PHYSICAL GEOGRAPHY OF THE PLEISTOCENE 273 



Vulcanism has been regarded as a factor which decreased the flora 

 in the western part of North America as compared with the eastern; 

 but since the floras were much the same throughout the Tertiary in 

 all northern lands, and since the climax of Cenozoic vulcanism came 

 as early as the Miocene, the importance of this factor in impoverishing 

 the Pleistocene life of the western part of the United States may be 

 questioned. Furthermore, it has little or no application to Europe, 

 where the flora was equally reduced. 



The to-and-fro movements of the land faunas and floras must have 

 introduced an elaborate series of superpositions, giving an elaborate, 

 orderly, and unusual succession. The record of this succession has 

 not been worked out in its completeness, and unfortunately there is 

 little chance that it will be worked out in its fulness unless by the 

 most persistent care. In the regions which were glaciated repeatedly, 

 the advance of each ice-sheet destroyed, in most places, the record of 

 the successive floras and faunas which had lived since the preceding 

 retreat, so that, within the area glaciated, the succession of successions 

 is hardly likely to be found in its entirety in any one place, and perhaps 

 not in all places. Outside the area which was glaciated, especially 

 near the borders of the regions occupied by the successive ice-sheets, 

 there is better chance that a complete record of the biological changes 

 may have been preserved. The peat bogs of such regions might be 

 expected to give complete records if they had endured continuously 

 since the time of the first glaciation; but peat bogs are themselves 

 temporary, and it is perhaps too much to expect that complete record 

 of the migrations of life during the successive epochs of the glacial 

 period will ever be found at any one place. 



The records, however, of the post-glacial peat bogs are such as to 

 give some indication of the results which would probably be found 

 if all the migrations could be ascertained. Thus in Scandinavia 

 and Denmark, we have a succession of post-glacial floras, the first cor- 

 responding in a general way to the present vegetation of the tundra, 

 the second a forest vegetation dominated by the birch and poplar, 

 the third a forest vegetation dominated by pines, the fourth, one 

 dominated by the oak, etc., the fifth a flora similar to that of the 

 Black Forest Mountains, indicating a temperature warmer than that 

 of today for the same region, and finally, a southward retreat of the 



