366 PAST succession: the ceneosere. 



loess of seolian origin, thus indicating arid conditions in the Great Plains. 

 This conclusion is confirmed by the appearance of plains grasses such as Stipa, 

 and the rapid evolution of soUd-hoofed animals. All of this evidence indi- 

 cates that the Oligo-Miocene cycle was marked by a general climate cooler and 

 drier than that of the Eocene, and hence by a differentiation of climates 

 approaching that of to-day. It is not in accord with the results of Heer and 

 Nathorst as to the existence of a warm temperate vegetation in Greenland, 

 Spitzbergen, and the Arctic regions generally. Chamberlin and Salisbury 

 (1906:3: 195, 282, 283) call in question the correctness of Heer's reference of 

 this flora to the Miocene, but Schuchert (1914 : 293) accepts without comment 

 Nathorst's conclusions that Taxodium, Sequoia, and Magnolia, as well as 

 beeches, maples, limes, oaks, pines, firs, poplars, and birches, flovuished as far 

 north as 79° to 82° during the Miocene. From the standpoint of the deforma- 

 tional sequence, as registered in the differentiation of climate and vegetation, 

 it is difficult to beKeve that Taxodium, Sequoia, and Magnolia could have 

 ranged from the southern United States to within 8° of the North Pole. One 

 can hardly resist the feeling that the reference to the Miocene is a mistake in 

 both cases, and that these formations belong to the Eocene, in which the pres- 

 ence of warm temperate species at high altitudes can be explained in harmony 

 with the deformational sequence. 



Coseres and clisere. — So far as dominants were concerned, there appears to 

 be little difference between the floras of the Eocene and Miocene. The domi- 

 nant tree genera appear to have been about equally represented in both, and 

 this is largely true of shrubs, though a few genera, such as Cercocarjms, Phila- 

 delphus, Ptelea, Ribes, etc., are first recorded in the Miocene. This suggestion 

 of greater aridity is confirmed by the occmrence of Stipa as the first repre- 

 sentative of the plains grasses. Thus, while the flora remained largely the 

 same, it must have imdergone marked differentiation and shifting as a result 

 of the deformation and cooling which initiated the cycle. The northerly 

 climax zones must have been broadened as well as pushed to the south. At 

 the same time, the further deformation of the CordiUeran system brought 

 about a marked differentiation of arid climates, and it seems certain that the 

 grassland of the Great Plains and the desert scrub of the Great Basin were 

 developed or at least greatly extended at this time. 



The shifting of climax zones to the south probably carried with it the dis- 

 appearance from North America of such tropical or subtropical genera as 

 Araucaria, Artocarpus, Eucalyptus, etc. The final disappearance of these 

 seems to have been due to the fact that the cooling from a tropical or sub- 

 tropical climate to a warm temperate one over much of the continent was 

 permanent. Before the climatic effect of Oligocene deformation had dis- 

 appeared, the deformation cycle of the late Miocene and Pliocene had begun 

 to culminate in the Ice Age. Thus the shifting of the climax zones took place 

 only to the southward, as well as downward on the mountains. There was 

 no reciprocal movement to the northward, such as occurred in the Eocene, 

 and again in the Pleistocene. The shifting of the clisere amoimted probably to 

 a zone, but this change became the permanent basis for the repeated cUseres 

 of the glacial period. This seems to be indicated by the Miocene flora of 

 Florissant, where to-day a cold temperate flora exists. The development of 

 seres in the forest climaxes must have been essentially identical with that of 



