THE PEAT CLISTASE. 



389 



Upon the basis of numerous phenological observations of the subfossil 

 plants concerned, Andersson obtains the following mean temperatures in C° 

 during the growing season: 



The last figures correspond approximately to the temperature conditions 

 which obtained at the time of the invasion of the birch forest into Sweden. 

 The dominance of the birch was short, for it was soon followed by the pine 

 which long controlled the forests. With the further rise of temperature the 

 oak appeared and extended further to the north than at present. The inva- 

 sion of the beech and spruce was relatively recent, and in consequence these 

 two dominants have not reached their cHmatic limits in Scandinavia. Their 

 late appearance is thought to be connected with an earher great extension of 

 the steppes to the west. 



Holmboe (1903) has summarized his earher work upon Norwegian peat 

 moors in an exhaustive treatise: 



Moors are found in all portions of Norway, forming altogether 3.7 per cent 

 of the total land surface. The depth seldom exceeds 6 to 7 m. As Blytt has 

 already indicated, moors at low altitude are commonly very shallow, but they 

 becom.e deeper with the rise of altitude, to the limit of late glacial subsidence. 

 The different kinds of deposits are distinguished as sand, mud, marine chalk, 

 slime, and drift. In these, remains are abundant, both of land and water 

 plants. The various kinds of peat are distinguished as moss-peat, including 

 Sphagnum peat and "fett"-peat, and vascular plant peat, Eguisetum peat, 

 rush-peat, reed-peat, sedge-peat, and heath-peat. The sequence of layers in 

 the moors gives an exact indication of changes of climate during their forma- 

 tion. Since the moisture can decrease or increase, there are distinguished two 

 kinds of different moor development characterized by the sequence. Many 

 moors have developed through the fiUing up of lakes and in consequence of 

 decreasing moisture. Various deposits shallow the water until swamp-plants 

 can develop in it, such as Sdrpus, Phragmites, Eguisetum, Iris, and AKsma. 

 With the increase of humic acid, the mosses, and especially Sphagnum, begin 

 to appear. The peat-moors which have been formed in some fashion through 

 swamping have in conunon the fact that the plant remains found in the lower 

 layers show greater moisture than those in the upper. In moors formed by 

 aJtemating moist and dry periods, the presence of forest layers seems conclu- 

 sive evidence of a climatic change, but it is then necessary to decide whether 

 this is due to periodic variation in rainfall or to local causes. A large number 

 of moors have been studied in profile, as well as in their present development 

 and the various horizons have been correlated. ' 



Warming (1904) has furnished a concise account of the development of 

 Danish vegetation since the glacial period: 



The first established formation was the arctic tundra, as still foimd in polar 

 lands. In Denmark, it has disappeared completely before heath, steppe and 

 forest. The steppe persisted for a short time relatively, giving way to scrub 

 and forest. The author regards heath as a prioaitive formation, older than 

 the forest. The succession of forests is that estabUshed by Steenstrup- 

 (1) willows and bu:ch, (2) pme, (3) oak, (4) beech. Warming dissents from 

 the view of Blytt and others that the species of a closed community migrate 



