66 MR FRANCIS J. LEWIS 



are found. The Vaccinium-Calluna association, now so constant a feature on moorlands 

 in Scotland and England, appears to be a comparatively recent feature. 



The Peat above the Upper Forest. — The peat of varying thickness lying above the 

 Upper Forest presents the same features as the Lower and Upper Peat Bog. Occasional 

 beds of Calluna or Eriophorum make their appearance, but are not continuous over 

 large areas, and are evidently due to local causes. The general flora is of a distinctly 

 wetter type than that characteristic of many of the peat areas of the present time, as it 

 consists mainly of Scirpus csespitosus and Sphagnum. Although this zone does not 

 indicate any widespread changes in the distribution of the flora it shows some features of 

 oecological interest in connection with the present denudation of the peat ; these will be 

 dealt with in another paper. 



The Relation of the Strata to the Glacial Succession. 



It is evident that the peat mosses do not give any information about the chain of 

 events during the maximum glaciation of the country, and it is immaterial whether we 

 regard the morainic material upon which so many of the older peat mosses in the south 

 of Scotland rest, as the deposits of the waning ice-sheet of maximum glaciation, or as 

 the deposits of an entirely distinct glacial stage, separated from the ice-sheet by a warm 

 inter-glacial phase. The fact remains that the First Arctic Bed contains an arctic-alpine 

 flora which existed over wide areas near sea-level. It is of course impossible to say 

 from the evidence of the plants whether all traces of glaciation had vanished from 

 Britain at that time, or whether certain regions were still under ice. An entirely 

 different flora makes its appearance in the Lower Forestian ; not only are all arctic- 

 alpine plants absent, but the flora is made up of well-grown trees and an assemblage 

 of plants quite typical of any marshy lowland forest region in Britain at the 

 present time. 



If it be contended that the Lower Forestian is due to local changes in climate — such 

 as variation in snowfall, direction of wind — how is it that the bed maintains its 

 character, not only in the south of Scotland, but also in the Outer Hebrides and Shetland 

 Islands ? 



If the succession of the First Arctic by the Lower Forestian, Lower Peat Bog and 

 Second Arctic Bed were really due to local causes, widely separated districts should show 

 different successions ; while, for instance, tundra vegetation would be represented in one 

 district the remains of a forest vegetation would occupy the same horizon in another. 

 But this is not the case, for nothing is more striking than the continuity of the horizons. 

 The chief points for consideration may be stated as follows ; — 



(a) First Arctic Bed. Lower limit of arctic-alpine vegetation depressed nearly 

 to sea-level. 



(b) Lower Forestian. Upper limit of deciduous trees raised to at least its present 

 level. 



