368 PAST succession: the ceneosere. 



up the bulk of the climax elsewhere. From the age of the trees the period 

 represented by the costase must have been from 5,000 to 10,000 years long 

 (fig. 35). 



THE PLEISTOCENE CLISERES AND COSERES. 



The deformation cycle. — ^From the close of the Miocene to the Pleistocene 

 there appears to have been a constant succession of deformation movements. 

 The elevation of mountains or plateaus appears to have taken place at the 

 close of the Miocene and one or more times during the Pliocene, as well as at 

 its close and at the beginning of the Pleistocene. The climatic effect of this 

 is recorded in the characteristic red to yellow colors of the extensive Lafayette 

 formation which bespeak an arid climate, in the further shifting of climax 

 zones to the south, and especially in the culmination in an ice age. Deposits 

 of salt and gypsum also indicate dry areas in Europe, and, while they are not 

 yet known for North America, it seems that they must have occurred. The 

 Plio-Pleistocene deformation cycle stands out as the greatest of all those of 

 the three great vegetation eras, and hence it is clear that it should have pro- 

 duced the most general and widespread of all known glaciations. The 

 Pliocene was distinguished by the abundance of aggradational rather than 

 marine, lacustrine, or fluvial deposits. Subaerial deposition of this sort is 

 unfavorable to the transport of plant remains for fossilization, as well as for 

 the process of fossilization itself. As a consequence, the record of land vege- 

 tation during the Pliocene is pecuUarly scanty. Falling as it does, however, 

 between the abundant records of the Miocene and Pleistocene, its flora and 

 vegetation may be readily inferred. Since the Pliocene was initiated by 

 deformation and consequent cooling, its flora must have approached that of 

 the Pleistocene rather than that of the Miocene, and the distribution of its 

 climax zones must have resembled closely that which is found to-day. 



The record of fossil plants in this country is almost wholly from California, 

 where Alnus, Amelanchier, Arbutus, Comus, Quercus, Rhamnus, Salix, Sequoia, 

 etc., point to conditions closely resembling those found there to-day, if indeed 

 not practically identical with them. In England the record of the PUocene 

 is much fuller, nearly 40 of the dominant genera of to-day being known for it. 

 It is significant, however, that while all of these occur in the Pleistocene, 14 of 

 them are not recorded for the Miocene. Moreover, since the species of many 

 of these dominants were the same for both the Pleistocene and Pliocene, the 

 general course of serai development must be regarded as essentially identical 

 for both. Hence the consideration of the seres and coseres of the glacial period 

 will serve for those of the Pliocene as weU. 



The causes of glaciation. — It has already been suggested that the great 

 periods of glaciation, such as the Permian and the Pleistocene, were due 

 primarily to great crustal deformation, and that the consequent climatic 

 effects were greatly modified by the grand sun-spot cycles, and to some 

 extent by long periods of violent volcanic eruption (p. 328). The convincing 

 summary of the evidence as to the basic effect of deformation, which Schuchert 

 has made (fig. 26), does not need fmther discussion. Deformation appears 

 by far the most probable of aU the causes assigned for glacial periods, and our 

 task is confined consequently to that of ascertaining the cause of the glacial- 

 interglacial cycles of the Permian and Pleistocene. The distinctly alternating 



