PRIMARY SUCCESSIONS 111 



in talus will depend upon the kind of rock in the latter. If 

 the rock is igneous or metamorphic , decomposition will be 

 slow, and the soil will be dysgeogenous. Successions on 

 such talus consist of many stages, and the formations are 

 for a long time open and xerophytic. In talus formed from 

 sedimentary rocks, especially shales, limestones, and calcar- 

 eous sandstones, decomposition is much more rapid, and the 

 successions are simpler and more mesophytic. 

 5. 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 shallowed by the material washed in 

 by the waves and tides. Such paludal deposits are invari- 

 ably 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: (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 successions 

 corresponding to these are radically different. In the first, 

 the pioneer vegetation is hydrophytic, consisting largely of 

 amphibious plants. The pioneer stages retard the move- 

 ment of the water more and more, and correspondingly 



