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 ailuvium. 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 sandhars 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: 
(1) 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, passing 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 
xerophytes, 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, 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, tnless 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- 
