INVASION AND SUCCESSION 



277 



formed from sedimentary rocks decomposition is usually much 

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

 296. Succession in alluvial soils. Alluvial soils are formed 

 when any obstacle retards the movement of water. This de- 

 creases its ability to carry sediment and causes the deposit of 

 all or part of its load. Alluvial soils consist of more or less rounded, 

 minute particles mingled with organic matter. Such deposits 

 are common at the mouth of streams or rivers and in the valleys 

 flooded by them. Alluvium is also formed by the filling of ponds 



Fig. 112. A meadow formation, the last stage of the alpine collu\ial 



succession. 



through surface wash and of lakes through the deposit of mate- 

 rial by streams entering them. It occurs along coasts where 

 bays and inlets are slowly converted into marshes, in consequence 

 of being shallowed by the material washed in by waves ami tides. 

 Two kinds of alluvial deposits are distinguisho<l: (1) those 

 black with organic matter and little (listurhod ])y walcr. ;iiid 

 (2) those of a light color, whicli are constantly swept l)y the 

 Avaves. The corresponding successions are radically dilTci-ciil . 

 In the first the pioneer vegetation is hydrophytic; in the second 



