Vegetation of Northern Cape Breton. 309 



the contrar}^, even in narrow ravines incipient flood plains may 

 be commonly observed in situations which in some way are pro- 

 tected from the swift current (foreground in Fig. 17), while in 

 wider valleys (Fig. 20) the stream is usually bordered by an 

 interrupted series of low, terrace-like flood plains, which have 

 been built up along the less exposed banks. The flood plains of 

 rapid streams, however, not only in Cape Breton, but elsewhere as 

 well, differ markedly from the familiar type of sluggish streams. 

 There the alluvial deposits consist largely of fine-grained sedi- 

 ments. Flood plains of this latter sort, in northern Cape Breton, 

 have been developed to a considerable extent locally, particularly 

 toward the mouths of some of the larger rivers. Along rapid 

 water courses, however, the deposits are much coarser, the swift- 

 ness of the current in times of flood being so great that most of 

 the finer material is washed away. Even where the conditions 

 for deposit are most favorable, the alluvial material along a 

 rapid stream is made up largely of coarse sand and gravel, while 

 miniature flood plains built up almost wholly of cobbles and 

 pebbles are frequent in less favorable situations. Incidentally, 

 it should be remarked that while, in a sense, a boulder plain 

 might be regarded as a flood plain, for obvious reasons it is best 

 treated separately. Typical flood plains commonly overlie 

 former boulder plains. 



b. THE ASSOCIATION-COMPLEXES OF ROCK RAVINES 



This is the only type of ravine which need be considered. The 

 associations here may be divided roughly into four groups, as 

 follows. (/) The stream bed association-types: comprising the 

 vegetation in areas where the bottom is submerged at all seasons. 

 (<?) The stream bank association-types : comprising the vegeta- 

 tion of areas, mostly along the margin of the stream, which are 

 flooded at times of high water but at other times, of variable 

 duration, are exposed to the air. (5) The association-types of 

 cliffs : comprising the vegetation of areas above the flood zone 

 which are too steep or unstable to support a forest. (4) The 

 ravine forest. With reference to their water relations, some of 

 these association-types are naturally classed under the xerarch 

 series, others under the hydrarch series, but this classification is 

 not always easy to apply. In the hydrarch category should of 



