212 N. S. SHALER — PHENOMENA OF BEACH AND DUNE-SANDS. 



may exhibit little or none of it. Watching the movement of dune-sands 

 we note that if the marching of the mass be considerable the amount of 

 fine dust blown to a distance by the wind is very noticeable. In fact 

 the indefinite advance of dunes from the shores is to a great extent 

 hindered by this process, by which a large part of their masses is con- 

 verted into dust which blows far away. 



Prevented by Vegetation. — The process of decomposition, which is indi- 

 cated in a large manner by the amount of the dust developed in dune- 

 materials serves in another way to bring these masses into stable condi- 

 tions. As the decay advances, the mass becomes more and more fitted to 

 sustain plant life, particularly the grasses that have become specially 

 adapted to the environment which these sands afi'ord ; of these the com- 

 mon beach-grass is a familiar type. This plant has the habit of sending 

 certain of its main roots to a great depth, where they are tolerably sure 

 of a water-supply ; its horizontal shoots are forced laterally at a consid- 

 erable depth beneath the surface, and are thus in a measure secured 

 against risk of exposure by the movement of the sand, while the leaves 

 by their hard nature and their order of growth are well fitted to resist 

 the cutting action of the blown sand and to bring it to rest in their inter- 

 spaces. In fact this plant of all the dune-bearing species is the most 

 eff'ective in defending the accumulations from the further action of the 

 wind. 



Limited as to Distance. — The decomposition of dune-materials is evi- 

 dently favored by the considerable amount of organic matter which is 

 derived with the sand from the beaches ; it is also rapidly promoted by 

 the vegetation which begins to feed in the lean soil, so that the distance 

 inland to which a dune can in ordinary climatal conditions be able to 

 journey is never very great. 



