SUCCESSION 245 



especially shales, limestones, and calcareous sandstones, decomposition is 

 much more rapid, and the successions are simpler and more mesophytic. 



296. Succession In alluvial soils. Alluvial soils are fluvial when laid down 

 by streams and rivers, and litoral when washed up by the waves or tides. 

 They are formed when any obstacle retards the movement of the water, de- 

 creasing its carrying power, and causing the deposit of part or all of its load. 

 They consist of more or less rounded, finely comminuted particles, mingled 

 with organic matter and detritus. Alluvial deposits are especially frequent 

 at the mouth of streams and rivers, on their terraces and flood plains, and 

 along silting banks as compared with the erosion banks of meanders. The 

 filling of ponds by the erosion due to surface drainage, and of lakes by the 

 deposition of the loads of streams that enter them, results in the formation 

 of new alluvium. A similar phenomenon occurs along coasts, where bays 

 and inlets are slowly converted into marshes in consequence of being shal- 

 lowed by the material washed in by the waves and tides. Such paludal de- 

 posits are invariably salt water or brackish. Contrasted with these, which 

 are uniformly black in consequence of the large amount of organic matter 

 present, are the sandbars and beaches, which, though due to the same agents, 

 are light grey or white in color, because of the constant leaching by the 

 waves. Two kinds of alluvial deposits may accordingly be distinguished: 

 (i) those black with organic matter, and little disturbed by water, and (2) 

 those of a light color, which are constantly swept by the waves. The suc- 

 cessions corresponding to these are radically different. In the first, the pio- 

 neer vegetation is hydrophytic, consisting largely of amphibious plants. 

 The pioneer stages retard the movement of the water more and more, and 

 correspondingly hasten the deposition of its load. The marsh bed slowly 

 rises in consequence, and finally the marsh begins to dry out, piassing first 

 into a wet meadow^ and then into a meadow of the normal type. A notable 

 exception to this sequence occurs when the swamp contains organic matter 

 or salts in excess, in which case the vegetation consists indefinitely of swamp 

 xerophvtes, or halophytes. The first vegetation on fresh water 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, Popnlus, 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 fresh-water floods change the character of the soil. The chem- 

 ical action of underground waters also produces new soils, which might be 

 classed as alluvial. These soils are essentially rock deposits, travertine, sili- 



