THE CRETACEOUS-EOCENE CLISERE. 365 



On the other hand, if the effect of a great sun-spot period were exerted in 

 opposition to the cooling action of deformation and vulcanism, the climate 

 might be merely cooled from tropical to temperate. 



Vegetation zones. — ^The earUest Eocene flora, that of Heers in Belgiimi, 

 indicates a temperate climate, characterized by Quercus, Castanea, Salix, 

 Lauras, Hedera, etc. Similar horizons are found in the Lower Eocene of 

 France and England. At a later stage, palms, bananas, figs, cinnamons, etc., 

 became dominant, indicating a return to tropical conditions. This change of 

 floras in successive formations of the Eocene in England seems to establish 

 the fact that tropical and temperate floras had been further differentiated, 

 and that the change to a cooler climate had gradually forced the corresponding 

 climax zones southward. With the gradual disappearance of the effects of 

 the deformation, the climate again grew warmer, and the tropical climax 

 dispossessed the temperate one. It thus seems highly probable that actual 

 glaciation would have carried the boreal and arctic climaxes of the high north 

 into the present temperate region, just as it did in the Pleistocene. While 

 the cooling was too slight to produce a complete cUsere, the difference is only 

 one of degree. From the north, each climax zone advanced upon the next, but 

 replaced it only in part, instead of driving it whoUy into a more southern posi- 

 tion. Similarly, after the cooling effect had disappeared, each climax regained 

 much or all of its former area. The cHsere, in consequence, was shifted less 

 than a zone in space, instead of several zones, as in the Pleistocene cliseres. 

 In time, it was modified by loss of the normal climax stage of each sere, and 

 its replacement by a preclimax, i. e., temperate for tropical, and boreal for 

 temperate. At the end of the cooled period, the climaxes had again shifted, 

 and the sere, which had terminated in a temperate climax, now developed 

 again to the original tropical climax found in the Cretaceous. 



Dominants of the eocene. — ^The chief changes seem to have been in the final 

 disappearance of cycads as occasional dominants, and in the increased number 

 of temperate and boreal genera. Among the trees, Alnus, CeUis, Larix, 

 Pseudotsuga, and Tilia are first recorded in the Eocene. This is likewise true 

 of Amelanchieri Berberis, Spiraea, and Vaccinium among the shrubs, and Thaiic- 

 trum, Sagittaria, Sparganium, Juncus, Poa, etc., among the herbs. The fern- 

 worts and the more primitive conifers seem to have become completely sub- 

 ordinate, and dominance appears to have passed finally to angiosperms and 

 the Abietineae especially among gymnosperms. The course of evolution as 

 well as the changes of dominance and of climax zones, seems to bespeak the 

 decisive influence of a cooled climate. 



THE OLIGOCENE-MIOCENE CLISERE. 

 The deformation cycle. — ^The period of gradation and sea invasion of the 

 Eocene and Lower OUgocene, with the corresponding tropical climate over 

 much of North America, was closed by marked deformation in the western 

 mountains and the accompanying withdrawal of the sea from the Atlantic and 

 Pacific coasts. The movmtain-making and vulcanism indicated in Schuchert's 

 chart (fig. 26) are accompanied by a fall in the temperatm-e curves and by a 

 marked rise in the curve of aridity. This corresponds with the occurrence of 

 gypsum-beds in the Grand Gulf formation of the Oligocene, and with Mat- 

 thew's suggestion that the White River beds of the Bad Lands are an ancient 



