TOPOGRAPHIC CAUSES. 37 



in another. The wind may sweep sand from a shore or blow-out and heap 

 it up elsewhere, or it may carry dust from dry lake-beds or flood-plains for 

 long distances and pile it in great masses of loess. Gravity in conjunction 

 with weathering removes the faces of cliffs and accumulates the coarse material 

 in taluB slopes at the base. 



Volcanoes and ground-waters in the form of hot springs and mineral springs 

 act similarly to the extent that material is taken from one place and added 

 to another. They differ from the agents cited above, however, in that the 

 removal is from the interior of the earth's crust as a rule, and bare areas are 

 consequently produced only by addition. Perhaps the formation of sink- 

 holes may well be regarded as an exception, where the collapse of the surface 

 results directly or indirectly in denudation. Volcanoes change land forms 

 principally by means of lava-flows and deposits of volcanic dust, and mineral 

 springs by deposition of dissolved material as travertine, sinter, etc. In the 

 case of weathering, the process itself neither adds nor subtracts, but is so 

 intimately and universally associated with transportive agents — ^water, wind, 

 ice, and gravity — that the effect is the same. Residuary soils furnish the 

 only example of weathering without transport, but these are of little impor- 

 tance in succession (plate 4, a, b). 



Kinds of processes. — The various processes which control land forms, and 

 hence the surface available for succession, are (1) erosion, (2) deposit, (3) 

 flooding, (4) drainage, (5) elevation, and (6) subsidence. From the standpoint 

 of physiography, it is evident that these are more or less related in pairs of 

 complementary processes. Erosion in the upper part of a valley has its inevi- 

 table effect in the deposition which characterizes the lower part. The forma- 

 tion of a lake by flooding has its normal outcome in drainage by the cutting 

 down of the stream which flows from it, unless filling or evaporation proceed 

 too rapidly. Elevation and subsidence are theoretically complementary at 

 least, and on the Scandinavian coast it is assumed that they are associated at 

 the present time. As wiU be shown later, elevation and subsidence have prac- 

 tically no effect upon succession at the present, except in the rare cases where 

 new land suddenly appears. Moreover, grave doubt has been thrown upon 

 many of the supposed evidences of coastal changes of level. 



While erosion and deposit, flooding and drainage are complementary in the 

 life-history of a river system, as processes they are opposite or antagonistic. 

 The clue to their influence upon vegetation is not to be found in the fact that 

 they are associated in the base-leveling of a region. It resides, on the contrary, 

 in the fact that one is destructive of vegetation or habitat and the other con- 

 structive as to habitat. In general, erosion lays bare or destroys an existing 

 habitat, deposition produces a new one. Flooding destroys an existing habi- 

 tat and drainage lays bare a new one. The fact that all produce bare areas 

 upon which successions can arise is no evidence of their relationship from the 

 standpoint of vegetation. Bare habitats are also produced by climate, fire, 

 man, or animals, without indicating any essential relationship among them. 

 Viewed as topographic processes merely, the sharp contrast between erosion 

 and deposition is obvious. Indeed, in this respect, they are exact opposites. 

 Erosion removes the surface of a land form or decreases its area, or it may 

 do both in the same case. Deposit adds to the surface, or increases the area 

 of the land form, or both. Their union in the development of a river system 



