356 MR FRANCIS J. LEWIS 



have caused considerable glaciation in the northern and mountainous districts. That 

 this cold stage was separated from the preceding glacial stage by a considerable period 

 is shown conclusively by the existence of the thick lower forest zone and overlyin^ 

 Sphagnum beds. This forest zone has been found in all the older peat hitherto ex- 

 amined in the South, its widespread character and thickness indicating a clear break 

 between the glacial deposits on which this peat rests and the intercalated Arctic plant 

 bed above. As the cold conditions which resulted in the extension of the local ice- 

 sheets and valley glaciers of the Southern Uplands (Fourth glacial stage) passed away 

 an Arctic flora would linger on over these upland valleys and hills — it may be for a 

 considerable time, — but nothing except a decided climatic change could bring such a 

 flora back again after the whole of the South of Scotland had become clothed with 

 temperate forest. The existence of these intercalated Arctic plant beds in the Southern 

 Uplands therefore strongly supports the view, already established from the geological 

 evidence — that the Mecklenburgian or Fourth glacial stage, whose glaciers deposited 

 the prominent sets of moraines at a general level of 900-1200 feet in so many of the 

 Southern Upland valleys, was followed by a genial interglacial period, represented by 

 the lower buried forest of birch and hazel, with accompanying temperate plants. This 

 gradually came to an end, its passing away being chronicled in the peat by thick beds 

 of Sphagnum, representing wet and cold conditions, — and was succeeded by Arctic 

 conditions, during which an Arctic flora flowed downwards and southwards from the high 

 northern ground to which it had been forced during the genial lower forest period, and 

 covered all the lower ground of the South of Scotland. 



The evidence afforded by the peat in those districts which have been examined in 

 the Highlands is no less conclusive. 



All the deposits examined at high elevations and in the extreme North begin their 

 history at a stage later than those at low elevations in the South. In the northern 

 deposits the lower forest bed, with the overlying Sphagnum beds, is entirely wanting ; on 

 the other hand, all the upper beds of the Southern Uplands, from the intercalated Arctic 

 zone to the present surface of the peat, find their representatives in the Highland peat. 

 These Highland deposits began to form at the passing away of the cold conditions which 

 gave rise to the intercalated Arctic bed of the Southern Uplands ; in other words, the 

 Arctic plants in the Southern Uplands represent the whole of the Fifth glacial episode, 

 whilst the basal Arctic bed in the Highlands represents the passing away of that phase. 

 That milder conditions again supervened after this stage is shown by the existence of 

 the upper forest-bed in all the Eastern and Northern Highland districts, — this corre- 

 sponding with the upper forest zone of the Southern Uplands. It is interesting to note 

 that Betula alba is the dominant tree in the lower buried forest, whilst Pinus sylvestris 

 is always dominant in the upper buried forest. This may not indicate any decided 

 difference in the climate during the two forest periods, but it does indicate a considerable 

 break between the two periods. 



It is perhaps too soon to attempt the complete correlation of these different strata 



