RELATION OF WIND TO TOPOGRAPHY 561 



and others, it is now a well-established fact that the movements of 

 the atmosphere are of the greatest importance for the plants, especially 

 because of the influence of winds on the transpiration processes. 

 With regard to vegetation on coastal sands wind is a factor of par- 

 ticular moment. The difference in wind velocities on open soil, 

 on a surface more or less uneven, or covered with a more or less dense 

 vegetation, is therefore of great significance, and we shall be able to 

 show some effects of this eolian influence in the following pages. 



The direction of wind on different shores is further a factor which 

 must not be overlooked as it plays an important rdle, not only in the 

 development and topography of the sand formations, but also in the 

 distribution of plants on the coast. 



As a general rule we can lay down the law that on every coast 

 where sand dunes occur the offshore wind is prevailing. There are, 

 however, a great many exceptions to this rule, where the local topog- 

 raphy or other factors have been the cause of dune development. 



On some coasts, as that of Jutland, where the supply of marine 

 sand seems to be inexhaustible, the prevailing westerly winds from 

 the sea drive the sand inland from the beach, continuously adding 

 to the volume of sand. On other coasts, as that of Gascony, strong 

 land winds occur, which often return considerable quantities of 

 sand to the sea. 



On the Pacific coast of America near San Francisco, and near 

 Salina Cruz in Mexico I noticed that the sand cast upon the beach 

 by the northerly and westerly winds was again driven back into the 

 sea by the winds from the south and east. Similar conditions exist 

 on many other coasts where the topography is favorable to such a 

 forward and backward movement. 



The force of the wind is augmented or diminished according 

 to the season, but the result as regards sand formations depends 

 greatly upon the direction of the coast. On the windward side of 

 many islands high dunes are formed, as for instance on the northern 

 coast of Oahu in the Hawaii group where they reach about 30 feet. 



Typical coastal sand formations. — Very little variation is noticed 

 in the formation of sand drifts on marine coasts. Wherever the 

 drifting sand encounters an obstacle in its way such as a shrub or a 

 piece of wood, or a rock, there it deposits on the leeward side. Gener- 



