STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 55 



between the trees was not the direct cause of their destruction ; but 

 accumulation in an increasingly moist climate might well contribute 

 indirectly by retention of moisture and thereby bringing about the 

 condition in which proper aeration of the roots could not take place. 



Lesquereux, Heer, Geikie, Grand'Eury and others have shown by 

 sections in Britain and central Europe this alternation of swamp and 

 forest conditions, while Steenstrup, Blytt, von Post, Andersson, Ser- 

 nander and others have made the matter abundantly clear for the 

 Scandinavian areas. ^"^ 



The causes of these alterations have been subject of much dis- 

 cussion, as they are among the most striking features of peat de- 

 posits. Andersson^' maintains that the presence of tree stumps in 

 a bog is not necessarily evidence of actual change in climate ; that 

 can be explained by the ability of peat bogs to invade forests and to 

 convert them into swamps, as has been proved by several Swedish 

 observers. But this familiar fact can explain only the presence of 

 trees rooted in the underclay; it does not explain the presence of a 

 forest layer with its roots wholly enclosed in the peat. This indi- 

 cates invasion of the swamp area by the forest. A change in direc- 

 tion or extent of drainage might answer well as explanation of local 

 appearance of trees, for only a slight lowering of the water-level 

 would suffice. But changes in drainage or the encroachment by 

 swamps, while accounting well for local variations in a swamp, great 

 or small, cannot suffice as explanation of widespread variation appear- 

 ing almost contemporaneously in immense areas. Some general 

 cause must be sought. Blytt and von Post have presented incontro- 

 vertible evidence of alternations in climatal conditions throughout 

 Scandinavia ; Lewis has done the same for Scotland as J. Geikie has 

 done for a wide area ; while Schreiber,^^ after detailed study within 

 the province of Salzburg, showed that the variations in bog life were 

 associated with climatal changes involving migration of the snow- 

 line in the Alpine regions. 



^<^ See " Formation of Coal Beds, II.," Proc. Amer. Phil. Soc, Vol. L., 

 pp. 604-613. 



5" G. Andersson, " Studier ofver Finlands torfmosser och fossila kvar- 

 tarflora," Bull. Com. Geo!, de Finlande, No. 8, 1898, p. 186. 



58 H. Schreiber, " Vergletscherung und Moorbildung in Salzburg." Sepa- 

 rate from Oester.-Moorzcitschr., Staab, 1912, pp. 14, 15. 



