PLANT REMAINS IN PEAT MOSSES 317 



be much later in the extreme north 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 is repre- 

 sented in every district so far examined from the Southern 

 Uplands to the Shetlands. 



, In the south of Scotland the zone contains Salix reticulata, 

 S. herbacea, Loiseleuria procumbens, Empctrum nigrum, Arcto- 

 staphylos alpina, but 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 High- 

 lands 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., Equisctum sps., Empetrum nigrum, Potentilla Comarum, 

 Viola palustris, Menyanthes trifoliata. 



In the district near Cape Wrath these arctic 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 repcns, Potamogeton sps., 

 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 districts lying in the extreme west, such as Skye, North 

 Uist, and Lewis, the second arctic bed is not recognisable, and 

 the first arctic bed is but poorly represented. In fact, the two 

 successive waves of arctic vegetation do not seem to have pene- 



