THE PEAT CLISTASE. 



383 



silvestris, with occasional cones. Here are found also leaves of birch, Vaccinium 

 vitis-Maea, and Linnaea borealis, but no trace of Picea excelsa, which grows in 

 the region to-day. Beneath the layer of mud is a thin layer of earthy tufa, 

 containing leaves of Dryas octopetala, remains of Salix reticulata, S. arbuseula, 

 etc. The lowermost portion of this driest tufa lacks pine remains, but the 

 upper layers contain scattered examples, indicating that the pine forest 

 gradually replaced the Dryas arctic flora. The limiting layer of mud between 

 the two tufa deposits, as well as the basal one, and the humus of the present 

 time point to the prevalence of a drier chmate in which the springs dried up. 

 The author regards this as convincing evidence in support of his theory of the 

 alternation of moist and dry periods. The Norwegian tufa deports are not 

 the only evidences, for there are similar deposits of tufa throughout Europe. 

 He hence concludes that tufa and peat are formed in rainy periods, and just 

 as the tufa deposits correspond to layers of peat, so the interpolated layers of 

 mud in the tufa correspond to the layers of roots and stumps in the peat beds. 

 The birch tufa is thought to correspond with the infraboreal peat and with 

 the pine period of Steenstrup, the pine tufa with the Atlantic peat and the oak 

 period of Steenstrup, while the Dryas tufa seems to have been formed at the 

 beginning of the boreal period. 



Nehring (1892) has found the following layers in a peat bed of northern 

 Germany: 



The lower layer contained Naias, Potamogeton, CeratophyUum, Nuphar, 

 Cratopleura, Thalictrum flavum, Cladium, Ilex, Corylus, Quercus, Carpinus, 

 Tilia, Acer, and Betula. The middle layer was characterized by Cratopleura 

 (Nymphaeaceae), and isolated fruits or seeds of Carpinits and Picea. The 

 upper layer contained Betula, Salix, Vaccinium oxycoccus, Picea, Pinus, etc. 

 No distinct levels for birch, spruce, or pine could be determined. 



Artdersson (1893) advances the following generalizations from the investi- 

 gation of seven peat moors in southern Skane, Sweden. The order of inva- 

 sion of the principal trees is the same as that determined by Steenstrup for 

 Denmark. The cosere can be traced in unbroken continuity in several places : 



I. The arctic and the birch-aspen vegetation. The arctic bed beneath the 

 peat contains Salix polaris, S. reticulata and S. herbacea, Dryas, Betula nana, 

 Diaperma lapponica, Andromeda polifolia, and species of Potamogeton. A 

 subalpine flora and arctic peat are also indicated., Peat formation proper 

 began with the birch and aspen, which appear to form a short transition period 

 to the next. On the contrary, they may be wet place concomitants of the pine. 



II. Pine vegetation. With the pine are foimd Betula odorata, Populus 

 tremula, Salix aurita, cinerea, caprea, and repens, Cornus sanguinea, Corylus 

 avellana, Prunus padus, Rhamnus frangula. Viburnum opuLus, Alisma plan- 

 tago, Carex, Nuphar, Potamogeton, and Scirpus lacustris. The water plants 

 show the presence of open water in the moors. 



III. Oak vegetation. Qv£rcus pedunculata followed the pine, and consti- 

 tutes the thickest layer of the moors examined. Its associates are the same 

 as to-day. The older layers contain Betvla odorata, Populus tremula, Salix, 

 Corylus, Nymphaea, Pteris, etc., the later Alnus glutinosa, Fraxinus excelsior, 

 Tilia europaea, Viscum album, etc. 



IV. Beech vegetation. While beech grows near at hand, it is lacking in the 

 moor, a fact explained perhaps by the removal of the later layers of the peat. 



Blytt (1893) has summarized his theory of vegetation development in 

 response to alternating changes of climate: 



The alternation of geological strata makes it evident that the relative posi- 

 tion of land and sea has changed periodically. The displacements of the 



