356 PAST succession: the ceneosere. 



explanation. The presence of six genera of palms in the Raton formation of 

 the Eocene in New Mexico does not mean they were necessarily associated 

 with Popvlus, Quercus, XJlmus, and other deciduous trees found in the same 

 horizon. Wherever fossiliferous deposits were laid down at the base of 

 mountain ranges or plateaus, it is clear that there would result a mixture of 

 genera belonging to different zones or climaxes of the clisere. This would be 

 true also of the transition area between arid and humid regions, and wherever 

 a mountain range rose in an extensive arid region, the mixing of the fossils 

 in the same formation would be extreme. The great interior invasion of the 

 sea during the Cretaceous and the moimtain-making of the late Cretaceous 

 and the Eocene must have resulted in a striking juxtaposition of different 

 climates and vegetations. Thus, while the current view of a mixed or undif- 

 ferentiated flora throughout North America dm-ing the Cretaceous and 

 Eocene must still be regarded as true in some degree, it will undergo inevitable 

 modification as a result of the further study of climatic cycles and of succession. 



The very presence of the larger number of the climatic and serai dominants 

 of to-day throughout the Cretaceous and Tertiary periods makes it certain 

 that the general features of Cenophytic succession were more or less uniform. 

 It is highly probable that there were broad-leaved evergreen, deciduous, and 

 coniferous climaxes, with one or more mixed climaxes, as well as scrub, grass- 

 land, and desert. The large nimiber of aquatic consocies recorded makes it 

 clear that the development of the hydrosere was essentially similar to that 

 known for the present. It is probable that this was equally true of the xero- 

 sere, but the especially unfavorable conditions for fossilization make the actual 

 evidence extremely fragmentary. 



Structure of the vegetation. — ^As just indicated, the major featiu-e of Ceno- 

 phytic vegetation must have been the existence of great climaxes, such as we 

 see to-day. Moreover, these chmaxes must have been arranged in zones 

 corresponding to the cHsere. The climax zones of evergreen, deciduous, and 

 coniferous trees extended much farther northward than at present, and this 

 must have been true also of their upward extension on mountain ranges. It 

 is probable that forest covered the entire Cordilleran system, except in the far 

 north, before the deformations of the later Tertiarj' epochs carried the crests 

 into a cooler climate. Zones of scrub and grassland must have existed in arid 

 areas, and probably also in arctic and high alpine regions. As a consequence 

 of active and repeated deformational and erosive processes, each climax must 

 have presented innumerable bare areas of rock and water, in which succession 

 was taking place. In the interior of the continent, with which we are espe- 

 cially concerned, the proportion of the climax area covered by developing seres 

 must have been much larger than at present, since it was here that sea invasion, 

 deformation, and erosion were most active throughout the era. With the 

 exception, then, of the greater northward and upward extension of zones, the 

 general structiu-e and appearance of Cenophytic vegetation must have been 

 very like that of to-day. The broad east-and-west zones of the continent 

 were matched by the corresponding north-and-south zones of the great 

 mountain ranges. Between these, in the west, the zones were more or less 

 broken up by arid areas of grassland, scrub, or desert. Finally, the climax 

 formations throughout were interrupted by numberless areas of all sizes, 

 from bare water or rock to subclimax conamunities, in which development was 



