260 THE FORMATION 



ness of the soil render erosion so extreme that it is all but impossible for 

 plants to obtain a foothold. Their reaction is practically negligible, and the 

 vegetation passes the pioneer stages only in the relatively stable valleys. 

 Mountain slopes (ancia), and ridges and hills (lophia) are readily eroded 

 in new or denuded areas. This is especially true of hill and mountain re- 

 gions which have been stripped of their forest or thicket cover by fires, 

 lumbering, cultivation, or grazing. Where the erosion is slight, the result- 

 ing succession may show initial xerophytic stages, or it may be completely 

 mesostatic. Excessively eroded habitats are xerostatic, as in the case of 

 bad lands, or, more frequently, they are mesotropic, passing first through 

 a long series of xerophytic formations. Sandbars (chcradia, syrtidia) 

 should be considered here, though they are eroded by currents and waves, 

 and not by run-oflF. They are fixed and built up by sand-binding grasses 

 and sedges, usually of a hydrophytic nature, and pass ultimately into meso- 

 phytic forest. 



319. Succession by filling with silt and plant remains. All aquatic hab- 

 itats into which silt, wash, or other detritus is borne by streams, currents, 

 floods, waves, or tides are slowly shallowed by the action of the water plants 

 present. These not only check the movement of the water, thus greatly de- 

 creasing its carrying power, and causing the deposition of a part or all of its 

 load, but they also retain and fix the particles deposited. In accordance 

 with the rule, each plant becomes the center of a stabilizing area, which rises 

 faster than the rest of the floor, producing the well-known hummocks of 

 lagoons and swamps. All aquatics produce this reaction. It is more pro- 

 nounced in submerged and amphibious forms than in floating ones, and it 

 takes place more rapidly with greatly branched or dissected plants than with 

 others. In pools (tipJiia) and lakes {Hinnia), debouching streams and sur- 

 face waters deposit their loads in consequence of the check exerted by the 

 still water and the marginal vegetation, and delta-like marshes are quickly 

 built up by filling. Springs (crcnia) likewise form marshes where they gush 

 forth in sands, the removal of which is impeded by vegetation. The flood 

 plains and deltas of rivers show a similar reaction. The heavily laden flood 

 waters are checked by the vegetation of meadows and marshes, and deposit 

 most of their load. The banks of streams (oclithia) and of ditches {taphria) 

 are often l>uilt up in the same fashion by the action of the marginal vegeta- 

 tion upon the current. The presence of marginal vegetation often deter- 

 mines the checking or deflecting of the current in such a way as to initiate 

 meanders, while natural levees owe their origin to it, in part at least. Along 

 low seacoasts, waves and tides hasten the deposit of river-lx)rne detritus, 

 causing the water to spread over the lowlands and form swamps. They 



