STABLE AND MIGRATORY I'LANT FORMATIONS 131 



the distribution of deposits, localising the shingle beach, sand- 

 dune, salt-marsh, and marine algal vegetation. 



5. The range of the tide in its influence on the marine 

 algal zones and the salt-marsh vegetation. 



6. The position of the river exits, as affecting the salinity 

 of the waters, and the relations of river-borne and tidal 

 deposits, of brackish and salt-water marsh and reed-swamp. 



7. The influence of seaward drainage, weathering, and 

 gravity on the coastal profile. The ratio of degradation 

 by weathering to the advance of sea erosion. 



In the stream belt the principal factors are perhaps 

 included in : — 



1. The sources of the permanent water supply ; their nature 

 and capacity, their distribution and seasonal variation. 



2. The physiography resulting from the interaction of the 

 drainage system and the surface, in the various segments of 

 the torrent, valley, and plain tracts. The rate of flow, 

 prevalence of erosion or deposition, sorting or silting, flushing 

 or sustained submergence. 



The blown sand and salt-marsh formations of the coastal 

 belt are described in the " Types of British Vegetation " (3), 

 and our shingle beaches are at present undergoing careful 

 analysis (38). The stream belt, apart from the fens (3) (25), 

 certain lakes (39) (40), and moorland districts in Scotland (26), 

 has at present received little attention in this country, owing 

 partly, no doubt, to the extensive artificial alterations it has 

 undergone. Our rock formations have also received some 

 attention (3) (26). 



Many of these migratory formations, though perhaps not 

 recognised as such, have been described by plant ecologists in 

 various parts of the world. 



Rock formations have formed a special study in Switzer- 

 land. The stream belt and coastal belt have been studied in 

 America, New Zealand, Belgium, the Faroes, Denmark, and 

 elsewhere. 



In the following generalisations, the migrator}' formations 

 are treated chiefly from the geological standpoint, as much 

 work is needed before an ecological classification of migratory 

 formations can be considered as placed on firm foundations. 

 The majority of them are perhaps included under Warming's 



