388 



PAST succession: the ceneosere. 



probable that it belongs to the subarctic period, as is probably true also of 

 layer (d), containing willow, birch, aspen, pine, Eguisetum, etc. The upper 

 black margin of layer (d) belongs to the boreal period, while layer (c), contain- 

 ing the hazel, linden, oak, elm, maple, etc., belongs to the Atlantic period. 

 Layer (b), though it contains no fossils, is assigned to the sub-boreal period, 

 and layer (a), with linden, hazel, ehn, etc., to the sub- Atlantic period. 



Sernander (1899) has considered in detail the causes of the invasion of 

 alpine and subalpine plants into the forest zone in Norway. He takes up in 

 full the composition of outpost commmiities of the different forests, and 

 discusses also the probable period of their invasion with especial reference to 

 changes of climate in Scandinavia. Much attention is given to the question 

 of relict communities and their relation to the different climatic periods of 

 Blytt. 



Weber (1900) has summarized the fossil remains of the diluvial period in 

 middle Europe. In all, 265 species are known, from widely distributed 

 deposits grouped by Weber as follows: 



I. Preglacial period: forest-bed of Cromer, meadows in the Harz Mountains. 

 II. First glacial period: Mundesley and Ostend, glacial sand of Honerdingen. 



III. First interglacial period: calcareous tufa of Thuringia, interglacial of Hoxne. 



IV. Second glacial period: till of KUnge and Hoxne. 



V. Second interglacial period: calcareous tufa of Cannstatt, Hottinger breccia, etc. 

 VI. Third glacial period: glacial peat of Ltitjenbjomholt, etc. 



Weber (1900) states that low moors, like high moors, may show both infra- 

 and supra-aquatic growth: 



The completely developed low moor shows three layers from the bottom 

 upward: (1) mud peat, arising from water plants; (2) swamp peat, produced 

 by swamp plants; and (3) swamp-forest peat, due to swamp trees. The swamp 

 peat appears most frequently as reed peat, consisting of Phragmites. Increased 

 accimiidations of peat handicap absorption, and bring about the conversion 

 of the swamp-forest moor into high moor. Weber has determined experi- 

 mentally that most of the species of Sphagnum thrive in water rich in lime, and 

 he concludes in consequence that Sphagnum is not found usually in calcareous 

 waters on account of the greater competition. The youngest high moors have 

 but a single layer, the older three, namely, older moss peat, the "horizon" 

 peat, and the new moss peat. The "horizon" peat is regarded as due to a 

 climatic change unfavorable for a time to Sphagnum. The age of the oldest 

 high moors of North Germany is estimated to be more than 10,000 years. 



In a discussion of the postglacial climate and flora of Sweden, Andersson 

 (1903) recognizes a single great glacial period, a late glacial period of tundra- 

 Hke arctic-alpine vegetation, and a postglacial period of cUmax forest. The 

 forest moors exhibit five layers of vegetation remains, corresponding to as 

 many periods, namely : 



4. Period of oak forests. 



5. Period of beech and spruce forests. 



1. Period of the Dryas or arctic-alpine flora. 



2. Period of birch forests. 



3. Period of pine forests. 



Such a succession must have been caused by climatic changes. That such a 

 gradual amelioration of climate took place within the Dryas period is shown 

 by the three successive "horizons" of Salix polaris, S. herbacea, and S. phyU 

 idfolia, the last or uppermost representing a large-leaved scrub vegetation. 



