituted by the shrub or stone around 

 which the sand is first deposited — and free extremities. 



On a receding coast, dunes will not attain so great a height as 

 on more secure shores, because they are undermined and carried 

 off before they have time to reach their greatest dimensions. 

 Hence, while at sheltered points in Southwestern France there 

 are dunes three hundred feet or more in height, those on the 

 Frisic Islands and the exposed parts of the coast of Schleswig- 

 Holstein range only from twenty to one hundred feet. On the 

 western shores of Africa, it is said that they sometimes attain an 



