102 TREE ANCESTORS 



is shown by their presence in the lowermost (oldest) Eocene of the 

 Paris basin. 



Birches are a widespread and common type in the Eocene floras, 

 occurring in the earliest rocks of this age in both Europe and Amer- 

 ica. About 30 Eocene species have been described, and during the 

 northward swing of temperate forests which is one of the most 

 spectacular events of Eocene times, birches penetrated almost to 

 the pole itself, in Spitzbergen, Banks Land, Grinnell Land, Green- 

 land and Iceland, in addition to being exceedingly abundant on 

 all the northern continents, especially in Alaska and the western 

 provinces of Canada. At this time they are supposed to have 

 reached Austraha and Tasmania, but these antipodal occurrences 

 are not above suspicion and may merely represent incorrect de- 

 terminations. In the region of the not yet elevated Rocky moun- 

 tains was an area still swept by humid Pacific winds, the dwelHng 

 place of several typical species of birch whose foliage doubtless 

 helped to furnish the fare of the early browsing mammals, not yet 

 learned in the habit of grazing. 



The Oligocene records include some ten or a dozen species, 

 almost entirely European for reasons explained in connection with 

 the history of other trees, and their remains are beautifully pre- 

 served in the Baltic amber, as well as in the gypsiferous shales of 

 southeastern France. During succeeding Miocene times the 

 birches reached the acme of their development. Over 40 Miocene 

 species are known, and they left their remains in the deposits of 

 this time on all the larger land masses of the Northern Hemisphere 

 from Japan westward to France, and in Colorado, Oregon and 

 Cahfornia, being especially abundant throughout southern Europe. 

 They continued in but slightly abated abundance in the last named 

 region throughout the Phocene, and a species named prenigra since 

 it appears to be ancestral to the modem American river birch, has 

 been found in the Phocene deposits formed at that time along the 

 then Gulf coast in Alabama. 



Birches were unusually numerous during the Pleistocene, about a 

 dozen species having been recorded, largely because they dwelt in 

 regions where their remains could take a part in the numerous peat 



