82G MR FRANCIS J. LEWIS 



Summary. 



I described the principal stages, in the history of the vegetation over peat-covered 

 areas since the later stages of the glacial period, in former papers during 1905, 1906, 

 1907. These stages are as follow : — 



1. An arctic-alpine vegetation resting on the moraine laid down by the last mer 



de glace. 



2. A forest of birch and hazel. 



3. A layer of arctic -alpine plants occurring down to sea-level in Shetland. 



4. A forest of pine, hazel, and birch occurring up to 3200 feet above sea-level. 



5. A layer of peat accumulated from the period of stage 4 to the present day, con- 



sisting entirely of moorland plants. 

 The districts described in this paper amply confirm this sequence. Further in- 

 vestigations in Shetland have shown the existence of the distinct stages of peat growth 

 — one beginning with arctic plants lying on the glacial drift and containing the remains 

 of birch, alder, and rowan, succeeded by a period when alpine-arctic plants again 

 spread over the peat ; the other consisting of unstratified peat formed from moorland 

 plants accumulated during a period when the bogs greatly enlarged their areas, covering 

 the surrounding land. 



The observations from Rhilochan in East Sutherlandshire show practically the same 

 broad succession for that area, with the addition of an Upper Forest, which, although 

 most widely spread throughout Scotland, has not been found farther north. 



Special attention has been paid to the character of the two forest beds, and detailed 

 measurements have been made which, as far as they go, prove that the trees occur only 

 at definite horizons in the peat. The fossils collected from the Lower Forest show that 

 the conditions at that time were, as far as temperature is concerned, not markedly 

 different from those of the present day. The Shetland areas, however, show that the 

 forest passed far beyond the present tree limit, and indicate that entirely different 

 meteorological conditions obtained there at that time. 



The view is taken that the First Arctic Bed, Lower Forest, and Second Arctic Bed 

 represent distinct climatic phases during the early post-glacial stages. The Upper 

 Forest may represent a stage due to edaphic causes, but if this is so it is difficult to 

 account for the existence of this bed so far above the present tree limit, unless the trees 

 in that stratum had altogether different ecological requirements from similar species 

 now existing. 



The view held by Axkl Blytt that the forest beds indicate dry periods is not 

 upheld by the plants collected from that layer, although the evidence is in complete 

 agreement with Blytt (16) and Sernander (17) as to the fact that those beds 

 occupy definite horizons in the peat. The view brought forward by Gunnar 

 A.NDERSSON in 1909, that "stool occurrences (tree roots) in the peat mosses of Scandi- 

 navia represent all the phases of the Post-glacial Period, and that even if they are 



