334 Proceedings Portland Society Natural History 



with the climax trees, are of very little importance and us- 

 ually represent, relic species from an earlier stage in the 

 succession of vegetation. The poplar (Populus grandiden- 

 tata) and the gray birch (Betula popidifolia) are pioneer 

 trees in clearings and they rarely appear in the denser for- 

 ests owing to their great intolerance. The white cedar 

 (Thuja Occident alls) and the white ash (Fraxinus ameri- 

 cana) are more typical of swamp forests, and in the upland 

 woods are to be found only in extremely moist places. The 

 white spruce (Picea canadensis) is primarily a pioneer in 

 the revegetation of old fields and is unable to compete in the 

 forest with deciduous trees, although it is of frequent oc- 

 currence in coniferous woods. 



IV. THE PHYSIOGRAPHIC FORMATION-COM- 

 PLEX OF THE PENOBSCOT BAY REGION 



A. PRIMARY FORMATIONS OF THE XERARCH AND 

 MESARCH SERIES 



1. Formations of Ordinary Well-Drained Uplands 



In general the succession of vegetation leading up to the 

 •climax type is the same on all ordinary uplands, whether 

 they be characterized by rock outcrops or glacial drift. 

 Occasional differences do occur depending on the nature of 

 the underlying rock, the succession on igneous rocks dis- 

 playing slightly different features from that on sedimentary 

 rocks. In the present discussion the succession on granite, 

 the igneous rock of most frequent occurrence, will be out- 

 lined in detail. The succession on schists, the most abun- 

 dant sedimentary rock, however, will be considered only in 

 those respects in which it differs from granite, whenever 

 these seem of sufficient importance to warrant it. 



a. Association-types of igneous rock outcrops 

 Owing to the paucity of the soil covering and the conse- 

 quent frequent occurrence of bare rocks, the Penobscot Bay 



