64 MR FRANCIS J. LEWIS 



of the peat strata. It is evident that these successive beds represent changes in plant 

 distribution on a great scale, or occupying presumably considerable spaces of time. The 

 Lower Peat Bog stage undoubtedly represents a great invasion of North Britain by 

 swamp and bog plants which previously had their centre either in the south or east of 

 Britain. If that be so, the arrival of this type of flora would be much later in the extreme 

 N. of Scotland than, for instance, in the Southern Uplands. It is perhaps not without 

 significance that in Shetland the Lower Peat Bog thins out to a bed only a few inches 

 in thickness or a layer of diatomaceous earth. The centre of distribution of these 

 swamp and bog associations could hardly have been in the north of Britain during the 

 growth of the lower forest, and the farther the areas were from this centre the longer it 

 would be before they were colonised. 



The Second Arctic Bed. — The Lower Peat Bog is succeeded by a flora composed 

 mainly of typical arctic plants and, as will be seen from Plate II. , is represented in 

 every district so far examined along the line A B on Plate I. from the Southern Uplands 

 to the Shetlands. In the south of Scotland the zone contains Salix reticulata, S. 

 herbacea, Loiseleuria procumbens, Empetrum nigrum, Arctostaphylos alpina ; but in 

 the Highlands and in the Shetlands Salix reticulata, Betula nana, Empetrum nigrum, 

 Salix Arbuscula, Erica Tetralix. In these areas the Second Arctic Bed occurs between 

 the Lower and the Upper Peat Bog, but in the areas which lie within the Highlands no 

 older beds lie below this zone, and it rests directly upon sands, gravels, and clays. In 

 such districts the bed is thicker and contains a richer flora than in the Shetlands and 

 south of Scotland. 



In the areas stretching from the Grampian mountains to Cape Wrath the peat of 

 this bed contains Dryas octopetala, Salix reticulata, S. herbacea, Arctostaphylos 

 alpina, Betula nana, Veronica alpina, Lychnis alpina, Salix Arbuscula, Carex sps., 

 Equisetum sps., Empetrum nigrum, Potentilla Comarum, Viola palustris, Menyanthes 

 trifoliata. 



In the district near Cape Wrath these plants are frequently underlaid by a thin 

 seam of peat a few inches in thickness, composed of the remains of aquatic or marsh 

 vegetation, consisting of Ranunculus repens, R. Jlammula, Potamogeton natans, 

 P. prsdongus, P. rufescens, Menyanthes trifoliata. This seam is not continuous but 

 appears to underlie the Salix beds in small patches and frequently in depressions of 

 the ground, and marks the position of small swampy patches during the Second 

 Arctic Period. 



In the districts of the extreme west, along the line B C, Plate I., such as Skye, N. 

 Uist and Lewis, the Second Arctic Bed is not recognisable, and the First Arctic Bed is but 

 poorly represented. After the close of the Lower Forestian the general type of vegeta- 

 tion over these peat mosses seems to have remained very uniform, through the Lower 

 Peat Bog, Second Arctic Stage, and the Upper Forestian to the present day. During 

 those periods the mosses in the Hebrides must have resembled the tundra of Northern 

 Siberia described by Sewell (15). 



