56 MR FRANCIS J. LEWIS 



beds remain the same. The most important point is the presence, in this area also, of 

 the lower forest and the intercalated arctic bed above. The presence of these arctic 

 plants above the birch forest affords a striking confirmation of the observations I made 

 in the south of Scotland, for the fact that these beds occur in the Shetland Islands tells 

 a still more emphatic story of widespread changes, owing to the isolation of the islands 

 from the mainland and their now unfavourable climatic conditions for forest growth. 

 The presence of a buried forest on the west Shetland coast below an arctic bed shows not 

 only that arctic conditions supervened after more genial forest conditions, but that the 

 Atlantic cyclones must have pursued an entirely different path when that forest spread 

 over the country, for the most favourable conditions of soil and temperature would not 

 permit forest under present climatic conditions, and indeed a more unfavourable position 

 for tree growth could hardly be found at the present time in North- West Europe. 



Previous Work in Britain on Pleistocene Plant-bearing Deposits. 



During the last thirty years much work has been done by various investigators upon 

 plant remains in Pleistocene beds, and such work is admirably summarised in The 

 Origin of the British Flora, by Clement Eeid. A detailed account would be out of 

 place here, but reference may be made to some of the districts yielding arctic plants. 



It is obvious that the number of arctic horizons in Pleistocene and post-Pleistocene 

 deposits cannot be indefinite, and therefore some of the arctic beds in peat and alluvial 

 deposits may be contemporaneous with either one of the arctic zones in the Scottish 

 peat mosses. For the purpose of this paper the post-glacial deposits containing arctic 

 plants group themselves into two sets : — 



(a) With arctic plants lying between an upper and lower temperate bed. 



(b) With arctic plants at the base overlaid by temperate deposits. 

 Type («) may be considered first. 



At Hoxne in Suffolk four distinct superposed strata containing plant remains have 

 been described by a Committee appointed by the British Association (6). 

 The sequence of the beds is as follows : — 



1 . Brick earth and gravel with temperate plants. 



2. Black earth with arctic plants. 



3. Lignite with temperate plants. 



4. Clay with temperate plants. 



Since the deposits fill a channel eroded through the chalky boulder clay the inter- 

 calated arctic plants must belong to late glacial times long after the general glaciation 

 of the country. 



It is sufficient to point out that the basal temperate beds contain such southern forms 

 as Lycopus europaeus, Rhamnus Frangula, Sparganium ramosum, Sambucus nigra, 

 Rosa canina, whilst the arctic bed above contains Salix polaris, S. herbacea, S. 

 Myrsinites, Betula nana, to show that there is definite evidence in the south of 



