DEPOSIT. 45 



Fragmentary and local deposit. — ^As has just been seen, deposits may be 

 local, as well as of extremely small area. The clearest examples of this are to 

 be found in weathered rocl^, cliffs, and ledges, where deposit occurs in tiny 

 cracks, in clefts, or in large &sures. Here the deposit is often so slight that the 

 plants growing in it seem to be growing on rock, and hence to belong to the 

 initial stage of the rock sere. A careful scrutiny shows that they are not true 

 rock-plants, comparable with Uchens and mosses, but that they are soil-plants, 

 or in some cases water-plants. Local deposit in small separate areas, Hke local 

 erosion, produces innxmierable small conmiunities, each with its proper place 

 in the sere, but often so sturounded or interrupted by plants of other stages 

 that great confusion results (plate 6 a). In much work that has been done 

 upon succession so far, the course of development, the movement of the popu- 

 lation, and the relationship to the physical factors have been lost or confused by 

 the failure to recognize how detailed and accm-ate this scrutiny must be. As 

 is shown later in full, only the use of exact quadrat and transect methods can 

 show the way in such cases. 



Sterility of deposits. — ^Deposits vary greatly in the numbers of disseminules 

 found in them, a factor of considerable importance in the development of the 

 first stages. The number of viable propagules depends upon the source of 

 the material as well as upon the agent. The deposits of wind-borne volcanic 

 dust and of sinter and travertine formed by groimd-water represent the one 

 extreme of almost absolute sterihty. Primary dune-sand, i. e., blown more or 

 less directly from the beach, probably comes next, while secondary dune-sand 

 from established dunes would contain more seeds and fruits. Glacial deposits 

 are sterile, though terminal and lateral moraines of existing glaciers are rela- 

 tively an exception. Water deposits contain disseminules in varying numbers, 

 but for the most part they are relatively rich, though the viability of many of 

 the seeds is usually low. Talus deposits, land-sUdes, etc., tend to contain the 

 maximum number of seeds and fruits, owing to the fact that plants and plant 

 parts are so often carried down with the falling material, and to the favorable 

 conditions for the preservation of seeds in a viable condition. 



Bare areas due to deposit by moving water. — ^Under this term are included 



(1) streams and run-off and (2) waves, tides, and shore-currents. The typical 

 areas of deposit by running water, which includes streams of all degrees as 

 well as surface run-off, are the following: (1) alluvial cones, fans, bajadas, etc.; 



(2) alluvial plains; (3) flood-plains; (4) channel deposits; (5) deltas; (6) beds 

 of lakes. Topographically, the first three are much more closely related in 

 the essentials of the process of formation than their names indicate. It is 

 practically impossible to distinguish between an alluvial plain and a flood- 

 plain, if they are not indeed identical. Alluvial cones and fans often merge 

 into a complex, which is called by SaHsbury (1907 : 183) a piedmont alluvial 

 plain. It is clear that the Sand-bars of a river differ in little but form from the 

 deltas made in it by lateral streams, and in the case of a braided river such as 

 the Platte, the different streams of the network may form deltas, lateral banks, 

 and median bars in the same chaimel. Moreover, deposition in the bed of a 

 stream is very similar to that in the bed of a lake, a similarity that becomes 

 identity when a stream is ponded anywhere in its course. Finally, alluvial 

 fans and deltas are very like in both form and development. The delta of 

 many a mountain stream is different in no essential feature from the alluvial 

 cones produced by surface wash or by temporary rivulets along its course. 



