3IO 



George E. Nichols, 



course be classed the stream bed association-types, and here also 

 it seems most appropriate to include those stream bank and cliff 

 association-types whose ecological aspect is obviously correlated 

 with the more or less constant presence of an abundant water 

 supply. Similarly, in the xerarch category should be classed the 

 ravine forest and such of the association-types of stream banks 

 and cliffs as are exposed for considerable periods of time to 

 more or less xerophytic conditions. In the present connection 



Figure 19. — Gorge along Indian Brook; the upper edge of the flood 

 zone is indicated b}' the lower margin of the forest. 



attention is directed primarily to ravine associations of the 

 xerarch series. Apropos, it may be remarked that, for reasons 

 which the author has pointed out elsewhere ('16^, pp. 237, 249, 

 250), in considering the vegetation of rock ravines from the 

 dynamic point of view, the question of an actual succession of 

 plant associations, in so far as it is correlated with the physiogra- 

 phic development of the ravine itself, may be virtually dis- 

 regarded. 



Stream bank association-txpcs. — Largely owing to the narrow- 

 ness of the channel to which the rushing flood water ordinarily is 



