316 SCIENCE PROGRESS 



why it should not occasionally have spread over the peat-covered 

 areas, but neither in the south of Scotland nor in the Hebrides 

 and Shetlands is any trace of it to be found. 



The Lower Peat Bog. — This zone is present in the low-lying 

 Wigtownshire mosses, and the other areas examined in the 

 Southern Uplands, in Skye, the Outer Hebrides, and in the 

 Shetlands. In the Southern Upland areas and the Shetlands 

 it exhibits the greatest uniformity, the bulk of the peat being 

 formed from the remains of Sphagnum, Scirpus ccespitosus, 

 Eriophorum vaginatum, Molinia ccerulea, Carices, E. angusti- 

 folium. Calluna is scarce or absent, and the peat areas at this 

 time seem to have formed extremely wet Scirpus and Sphagnum 

 moors, and trees appear to have been entirely absent. The 

 peat bears every sign of having been formed under conditions 

 eminently favourable for peat growth, and although this zone 

 frequently attains a thickness of 5 — 7 ft., it is probable that it 

 was formed in less time than the first arctic bed, which is only 

 18 in. in average thickness. 



In the low-lying mosses of Wigtownshire, Skye, and North 

 Uist a different type of flora overspread the peat during this 

 stage, for the lower peat bog there consists of the remains of 

 Phragmites communis, Equisctum sps., Menyanthes trifoliata. In 

 fact, these areas must have been covered with a series of shallow 

 lakes and swamps, possibly explained by the fact that the peat 

 rests upon stiff clays, whilst many of the Southern Upland 

 mosses rest upon sands and gravels. In Lewis, the lower peat 

 bog approximates more to the Southern Upland type, being 

 composed of Sphagnum, Eriophorum angustifolium, Erica 

 tetralix. 



A noticeable feature is the fact that the lower peat bog tends 

 to thin out as it is traced across Scotland from the south to the 

 north. This is a phenomenon of some interest, particularly 

 when considered in connection with the significance of the 

 succession of the peat strata. It is evident that these suc- 

 cessive beds represent changes in plant distribution on a 

 great scale and occupying presumably considerable spaces of 

 time. 



The lower peat bog stage undoubtedly represents a great 

 invasion of North Britain by swamp and bog plants which 

 previously had their centre either in the south or east of 

 Britain. If that be so, the arrival of this type of flora would 



