Chapter IX —159— On Migration 



became grown over with forests. Alternating with these dry periods 

 there were humid periods, when the forests perished and their place 

 was occupied by bog moss (Sphagnum) and the peat deposits again 

 steadily grew over long periods of time. 



Steenstrup (cited by Blytt) established in peat deposits in Den- 

 mark four layers, corresponding to four stages in the history of the 

 invasion of this region by vegetation. The lowest layer is characterized 

 by the predominance of aspen leaves {Populus tremula); above it lies a 

 layer in which debris of pine trunks predominates; above that a layer 

 in which the oak, Quercus sessiliflora, predominates; and, lastly, in the 

 uppermost layer we find the alder (Alnus glutinosa) to be predominant. 

 He designates these layers, found in the same sequence in many peat- 

 beds in Denmark, as the aspen, pine, oak, and alder layers. The con- 

 clusion necessarily drawn is that these four layers in the peat-beds of 

 Denmark correspond, in the sequence of their stratification, to the four 

 layers in the peat-beds of southern Norway. Analyzing these data, 

 Blytt considers it possible to presume the following sequence in the 

 alternation of climatic conditions in Denmark and southern Norway: — 



/. End of Ice Age. Climate humid. 



2. .\rctic flora: Dryas, arctic willows and birches. Climate dry, continental, arctic. 



J. Peat with leaves of aspen and birch {Betula odorala). 



4. Remains of roots and other parts of forest trees. 



5. Peat with remains of Piniis. 



(During the deposition of the 3rd, 4th, and sth layers there occurred the invasion of 

 these regions by a subarctic flora.) 



6. Remains of trunks and other parts of forest trees, including the hazelnut, oak, and 



other heat-loving species. (Invasion by boreal flora.) 



7. Peat with remains of the trunks of the oak, Quercus sessiliflora, which at that time was 



more widely distributed than now, attesting a warmer cUmate. (Invasion by 

 Atlantic flora.) 



8. Remains of roots and other parts of forest trees. (Invasion by sub-boreal flora.) 

 g. Peat — almost pure Sphagnum. (Invasion by sub-Atlantic flora.) 



10. Contemporary. The peat bogs are, for the most part, dried up and partially over- 

 grown with forests; a new layer of roots is in process of formation, which will again 

 be buried under layers of peat as soon as there ensues a new pluvial period. {Con- 

 temporary flora.) 



This alternation of climatic periods Blytt associated with changes 

 in the direction of ocean currents, caused by shiftings of the Scandi- 

 navian shield and consequent changes in the temperature of the water 

 of the seas washing the coasts of Scandinavia. These views have been 

 confirmed by investigations as to variations in the level of the North 

 and Baltic Seas. 



Blytt's theory was definitely substantiated by the investigations of 

 peat-beds made by Sernander (1892, 1910) and his followers, who 

 employed very exact and improved methods of investigation. Basing 

 himself on the fundamental principles of Blytt's system, Sernander 

 gives his own scheme of the alternations of climatic periods, in con- 

 structing which he also employs archaeological data. Besides the fact 

 that the alternations in cUmatic periods are provided with more sub- 

 stantial proof, Sernander's scheme has the advantage of being simpler 

 than Blytt's. This scheme, known as the Blytt-Sernander scheme, 

 with some modifications based on subsequent investigations, is at 

 present the one most widely accepted. However, many other view- 



