39^ George E. Nichols, 



that here many soils, which in a less humid region would be too 

 dry to permit the development of the types of association which 

 characterize the better soils, are kept constantly moist. It may 

 also be due to the fact that bare rock outcrops, which in them- 

 selves are unable to retain water except in crevices, become 

 rapidly overgrown, except where they are too steep, by a layer 

 of lichens and bryophytes which create a water-retaining sub- 

 stratum and thus tend to produce ground conditions similar to 

 those found in soils which naturally would be more favorable to 

 plant growth. In the coniferous forest region of northern Cape 

 Breton, and probably in other similar regions as well, this 

 tendency toward uniformity is accentuated by the fact that in the 

 upland forests, owing largely to the prolific development of 

 mosses and liverworts and the copious accumulation of humus, 

 not only is the substratum kept constantly moist, but it is invaria- 

 bly acid to litmus, thus approximating the conditions which 

 prevail in bogs and in the majority of the swamps. In less 

 humid climates, many of the species w^hich here are characteristic 

 of uplands, or which grow both on uplands and in bogs and 

 swamps, are restricted to situations of the latter sort. The 

 tendency for different edaphic formation-types to merge into one 

 another is exhibited to some degree in the lowland region of 

 northern Cape Breton; it is quite pronounced in the forested 

 portion of the highlands ; but it reaches its culmination in the 

 barrens, where it is almost impossible to draw a sharp line 

 between the vegetation of uplands and that of the swamps. 



B. Formations of the Xerarch Series 



I. The Formation- types of Ordinary Uplands in the Forested 



Region 



a. THE ASSOCIATION-COMPLEXES OF WELL-DRAINED UPLANDS 



In comparing xerarch successions on ordinary, well-drained 

 uplands here with those of the lowland, the most striking differ- 

 ence is seen in the character of the climax association-type. 

 There, a balsam fir-spruce-paper birch forest may represent 

 merely a passing stage in the succession : in all edaphically 

 favorable situations it is a temporary association-type, destined 

 in the course of time to be superseded by a forest of the 

 deciduous type. Here, however, a forest of this sort represents 



