824 MR FRANCIS J. LEWIS 



districts are homotaxial, or whether they represent stages in the evolution of the 

 vegetation on peat due solely to ecological causes. This is really a most important 

 problem, and upon it hangs the whole question of the stratification of the peat. If 

 Blytt's conception of each forest bed as representing a dry period could be proved, then 

 indeed we have a proof of the contemporaneity of these strata ; but at any rate in the 

 case of the Lower Forest the evidence from this country does not give any support 

 to that theory. How then is it possible to show with any degree of certainty that 

 forest beds in two widely separated districts are really contemporaneous. 



From the details which have already been given, it will be seen that the Lower 

 Forest at any rate does not necessarily indicate any important difference in temperature 

 from that of the present day, but its distribution in Shetland indicates entirely different 

 meteorological conditions. In the south of Scotland the forest which is correlated with 

 the Shetland stratum lies within the present altitudinal range of trees, although the 

 areas in which it occurs are at present treeless. The absence of trees may, however, 

 be caused by many factors other than changed meteorological conditions, such as the 

 grazing of sheep and the influence of man. In the correlation of these two areas, then, 

 we have in the south of Scotland a buried forest within the present range of tree 

 distribution, and indicating no important change in climate ; in Shetland, a forest which 

 lies outside the present limits of woodland. Obviously the most detailed study of the 

 flora contained in these two widely separated strata would not prove or disprove their 

 contemporaneity, since both contain temperate forest trees and plants which might be 

 found in any lowland deciduous wood at the present time. The relation of these two 

 beds to the strata above and below is the important point. We know that before the 

 existence of either forest there was a cold period or series of cold periods marking the 

 glacial epoch. In the south of Scotland the Lower Forest rests upon the last moraine 

 laid down, in Shetland it rests upon peat formed of the arctic plants which marked the 

 passing away of that period. There is nothing remarkable in this ; peat formation 

 depends upon the delicate adjustment of several factors, such as soil, drainage, tempera- 

 ture, and precipitation — especially humidity of the air. In some parts of Sweden the 

 arctic vegetation which flourished during the later stages of the glacial epoch has not 

 been preserved as peat, although it is found in fresh-water clays underlying considerable 

 thicknesses of later formed peat. In both the south of Scotland and in Shetland an 

 arctic bed overlies the forest zone, so that in both cases the forest lies between strata 

 indicating without doubt much colder conditions than the present. If the forest bed 

 in both districts occurred between two strata of Sphagnum peat, then any certain 

 correlation would be more difficult ; but the changes which brought about the replace- 

 ment of temperate forest by arctic plants in one area must also have operated over 

 the other. It is quite impossible that so many climatic fluctuations occurred after 

 the deposition of the moraine that the arctic bed intercalated in the peat of S. Scotland 

 should not be contemporaneous with that intercalated in the Shetland peat. It is only 

 upon such similarity in the stratification that any sure correlation of the forest beds 



