Vegetation of Northern Cape Breton. ' 439 



has been formed, the lower portion of the complex (Fig. 65, C, 

 a-f) being neglected for the moment. 



The rock basin in question is situated at the summit of a low, 

 rounded hillock which is bounded laterally by slightly higher 

 hills. Partly as the result of direct precipitation, partly perhaps 

 through surface drainage from the adjoining higher ground, 

 this basin originally was kept filled with water, which spilled out 

 over the rim at g. The manner in which this pond became 

 obliterated was doubtless similar to what has been described in 

 the general discussion of bogs a few pages back. The open 

 water may have become filled in through the activity of the 

 aquatic sphagnums, or through the encroachment from the 

 margin of a floating mat, or through a combination of both 

 these methods. Assuming the aquatic sphagnums to have been 

 the pioneers, and that through their activity the pond had been 

 more or less completely clogged up, the second stage in the suc- 

 cession was probably dominated by the mesophytic cushion- 

 forming species {S. papillosum, S. magellanicum, S. pulchrum), 

 although there may have been an intermediate stage of semi- 

 aquatic species (S. pulchrum, S. Dusenii). Largely through the 

 activity of the mesophytic cushion-forming sphagnums the sur- 

 face may have been raised to a height of one or two feet above 

 the former pond surface, at which point the xerophytic cushion- 

 forming species {S. fuscum, S. capillaceum tenellum, S. 

 tenerum) asserted themselves. It is to the species of this latter 

 group that the further elevation of the bog surface to its present 

 height has been largely due. Throughout this series of changes, 

 various seed plants have occupied a more or less prominent 

 position, and have played an important role by binding together 

 and strengthening the ground-work formed by the sphagnums. 

 The stages characterized by the predominance of the mesophytic 

 and of the xerophytic sphagnums may be designated respectively 

 the wet bog and the dry bog stages. The character of the sur- 

 face vegetation in these two stages will be described later. 



General observations. — The exact stage at which the central 

 water body becomes obliterated in successions of the sort just 

 described varies. In the present case, the pond has been over- 

 whelmed so completely that there is absolutely nothing on the 

 surface of the bog that even suggests its former presence. In 

 other cases, however, the pond may persist for an indefinite 



