1886.] ON YARIATIONS OF CLIMATE IN THE COURSE OF TIME. 



9 



bogs has not growu very much within historical times, and tliat 

 the layer of stumps of trees, which are foimd on the surface 

 in the knolls, indicates an arrest of the growth of the peat, 

 the duration of which may probably be measured by many hun- 

 dreds, perhaps by thousands of years. It might be argued 

 that the present drier state of the bogs was simply due to the 

 circumstance that the peat had grown so high that the moisture 

 had run off. But this is not an acceptable explanation, because 

 if we bore deeper in the peat, we find that the oldest hogs are 

 huilt of four layers of i)eat. and hetwcen these stand tJiree layers 

 of stumps^ so that these bogs are for the fourth time covered 

 with trees since they began to form. And as most of the hogs, 

 if not all are at present drier than they were hefore^ the theory 

 of merely local variations of the moisture is also insvfficient to 

 explain the phenomena. It remains, therefore, only to assume, 

 that periods of dry and wet have alternated during ages. The 

 peat layers generally belong to the latter, and the stump layers 

 speak of drier periods, when the bogs were covered with trees. 



Of these four layers of peat, which in some places measure 

 upwards of twentysix feet in thickness, only the two youngest 

 enclose, as far the researches in Norway go to show, remains of 

 foliferous trees sensitive to cold. And this justifies the assump- 

 tion that they correspond to the four layers which Steenstrup 

 has shown in the bogs of Danemark, and which appear like 

 geological strata with distinct fossils, viz., the aspen, the fir, 

 the oak. and the black alder. This comparison of the peat layers 

 of Norway and Danemark is further supported by the circum- 

 stance that layers of stumps are also found in the Danish bogs, 

 and here they stand between the peat layers of the different 

 periods. They indicate long periods, during which also the 

 Danish bogs were dry and partly covered with forests, when the 

 peat ceased to grow. But during these dry times the flora was 

 changed through the immigration of new species, and when a 

 wet time again set in, it was other trees which grew around 

 the bogs and which spread their boughs, leaves and fruits over 



