''POSTGLACIAL FORMATIONS" OF SCOTLAND 679 



plants. It is quite possible, he thinks, that the structureless peat 

 referred to may represent the primitive vegetation of the district.^ 

 I may remark that similar structureless peat not infrequently under- 

 lies the lower forest-zone in the peat-bogs of Scandinavia. 



The birch zone is directly overlain by a thick stratum of peat, 

 composed entirely of bog-moss (sphagnum). This succession is 

 constantly repeated throughout the Southern Uplands — alike in the 

 peat at the bottoms of valleys, and in that upon steep hillsides and 

 fiat hilltops. The sphagnum bed thus bears witness to a general 

 increase of precipitation — it represents, in short, a change from 

 birch-forest conditions to wet moorland. 



As successive layers of the peat are followed upward, Mr. Lewis 

 finds that the bog-moss gradually gives place to cotton-grass {Eri- 

 ophorum vaginatum) and rushes (Scirpus). After these plants 

 had flourished for some considerable time, a decided change of 

 climate supervened. In the Merrick district the cotton-grass peat 

 is covered by a dense layer of the stems of crowberry (Empetrum 

 nigrum) J and two characteristic Arctic willows {Salix herhacea and 

 S. reticulata). The same zone is represented in the peat of Tweeds- 

 muir by the crowberry, and the creeping azalea (Loiseleuria pro- 

 cumhens) — the latter being a typical Arctic form. The constant 

 appearance of this remarkable zone throughout the Southern Uplands 

 can have only one meaning — it points unmistakably to a decided 

 decrease of temperature. It indicates a stage during which the valleys 

 of Southern Scotland were characterized by a climate as rigorous as 

 that now experienced on the summits of our loftiest mountains. 



The gradual dying-away of this cold epoch, and the reappearance 

 of forest vegetation, are, according to Mr, Lewis, faithfully chron- 

 icled by the peat. The crowberry, the arctic willows, and the creep- 

 ing azalea give place above to cotton-grass, and this in its turn to 

 bog-moss or sphagnum — a succession common to the peat through- 

 out the Southern Uplands. Eventually the wet moorland conditions 

 indicated by the sphagnum peat passed away; the bogs dried up, 

 and were invaded by trees — by forests of pine in the Merrick district 

 and forests of white birch in Tweedsmuir. 



I The evidence wanting at the base of the peat is supphed by the lacustrine allu- 

 via of the central Lowlands already referred to. See p. 5. 



