of Mississippi and Louisiana 75 



lation which this dune in particular holds to the vegetation is that 

 of a destructive agent, for the pines especially are not able to with- 

 stand the burial in sand. The live-oaks are, however, more re- 

 sistant, and many examples of trees may be seen with only the 

 ends of their branches protruding above the sand, and having the 

 appearance of vigorous shrubbery. The branches of these oaks 

 are covered with a growth of crustaceous lichens and a species of 

 Ramaluia, while an occasional annual grows in their shade upon 

 the dune surface. 



The other dunes of the series have on their windward slopes a 

 vegetation of annuals and perennials composed chiefly of Iva im- 

 bricata and Serenoa serrulata, with occasional clumps of Uniola 

 paniculate^ Siplionychia corymbosa and species of Euphorbia, plants 

 which are by no means confined to this habitat, as the reader has 

 already seen. The lee slope, which in all cases observed is very 

 steep, recalling the appearance of the dunes of Figs. 15 and 16 of 

 Cowles' paper supports no plants excepting a meagre, but never 

 too meagre, growth of Cenchms. These dunes are in rapid motion, 

 and have left behind them a "graveyard" of dead pines. Their 

 motion is evidenced not only by the character of the lee slope but 

 also by the distorted, protruding, but still living leaves of engulfed 

 plants of Serenoa. Reference has been made to the low sand 

 mounds encountered in the pine-clad plain as having originated as 

 dunes, but which are now beyond the effect of the winds. Upon 

 is explanation they may be called fossil dunes in a somewhat 



th 



exact sense. 



Shell Strand Vegetation (Plate ii) 



In the general account of the physiographic features of the 

 islands attention was drawn to the origin and presence of a dune 

 of shells on the seaward margin of many of the low islands. 

 There are two ways in which the dune may lie with reference to 

 the margin of the alluvial plain upon which it is superposed. The 

 figures here presented will show at a glance the difference. In the 

 u Pper diagram (Fig. 2, a) the edge of the alluvium is bare, so that 



pari p. 



oward the lee edge of the island. Under some conditions the 

 e <lge is eroded more rapidly than the shell dune is moved forward. 



