62 MR FRANCIS J. LEWIS 



But the most significant fact is that none of the characteristic arctic plants of the 

 First and Second Arctic beds are to be found in the Lower Forest Bed, though quantities 

 of material from the south of Scotland, the Hebrides and the Shetlands, have been worked 

 through. If it still be urged that there is no direct proof that the Lower Forest Bed was 

 accumulated under temperate conditions, one may ask what was the reason for the 

 disappearance of the characteristic plants of the First Arctic Bed during this period, and 

 why should these typical arctic species again appear as the strata above the Lower 

 Forest are reached ? 



The upper limit of tree growth has been used both in Britain and on the Continent 

 as the approximate boundary between the sub-alpine and alpine floras, and it is found 

 that a considerable difference in altitude separates the zone of natural woodland from 

 the alpine associations. 



From this, the First Arctic Bed appears to be the result of the depression of the 

 present alpine-arctic boundary in Britain to the extent of about 2000 feet, whilst the 

 Lower Forest represents the return of that boundary to at least its present altitude. 



The chief features in the distribution of the First Arctic Bed and the Lower Forestian 

 are shown in Plate II. The absence of these two beds from the Highland districts is 

 remarkable. Either conditions were not favourable for peat formation in those areas 

 during the First Arctic, the Lower Forestian, and the Lower Peat Bog period, or peat 

 formed during those periods was subsequently removed either by glaciation or by 

 atmospheric agencies. Further areas must be examined before it is possible to say 

 which view is correct. 



At the same time it must not be forgotten that water action frequently plays a great 

 part in the removal of peat beds. Many cases have come under my notice in various 

 parts of Scotland and the north of England where the older beds along a stream-side 

 have been swept away and replaced by mounds of sand and gravel. This is illustrated 

 in fig. 4, showing a sharp bend in the Abhuinn-a-Coire-Bhuig in Easter Eoss, where all 

 the older peat beds have been washed away and replaced with sand and gravel. This 

 event occurred before the Upper Forestian, and most probably during the Upper Peat 

 Bog stage. It is not, of course, suggested that the absence of the older peat beds in the 

 Highlauds can be due to water action of this character, but the example is given in 

 order to emphasise the many occurrences which may be responsible for the absence of 

 the older beds of peat in some districts. It is quite evident that peat formation is 

 dependent upon many factors, such as temperature, humidity of climate, drainage, 

 character of soil and the flora, or the peat deposits of Britain would not be so local in 

 distribution. 



No trace of Pinus sylvestris has been found in the Lower Forest, although it is the 

 most characteristic tree of the Upper Forest. Tf native in Scotland at that time it is 

 difficult to see why it should not occasionally have spread over the peat-covered areas, 

 but neither in the south of Scotland nor in the Hebrides and Shetlands is any trace 

 of it to be found. 



