112 SUCCESSION 



hasten the deposition of its load. The marsh bed slowly 

 rises in consequence, and finally the marsh begins to dry 

 out, passing first into a wet meadow, and then into a meadow 

 of the normal type. A Inotable exception to this sequence 

 occurs when the swamp contains organic matter or salts in 

 excess, in which case the vegetation consists indefinitely of 

 swamp xerophytes, or halophytes. The first vegetation on 

 freshwater sandbars is xerophytic, or, properly, dissophytic, 

 unless they remain water-swept, and the ultimate stages of 

 their successions are mesophytic woodlands composed of 

 water-loving genera, Populus, Salix, etc. It seems certain, 

 however, that these will finally give way to longer-lived 

 hardwoods. Maritime sandbars and beaches are always 

 saline, and their successions run their short course of 

 development entirely within the group of halophytes, unless 

 the retreat of the sea, or freshwater floods change the char- 

 acter of the soil. The chemical action of underground 

 waters also produces new soils, which might be classed as 

 alluvial. These soils are essentially rock deposits, traver- 

 tine, silicious sinter, etc., made by iron and lime springs, 

 and by geysers, and they must be changed by decomposition 

 into soils proper to be comparable with alluvial soils. 

 6. Succession in aeolian soils. The only wind- 

 borne soils of geological importance at the present time are 

 those which form dunes, both inland and coastal. Aeolian 

 deposits consist largely of rounded sand particles, which 

 are of almost uniform size in any particular dune, but vary 

 greatly in dunes of different ages. The reaction of the 

 pioneers on dunes plays an important part in building the 

 latter, but the immense dunes of inland deserts, which are 

 entirely destitute of vegetation, seem to indicate that its 

 value has been overestimated. The first stages in dune suc- 

 cessions are dissophytic, i. e., the plants grow in a soil of 

 medium or high water content, but in an atmosphere that is 

 extremely xerophytic. The ultimate stages vary widely in 

 accordance with the region in which they occur : they may be 

 xerophytic heaths, or mesoohytic meadows and forests. Be- 

 cause of their striking character and economic significance. 



