38 MR FKANCIS J. LEWIS 



Section III. : — 



Characteristic Plants. 



1. Scirpus c&spitosus. 



2. Stiff structureless peat, having apparently been 



much exposed to denudation before being 

 covered by the overlying Scirpus. 



3. Calluna peat. 



4. Betula alba (scrub). 



5. Empetrum nigrum. 



Accompanying Plants. 



1. Sphagnum. 

 2. 



3. 



4. 



5. Salix Arbuscula. 



Peaty sand and rock. 



An interesting feature here is the frequent presence of peat underlying the 

 Empetrum and Salix Arbuscula and S. reticulata zones. An examination of this basal 

 layer in the field showed no recognisable plant remains, but samples tested with nitric 

 acid in the laboratory yielded seeds of Potamogeton natans, P. preelongus, P. 

 rufescens, Menyanthes trifoliata, Ranunculus repens, R. fiammula — in fact, the remains 

 of an aquatic vegetation. All these species have a wide distribution at the present 

 time, but their presence at the base of the peat suggests that the summers of that time 

 were fairly warm and were long enough to allow these aquatic plants to seed freely. 

 The shallow character of this bed and its absence in many of the sections rather points 

 to its belonging to the same stage as the overlying Salix and Empetrum beds. The 

 same feature has been met with underlying the arctic bed in the Shetlands, and its 

 significance will be discussed more fully when dealing with that area. 



The general succession of vegetation here compares very closely with the succession 

 in the Inverness-shire, Easter Eoss, and Caithness-shire areas described in 1906 (3). An 

 important difference is the absence of the upper forest bed. This zone is generally 

 present in the south of Scotland and in the Highlands ; it has been noted as far north 

 as the valley of the river Dionard immediately to the south of this district, and is 

 occasionally present in the peat on the moors round Loch Eriboll, but it has not been 

 found in any of the peat areas examined in the Hebrides and the Shetlands. During 

 the examination of the peat on the Caithness- Sutherland border the upper forest zone 

 is generally found to be present but the pine is frequently replaced by birch. The 

 northern boundary of this forest bed is evidently approached in the Cape AVrath district, 

 as here, pine remains tend to become stunted and birch frequently takes its place. In 

 most districts the upper forest bed dies out before the northern coast is reached. 



The scrub birch described from this district must not be confused with the lower or 

 the upper forest bed, for the wood in all cases is very small, not plentiful, and is always 

 mixed with quantities of Eriophorum, Sphagnum, and Scirpus remains. Moreover, 

 it does not correspond in position to either the lower or upper forest bed. The 

 sections show that the basal Salix and Empetrum beds were gradually replaced by peat 

 bog associations, amongst which scrub birch managed to find a footing. 



As the peat deepened, the birch disappeared and Eriophorum and Sphagnum 

 became dominant over the area. That conditions at this period were not suitable for 

 the growth of birch forest is shown by the fact that the birch remains are often absent 



