ON THE PLANT REMAINS OF THE SCOTTISH PEAT MOSSES. 807 



still living, growth has been very slow, and none of them reach above the level of the. 

 sheltering wall. None are vigorous ; they bear many dead branches, and even the best 

 have but few leaves. Many of them show nanism ; the main stem having bent down- 

 wards gives rise to small distorted branches nearly bare of leaves. It may be mentioned 

 that this spot is situated 80 feet above sea-level in one of the most sheltered places in 

 the island. It is absolutely impossible that the well-grown trunks of birch found in 

 exposed places at 700 feet could have grown there under meteorological conditions 

 similar to those obtaining there at present. 



The sections described from Shetland Mainland and Foula show the general 

 sequence to be found in those areas, and the strata may be briefly reviewed from a 

 phytogeographical point of view. 



The First Arctic Stratum in Shetland. — Though this layer represents the earliest 

 peat found in Shetland, it does not follow that its growth took place immediately after 

 the retreat of the glaciers which deposited the boulder clay. The newer deposits of 

 peat on the Mainland were clearly deposited a considerable time after the deeper and 

 older peat strata began their growth, and they contain no arctic plants at the base ; but 

 the vegetation which existed over those areas between the deposition of the boulder 

 clay and the first formation of peat has left no trace, except exceedingly small frag- 

 ments of birch bark. In the same way, the period immediately following the deposi- 

 tion of the boulder clay may not have been favourable for peat growth, and so there 

 may possibly be a hiatus between the boulder clay and the arctic plant bed. But there 

 is no direct evidence that this is the case ; the boulder clay lying under the arctic bed 

 appears to show signs of weathering, but this point was not easy to determine with 

 certainty, owing to the depth of peat which had to be excavated, and the difficulty of 

 keeping water out after the glacial deposits were entered. The find of Pinus sylvestris 

 pollen grains below the surface of the boulder clay, although interesting in itself, is not 

 sufficient evidence that pine occurred in Shetland during that time, owing to the 

 distances that pollen may be carried by wind. The occasional presence of aquatic 

 plants below the first arctic bed is an interesting feature, but is hardly sufficient 

 evidence to bridge over a hypothetical gap between the time of the boulder clay and 

 the arctic plants above. Moreover, the leaves of Sahx reticulata are occasionally 

 found amongst the aquatic plants, suggesting that they were laid down in shallow 

 pools round which grew the willow and other arctic plants. I described this feature 

 in a former paper (3), and my last visit to Shetland did not result in any evidence 

 which would tend to show that the Potamogeton, Menyanthes, Ranunculus, and 

 Equisetum which sometimes underlie the lowest arctic plants, belong to a separate 

 period. The presence of Diatoms and Desmids in the boulder clay, however, proves 

 that a fairly rich fresh-water flora existed in pools on the boulder clay before the 

 arctic vegetation began to be preserved in the form of peat. 



After the growth of peat began by the accumulation of the remains of willow, etc. , 

 we have a fairly continuous record of the succession to the present day. The two 



