368 George E. Nichols, 



of boggy swamps may be attributed in large measure, indirectly, 

 to the influence of the cool, humid climate of the region. 

 The climate favors the luxuriant development of the sphagnums 

 and other peat-forming plants, and it seems to be very largely 

 through the direct influence of the layer of peat to which these 

 give rise, in retaining the water and thereby extending the 

 swampy condition throughout the season, that the boggy condi- 

 tion is brought about. Very often, in this way, through the 

 obstruction of the drainage which results from the activity of 

 the vegetation, a swamp which, during the early stages of its 

 development, would be classed as well-drained, in the course of 

 time becomes increasingly boggy. This is well illustrated by the 

 examples just described. 



It has been mentioned earlier that peat accumulation may occur 

 in connection with well-drained swamps ; but there, as already 

 suggested, the deposit is mucky and invariably shallow. Again, 

 the bog has been cited as the characteristic swamp-type of 

 undrained depressions ; but not infrequently shallow, undrained 

 depressions are occupied by swamps of the poorly drained type. 

 Sink hole swamps are often of this character. Fluctuations in 

 water level, underground drainage, alkalinity of the soil water, 

 or some such factors may perhaps explain the discrepancy here. 

 But, after all, the whole swamp situation is an extremely com- 

 plex one, and it is candidly admitted that there are any number 

 of questions which must be left unanswered. 



4. The formation-types in and along Rivers and Streams 



a. INTRODUCTORY 



Under this head are included fundamentally those association- 

 complexes of hydrarch origin whose ecological aspect manifestly 

 is correlated with the activity of rivers and streams. In so far 

 as it affects associations of the hydrarch series, the influence of 

 a stream on the vegetation in and along its course is expressed 

 primarily at times of high water, and then in two ways : first, 

 through the deposition of sediment, which leads to the develop- 

 ment of flood plains ; second, through the erosive activity of 

 the current, which may affect the vegetation directly, particularly 

 through the abrading action of ice, or indirectly, as seen in the 

 formation of oxbows. Under this head have also been included 



