384 PAST succession: the ceneosere. 



shore-line can be traced through all formations, and have produced corre- 

 sponding climatic changes, which find expression in the character of the 

 vegetation. 



In the oldest Norwegian peat-moors, the presence of four layers of peat, 

 frequently with three intercalated layers of trunk and root remains, is an 

 indication that three drier periods of climate alternated with four moist ones. 

 The regular repetition of these changes indicates a cause of imiversal character, 

 such as can not be found, at least for postglacial time, in the change of geo- 

 graphical relations. On the contrary, it is the regularly recurrent astronomical 

 periods which are determining in climate, and since such periods can be 

 recl^oned in years, we find in the alternating strata due to them the key to 

 geological chronology. 



The direction and strength of ocean currents is dependent upon astronomical 

 periods, but while the latter cause small climatic changes, the great modifica- 

 tions of climate seem rather to be due to changes in land surface, i. e., to defor- 

 mational changes in the earth's crust. Such was the effect of the elevation of the 

 submerged land-bridge which extended from Scotland to Greenland, The 

 evidence of plant geography makes it highly probable that such a bridge 

 existed during or after the Pliocene. The warm Gulf Stream was thereby 

 barred from the North Sea, which became a sea of ice. Conversely, when 

 warm currents were able to penetrate directly into the polar basin, a mild 

 climate in polar regions permitted the development of great forests, such as 

 actually existed during Tertiary times. 



In further studies of Swedish peat-moors, Andersson (1893) has concluded 

 that — 



Scyrbus aucuparia, Prunus padus, and Rkamnus frangvla appeared at least 

 as early as Pinus sikestris. In the last part of the pine period the invaders 

 were Tilia europaea, Ulmus montana, Alnvs glutinosa, Comus sanguinea, 

 Cwylus avellana, and Crataegus, though these reached their maximum in the 

 oak period. The pine appears to have invaded before the maximum of the 

 Ancylus period, when Scandinavia was still elevated. The pine period fol- 

 lowed the birch period, the remains of which rest upon an arctic bed of plants 

 that invaded the glacial soils and the exposed sea bottom after the with- 

 drawal of the late glacial sea. Andersson has also determined that the fossil- 

 bearing sand and clay layers of the river valleys of middle Norrland in Sweden 

 belong to two periods. The older were deposited in fresh water in the Ancylus 

 period. They contain remains of Pinus sUvestris, Alnus incana, Betula 

 odarata, and B. verrucosa, more rarely Populus tremula, Sorbv^ aucuparia, 

 Ulmus montana, Prunus padus, Rhamnv^frangula, Juniperus communis, Ridms 

 idaeus, Vacdnium vitis-'idaea, Ulmaria pentapetala, Oxalis acetoseUa, Comarum 

 palustre, Mantia fontana, Carex vesicaria, filiformis, and ampullacea, Nuphar, 

 Myriophyllum, etc. The younger layers are of the Idtorina period, deposited 

 in salt water. They contain Picea excelsa, Pinus silvestris, Alnus incana and 

 A. glutinosa, Betula odarata, verrucosa, and nana, Juniperus communis, Rubus 

 idams, Vacdnium vitis-4daea, Ulmaria pentapetdUi, ZannicheUia polycarpa, etc. 



Sernander (1894) has found the following sequence of zones in the moors of 

 Gotland: 



The marginal shallow portions exhibit Schoenus and Carex panicea. These 

 are followed usually by transition zones of C. filiformis and C. stricta. The 

 Myrica and Rhamnus communities form islands which spread concentrically, 

 and in the Schoenus areas appear commonly large areas of Molinia. The 

 central area consists of Cladium or of Cladium and Phragmites. The peat is 

 divided into grass-peat and moss-peat, and the latter into Sphagnum peat and 

 Amblystegium peat. The last played an important part in the composition 



