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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 undifferentiated
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 Cenophytic
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