364 PAST succession: the ceneoseee. 



Because of the practical impossibility of fossilization in xerophytic habitats, 

 the record of the xerosere is extremely scanty. It is suflBcient, however, to 

 warrant the conclusion that all of the initial stages were present. Aerial algae, 

 such as Gloeocapsa and Pleurococcus, must have long been in existence, and 

 have given rise to crustose lichens, and these had doubtless produced foliose 

 forms long before the chance records of the Tertiary. Mosses and liverworts 

 had appeared, and must have been ready to follow the lichens, the mosses 

 on the bare rock, and the liverworts in the shady clefts and nooks. Even 

 though angiospermous herbs may have been scarce at this time, ferns were 

 available to serve as the next stage. Forms such as Cheilanthes, PeUaea, etc., 

 grew in the dry clefts, and others, such as Filix and Woodsia, in moist shady 

 spots among the rocks. There is no record of the grass stage, but the presence 

 of Phragmites in the swamps suggests the existence of both Festuca and Poa, 

 which are typical rock-grasses. The presence of Juniperus and Ceanothus 

 suggests the development of rock-scrub, which would have changed into a 

 coniferous, deciduous, or mixed climax. 



The discovery of charred wood and charcoal in the Colorado series is fairly 

 definite proof of forest fires, and hence of subseres. Theoretically, there can 

 be no question that lightning and volcanic eruptions frequently produced fires, 

 and thus led to the formation of bare areas. Subseres may also have arisen 

 in denuded areas due to all the usual topographic causes, as well as to some 

 biotic ones as well. Coseres must have been especially characteristic of the 

 whole period, since coal stases are found in all of the epochs. The coal-beds 

 must have been the result of the hydrosere traced above. In some cases, thin 

 seams perhaps represented only the accumulated remains of a single sere, but 

 as a rule each bed must be regarded as a costase, i. e., the fossilized populations 

 of a cosere, consisting of two or more seres developed in the same spot. There 

 is no evidence of clistases, and it seems that these were lacking, only to appear 

 with the shifting of zones incident to the cooling of the next deformation cycle. 



THE CRETACEOUS-EOCENE CLISERE. 



The deformation cycle. — The close of the Cretaceous and the opening of 

 the Eocene were marked by a major deformation, extending from Alaska to 

 Cape Horn. This great mountain-making movement was felt especially in 

 western North America, and the Rocky Mountain system is regarded as 

 dating from this time. It is estimated that the mountains reached a maximmn 

 height of 20,000 feet in consequence. Orogenic movements also occurred in 

 parallel tracts to the west, as far as California and Oregon, and the land began 

 to take on much of its present character of alternating mountaiu-ranges and 

 basins. Such a pronounced deformation should have produced a marked 

 change of climate, amounting to glaciation it would seem. A distinct cooHng 

 is indicated by the flora of the early Eocene, and the usual accompaniment of 

 aridity is suggested by the salt and gypsmn beds of the Texas formations of 

 this period. From the standpoint of the intensity of the deformation (c/. fig. 

 26), there should have been a glacial period equaling those of the Permian 

 and Pleistocene. The most plausible explanation of the absence of glaciation 

 must be sought in connection with solar cycles. The plus coincidence of grand 

 sun-spot cycles with deformation and vulcanism in the Permian and the PHo- 

 Pleistocene would seem to account for the intensity of the change of climate. 



