68o JAMES GEIKIE 



Finally, the conditions again became adverse to forest growth, 

 and the trees of this Upper Zone were gradually buried under a 

 stratum of peat, consisting chiefly of rushes, bog-moss, and cotton- 

 grass. ' 



In comparing the peat-mosses of the Southern Uplands with 

 those of the Northern Highlands, Mr. Lewis finds that the latter 

 begin their history at a later stage than the former. At high levels 

 in the highlands none of the beds underlying the zone of arctic 

 plants in the Southern Uplands puts in an appearance. The reason 

 for this is obvious. The recurrence of cold conditions, indicated 

 by the arctic plants of the Southern Uplands, was more strongly 

 marked in the Northern Highlands. In those elevated regions 

 considerable snow-fields and glaciers reappeared, and all peat-beds 

 representing the lower forest zone of southern Scotland were swept 

 away. As these glaciers in the north began to retreat, a tundra 

 vegetation invaded the formerly glaciated tracts. Arctic willows 

 {Salix reticulata and S. herbacea) at first were dominant forms, 

 but these gradually gave place to subarctic types (Salix Arbuscula, 

 Betula nana, Empetrum nigrum, etc.). By and by this subarctic 

 brush-wood disappeared, and was succeeded by a close growth of 

 cotton-grass and bog-moss, interspersed with some scraggy birch. 

 That the climate eventually became more humid is suggested by 

 the fact that the birch in its turn vanished, and sphagnum alone 

 continued for a long time to occupy the ground. These wet moor- 

 land conditions next passed away — the thick sphagnum peat drying 

 up, and eventually supporting a forest of large pines, with an under- 

 growth of common heather. The great pine forests of this Upper 

 Zone, it may be mentioned, flourished at elevations between 2,000 

 and 3,000 feet above the present sea-level. Finally they decayed, 

 and were gradually buried under peat consisting chiefly of bog- 

 moss and rushes. 



My limits will not allow me to enter into other interesting evidence 

 adduced by Mr. Lewis. I may just mention, however, that he 

 finds everywhere evidence that existing conditions no longer favor 

 the general growth of peat. On hilltop, hillside, and in upland 

 valleys alike, the peat, he says, is almost without exception being 

 rapidly wasted away.%^The vegetation at present covering the peat 



