46 INITIAL CAUSES. 



It seems evident, therefore, that the above six forms due to water deposit 

 present no inherent topographic differences capable of controlling successional 

 development. As initial causes they are practically identical, and it is neces- 

 sary to turn to the materials and water relations of the new soil to discern the 

 real factors (plate 6). 



Bare areas due to waves and tides. — These are (1) beaches, and (2) reefs, 

 bars, and spits. Beaches are produced by the daily interaction of erosion and 

 deposit, but their soil is chiefly a result of deposit. They are peculiar to lakes 

 and seas in name only, for many rivers possess shores identical with them in 

 formation and structure. Bars, reefs, and spits are merely different forms 

 resulting from the same process, and, when adjacent, exhibit the same develop- 

 ment of vegetation. Such differences as exist are the result of variations in 

 height, composition, kind and amount of water, etc., and are often found in 

 different parts of the same spit or bar (plate 6 c). 



Composition and water-content of alluvial deposits. — ^The material of recent 

 or existing water-laid deposits may consist of (1) silt, (2) clay, (3) sand, (4) 

 gravel, (5) rubble or shingle, (6) boulders, or (7) marl. It is sorted in such a 

 way as to be essentially homogeneous in any one place, though it may vary 

 much between distal and proximal areas. The various materials differ chiefly 

 in the size of the particles, and through the latter influence the course of suc- 

 cession. The size of particles affects the water-content, usually in decisive 

 fashion, and it also determines the cohesion and resulting stability. It also 

 has some relation to the solutes present, though these are dependent upon the 

 kind of material as well as the source of the water. Here, as elsewhere, water 

 exercises the controlling influence in directing development, either by its 

 amount or its quaUty. As to the latter, it may be (1) fresh, (2) saline, (3) 

 alkaline, (4) acid. When it is other than fresh, the early stages of the sere are 

 characterized by a decreasing extremeness and a final return to fresh water as 

 water-content, after which the amount of the holard is decisive (plate 6 b). 



Bare areas due to deposit by ground waters. — Characteristic deposits are 

 made by mineral springs, especially hot springs, and geysers. WhUe deposi- 

 tion occurs usually about or near the spring or opening, it is also frequent in the 

 resulting streams, and may even occur in ponds or lakes at some distance from 

 the source. Such deposits consist of (1) travertine or tufa, (2) siliceous sinter 

 or geyserite, and (3) salt. Travertine is formed from waters highly charged 

 with lime, and is deposited in lakes of dry regions, as well as from spring-waters 

 and their streams. In a large number of cases its formation is due to algae, 

 but it also arises directly from chemical solution. Sinter or geyserite is typical 

 of the areas about geysers, where it arises by deposition from the hot siliceous 

 waters, through the action of algse. It also results from the decomposition of 

 siliceous minerals about the fumaroles of volcanic regions. Bothtravertine 

 and sinter are rocks and exhibit the general relation of rocks to succession. 

 Their first colonists are algae and lichens, which slowly weather the surface 

 and collect organic material for later stages. Salt may be deposited from 

 spring-waters, as in salt basins, or by the water of lakes in arid regions where 

 evaporation exceeds the inflow. In moist and semiarid regions the salt crust 

 is usually thin, and hence readily dissolved or weathered away, permitting 

 halophytes to enter and begin the succession. In arid regions, on the contrary, 

 the deposits are thicker, and removal by weathering or solution is nearly 



