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wood growing on a fertile clayey soil. Therefore it is 

 quite possible, that the whole coniferous wood bas disap- 

 peared, if only there was time enough available, before 

 in our country mankind began to affect nature. Thosc 

 readers, who know the Alps and hâve seen, in descending 

 a mountain, how a coniferous zone preceded a zone of 

 beeches, will now only expect, that the coniferous wood 

 has given way to the actual deciduous forest of Central 

 Europe, with the beech as its principal représentative. 

 This, however, did not happen. After the âge of Conifers 

 came a period of oaks, as with the dunes of Michigan, 

 and in this period a dry and warm climate prevailed. 

 The gênerai conditions in our régions must hâve been 

 about the same then as those found actually in Galicia, 

 where there is to this very day a zone of oak-woods 

 between the Central European wood of beeches in the 

 west and the steppes in the east. Somewhere about the 

 middle of this oak-period began the définitive sinking of the 

 soil, Vv^hich led to the North Sea breaking through the 

 Pas de Calais. The results were in our country, first a 

 climate much like the actual one. then the appearance of 

 the so-called atlantic and mediterranean-atlantic plants, 

 i. e. plants from the atlantic and mediterranean régions 

 which, owing to the mild climate on the west-coast of 

 Europe, could by this way reach our country ; nnally a 

 destruction of the oak-woods by heath and moors, and 

 — although sporadic owing to edaphic factors — the 

 appearance of beeches. In .any case — and this is the 

 principal point of interest to us — during the oak-period 

 there w^as for the last time the possibility of an introduc- 

 tion of typical river-plants into England. 



Let us now treat the botanical side of our problem ! 

 In order to do so, I hâve in the first place read the ar- 

 ticles of Van Eeden concerning a supposed former course 

 of the Rhine past Harlem to the north in old volumes of 



