54 INITIAL CAUSES. 



of marine areas by fresh vegetation when the high-tide level is lowered, and 

 because in the cycle of shore-line development retrograding exceeds pro- 

 grading, and retrograding tends to carry h%her tide-levels into low lands, 

 where apparent changes of level are most easily recognized. 



The above conclusions have been given in some detail because there can be 

 no question of the existence of fictitious evidences of subsidence. On the 

 other hand, it is equally clear that existing subsidence would produce similar 

 or identical phenomena, as Davis (1910:635) has well shown in the case of the 

 layers of salt-marsh grasses. To the ecologist the actual facts of coastal seres 

 and coseres are the important ones, upon which must be based the final 

 decision as to the causal action of subsidence or of change of tide-level. It 

 is clear, however, that slow subsidence, whether recent or remote, can only 

 destroy amphibious or land vegetation and preserve the plants more or less 

 completely in the form of peat as an evidence of former communities. It 

 can not initiate a new area for colonization, except in so far as the ocean itself 

 may be so regarded. 



Earthquakes. — Practically no attention has been paid to the effects of earth- 

 quakes in producing new areas. It is obvious that a variety of different areas 

 may arise from the action of earthquakes, either directly or indirectly. The 

 direct effects are seen in the emergence of land from water and the subsidence 

 of sea-coasts and deltas, and in the formation of small craters and mud-cones. 

 Indirect effects arise in valleys where the drainage is disturbed by faults or 

 otherwise, and new areas are consequently formed by ponding or draining. 

 Earthquakes also loosen masses of rock or soil from cliffs and slopes, producing 

 talus and slide masses. The great tidal-waves of earthquakes must also pro- 

 duce striking effects in denudation, erosion, and ponding on coast lands and 

 islands in their path. An earthquake is thus a primary cause which has 

 erosion, deposit, flooding, draining, elevation, and subsidence at its command in 

 producing bare areas on which succession will occur. 



Similarity of topographic processes. — The paired character of topographic 

 processes has already been remarked. Erosion and deposit, flooding and 

 draining, elevation and subsidence are all pairs of opposite and more or less 

 related processes from the standpoint of topography (plates 2 a, 7 b). To the 

 ecologist, however, they are alike in being initial processes which produce new 

 or denuded areas for succession. From this viewpoint their similarity depends 

 upon the water relations of the new area. Flooding and subsidence produce 

 new water areas, draining and elevation new land areas. Gradual deposition 

 in water makes the latter susceptible to colonization, while erosion exposes the 

 land surfaces to invasion. Theoretically at least, it is possible for all of these 

 six processes to produce bare areas of essentially the same water-content 

 within the same climax region, and hence to initiate the same succession. The 

 shores of large lakes do actually exhibit the same water succession in initial 

 areas produced by each of the four processes, deposit, flooding, draining, and 

 erosion. It would be altogether unusual for elevation and subsidence to be 

 added to these, but it could at least happen in a region subject to earthquakes 

 or volcanic disturbances. It must have occurred repeatedly in geological 

 periods characterized by great diastrophic changes. 



