360 PAST succession: the ceneosere. 



makes it possible to trace the successional development of this clisere and 

 cosere with some degree of detail and certainty. In the following sketch of 

 the eosere of the Cenophytic era, in consequence, a general account is given 

 of the probable development during the Cretaceous, and then attention is 

 paid chiefly to the successive cUseres, but especially to that of the Pleistocene. 



THE CENEOSERE. 



For a number of reasons it proves necessary to confine the discussion of the 

 ceneosere chiefly to the Great Plains and Rocky Moimtains, with some con- 

 sideration of the Pacific Coast. Eastern North America and Europe are 

 taken into account only to throw light upon the probable flora of the region 

 concerned. In the case of the Pleistocene, however, the European records 

 are so much more complete and reliable that the treatment is based largely 

 upon them. As to the discussion itself, it must be constantly borne in mind 

 that the standpoint of the ecologist is very different from that of the paleo- 

 botanist. For the latter, certainty of identification is of the first importance, 

 and plants can be accepted only when their determination is beyond all 

 reasonable question. But to the ecologist even the doubtful existence or 

 presence of a certain genus is of much importance, since it tends to support 

 the inferential evidence from phylogeny and association. This is especially 

 true of cryptogams, which even the paleo-botanist assumes have been long 

 in existence in spite of the extreme poverty of the record. Moreover, the 

 difficulty of specific limitations is also Uttle felt by the ecologist, since he is 

 concerned primarily with dominant genera. The number of species is impor- 

 tant at present chiefly in giving some idea of the degree of dominance and 

 differentiation. Ultimately, they will assume something of the importance 

 of existing species, as it becomes possible to deal more in detail with the suc- 

 cessional relations of different geological formations. 



The Cretaceous period. — ^The earhest formation or series, the Dakota, 

 extends throughout the Great Plains and Rocky Mountains, from Texas and 

 New Mexico to Utah, Alberta, Minnesota, and Iowa. It is mainly fresh-water in 

 origin, either lacustrine or subaerial and fluviatile, or, more probably, all three 

 kinds of deposition have helped to form it. From its widespread occurrence 

 it is assumed that the depositional region abounded in lakes, marshes, and 

 river flats, even among the motmtains. The Colorado epoch corresponds to 

 the great invasion of the sea, by which a vast mediterranean stretched from 

 the Gulf of Mexico to the Arctic Ocean. The eastern shore seems to have 

 passed through Oklahoma, Kansas, Iowa, Minnesota, and Manitoba, the 

 western through Arizona, Utah, Idaho, Montana, and British Columbia. 

 The deposits were largely marine, and this, with the vast extent of the sea, 

 explains the almost complete absence of the fossils of land plants. Beds of 

 coal are occasional, however, and charred wood and charcoal are thought to 

 indicate the presence of forest fires. The sea persisted during the Montana 

 epoch, and the beds are consequently also marine for the most part. The 

 presence of marshes is attested by local beds of coal. During the late Montana 

 and the Laramie epoch, the withdrawal of the sea was more marked, but it 

 appears to have oscillated constantly. As a consequence, shallow water and 

 marshes were characteristic, and deposit seems to have been largely in brackish 



