392 



PAST succession: the ceneosere. 



"It is evident that the peat mosses do not give any information about the 

 chain of events during the maximmn glaciation of the country, and it is imma- 

 terial whether we regard the morainic material upon which so many of the 

 older peat mosses in the south of Scotland rest, as the deposits of the waning 

 ice-sheet of maximimi glaciation, or as the deposits of an entirely distinct 

 glacial stage, separated from the ice-sheet by a warm interglacial phase. The 

 fact remains that the First Arctic Bed contains an arctic-alpine flora which 

 existed over wide areas near sea-level. It is of coutse impossible to say from 

 the evidence of the plants whether all traces of glaciation had vanished from 

 Britain at that time, or whether certain regions were still under ice. An 

 entirely different flora makes its appearance in the Lower Forestian; not only 

 are all arctic-alpine plants absent, but the flora is made up of well-grown trees 

 and an assemblage of plants quite typical of any marshy lowland forest region 

 in^Britain at the present time. 



fltCENT PE-AT 



UPPLR FORE.STIAN 



upper peat b06 



Second arctic bed. 



Lower peat bo5 



Lower forestian... 

 fiRST Arctic bed—. 



Wigtownshire. 



Grampian Mts. 







^S^p 



li^^i^^^s^;c?3;;l 



Fig. 42. — ^Portion of diagram by Lewis, illustrating layers of costase in Scottish peat-bogs. 



"If it be contended that the Lower Forestian is due to local changes in 

 climate — such as variation in snowfaU, direction of wind — how is it that the 

 bed maintains its character, not only in the south of Scotland, but also in the 

 Outer Hebrides and Shetland Islands? 



"If the succession of the First Arctic by the Lower Forestian, Lower Peat 

 Bog, and Second Arctic Bed were really due to local causes, widely separated 

 districts should show different succesisons; while, for instance, timdra vege- 

 tation would be represented in one district the remains of a forest vegetation 

 would occupy the same horizon in another. But this is not the case, for 

 nothing is more striking than the continuity of the horizons. The chief points 

 for consideration may be stated as follows: 



"(a) First Arctic Bed. — Lower limit of arctic-alpine vegetation depressed nearly to sea- 

 level. 



"(6) Lower Forestian. — Upper limit of deciduous trees raised to at least its present level. 



"(c) Second Arctic Bed. — Lower limit of arctic-alpine vegetation depressed nearly to 

 sea-level. 



"(d) Upper Forestian. — ^Upper limit of trees raised to over 3,000 feet above sea-level. 



" These changes are post-glacial in the sense that they occurred later than 

 the last ice-sheet. It is difficult to reconcile these changes with the ' gradually 

 waning glaciation, dtiring which there were occasional local advances of the 



