The Origin of the Desert Climax and Climate 121 



glacial alternations more recently regarded as five in number, the 

 glacial phases known as Nebraskan, Kansan, lUinoian, lowan, 

 and Wisconsin, with the four interglacial ones. There is also 

 increasing evidence of smaller cycles, such as can be discerned 

 in varves, tree rings, and other phenomena, but none of these 

 was sufficiently long to lead to the evolution or mass migration 

 of a climax. 



The Paleocene deformation appears to have been greater in 

 the region of the Rocky Mountains than along the Pacific, with 

 its efFect to be seen in a warm-temperate deciduous forest, 

 while the Pacific area was clothed with a subtropical forest 

 (Chaney, 1933). In the long quiescent period of the Eocene and 

 Lower Oligocene, swamp sedges and grasses seem to have spread 

 widely over the interior of the continent, as indicated by the 

 presence of such genera as Carex and Phragmites in the Creta- 

 ceous. It is probable that the evolution of grassland proceeded 

 more rapidly in the period of mountain-making in the Upper 

 Oligocene to produce the forerunner of the modern prairie in 

 the Miocene, where the typical genus Stipa is recorded, along 

 with horses of the grazing type, Merychippus and Protohippus. 

 It is a plausible assumption that, by the time of the Miocene, 

 grassland had become the distinctive climax in at least the central 

 and southern parts of the Great Plains and had spread westward 

 through the present desert, as attested by the ruling abundance 

 of Merychippus in the Barstow and the appearance of a still 

 more advanced grazing type, Protohippus or Pliohippus (Mer- 

 riam, 1919:479). With the rise of the Cascadian axis in the late 

 Miocene, this dominance of prairie must have been gradually 

 extended, probably with the general replacement of tall sub- 

 tropical grasses by the northern mid-grasses and western short- 

 grasses. 



