:*52 Proceedings Portland Society Natural History 



successions resulting from cultivation, fire, or lumbering 

 have brought about changes in the character of the forest. 

 A tract of woodland may consist of representatives of a sin- 

 gle species, such as the beech, or of two or three species ; and 

 again trees, ordinarily of secondary importance, may play a 

 prominent part and even dominate a given area. In con- 

 sequence there is extreme variation between the different 

 tracts of woods. 



2, Formations of Well-drained Uplani: long 

 Streams 

 • ne viycrs of the Penobscot Bay region are insignificant 

 sluggish streams and have had very little elfect in altering 

 the topography of the country. Most of them are tidal in 

 nature and the vegetation along their banks is of a lowland 

 type. In only one instance has a river developed sufficient 

 erosive action to carve out a valley for itself. The Duck- 

 trap River in Lincolnville, along its lower course, flows 

 through such a valley (Figure 15), which it has cut through 

 a ridge of trap rock. Both of its banks are covered with 

 a growth of spruce, fir, and hardwoods differing in no re- 

 spect from the forests on ordinary uplands. 



a. Association-types of boulder plains 

 The river has, however, been able to carry away the ac- 

 cumulated glacial debris along its course and has developed 

 a well-marked boulder plain (Figure 16) with a character- 

 istic flora growing between the rocks. The conspicuous pi- 

 oneers in such a habitat are Apocynnm androsaemifolium 

 and the grasses, Calamagrostis canadensis and Glyceria 

 nervata, while other abundant species include: 



Juncus brevicaudatus Lysimachia terrestris 



Iris versicolor Lycopus amcricanus 



Radicnla palustris Eupatorium perfoliatum 



Hypericum ellipticum Eupatorium purpureum var. 



mac id at urn 



