68 



MR FRANCIS J. LEWIS 



Peat Bog would mark the gradual passage of the Mecklenburgian stage into the Lower 

 Forestian, the Second Arctic Bed would represent the Lower Turbarian, and the Upper 

 Forest in the peat would correspond with the Upper Forestian or Fifth inter-glacial 

 stage. 



Summary. 



All the Scottish peat mosses show a definite succession of plant remains. The 

 oldest, in the south of Scotland and the Shetland Islands, have an arctic plant bed at 

 the base. This is succeeded by a forest of birch, hazel, and alder containing temperate 

 plants. A second arctic plant bed occurs above the Lower Forest and is overlaid in all 



5ea /ere/ 



Lower limit of 

 I" Arctic be J. 



Upper limit of 

 lower forest 



Lower limit of 

 Z°* Arctic bed 



Upper limit of 

 Upper foresf 



Fig. 5.— Broken lines represent the present limits of Arctic-Alpine and forest vegetation ; thick 

 lines show the limits attained by these formations during the growth of the peat. 



districts (except the Hebrides, Cape Wrath, and the Shetland Islands) by an Upper 

 Forest covered by several feet of peat-bog plants. 



The altitudinal limits of these successive zones is shown in fig. 5. It will be seen 

 that the two arctic beds descend to within about 150 feet of sea-level, whilst the Lower 

 Forest rises to 1500 feet and the Upper Forest to close upon 3500 feet. The present 

 lower limit of an arctic-alpine vegetation in Britain is taken as 2000 feet, and the upper 

 limit of forest (pine and birch) as 2000 feet, although in many districts it lies consider- 

 ably lower than this. The upper limit of the Lower Forest is given at 1500 feet, owing 

 to the fact that all the beds below the second arctic zone are wanting above this 

 altitude ; but as the Lower Forest is present in many exposed situations, such as the 

 west coast of Shetland, it is probable that the upper limit of forest lay considerably 

 above this level. 



