52 MR FRANCIS J. LEWIS 



that the peat bluffs along stream sides show a band of peat formed from aquatic plants 

 lying below the first arctic bed, either marking the position of a large pool or a former 

 stream channel whose margin must have been fringed with M enyanthes and Potamogeton 

 whilst the surrounding moorland was covered with willow and dwarf birch. 



The arctic bed is overlaid first by 14 inches of Sphagnum and Eriophorum remains, 

 and then by a thick bed of Betula alba and Corylus Avellana. The trees are large, and 

 represent what must have been a forest growing over the remains of the primitive 

 arctic vegetation. 



But of still greater interest is the fact that above the forest remains the peat shows 

 a return to arctic conditions by the occurrence of Salix reticulata, Salix Arbuscula, 

 Empetrum nigrum, and Betula nana, in a perfectly distinct bed lying over the birch 

 forest. This is the same sequence as I found in the Southern Uplands of Scotland, where 

 an arctic plant bed overlaid the birch forest. There can, I think, be no doubt that the 

 intercalated arctic bed above the birch forest in Shetland represents the same epoch as 

 the intercalated arctic bed above birch forest in the Southern Uplands. 



A distinct change of climatic conditions is necessary to produce a transition from 

 moorland and forest vegetation to moorland covered with creeping willow and other 

 arctic plants ; and further, such climatic conditions must have operated over large areas. 

 If that be so, the arctic zones in the peat are true horizons, and therefore a trustworthy 

 guide to the comparative age of the beds lying below and above them. 



The sections here contain two arctic beds ; the Merrick-Kells district and Tweeds- 

 muir contain only one. But the intercalated arctic bed above the birch forest is present 

 in all, and I have little doubt that extended search would yield the basal arctic bed in 

 the Southern Uplands. Indeed, it is quite certain that the absence of the basal arctic 

 plants from districts in the south of Scotland is due to the imperfection of the record in 

 the places examined, for we may be sure that a forest of birch, hazel, alder, with 

 accompanying temperate plants, did not immediately follow the retreating ice. 



As the birch forest in Shetland occurs below the intercalated arctic plants, it clearly 

 belongs to the lower forest period, and is contemporaneous with the birch of the districts 

 in the south of Scotland. 



The lower forest bed is the only evidence of woodland conditions met with in the 

 peat of Shetland, for, instead of the intercalated arctic plant bed being overlaid with the 

 remains of the upper forest as in the south of Scotland, it is covered with a thick bed of 

 dense structureless peat crowded with the stems of Calluna. This stratum bears every 

 sign of having been accumulated under conditions which were not favourable for the 

 rapid growth of peat. The Calluna stems are much shrunken, and frequently the 

 interior of the stems has disappeared, only leaving the epidermis. Further, the peat 

 itself is similar to the black crumbly peat now met with on the sides of old peat-hags, 

 save that pressure and age have consolidated it into a hard mass. As this layer is 

 traced along the banks of streams it is found to be of unequal thickness ; carefully 

 examined, it closely resembles the appearance of buried peat-hags. From an examina- 



