302 PAST CLIMATES AND CLIMAXES. 



Deformation and gradation must have destroyed large areas of vegetation 

 directly, at the same time that they produced new areas for colonization. The 

 climatic effects must have consisted partly in the destruction of existing 

 vegetation, and partly in the shifting of the population to an adjacent but 

 more favorable region. In addition, the great restriction of plant life was 

 probably an essential and perhaps a causal phase of the evolutionary cycle 

 which terminated in the expansion of the Mesophytic era. It is doubtless not 

 without significance that Cambrian glaciation was followed by a great land 

 flora, and Permian glaciation by the replacement of one great flora by another. 

 The destruction, shifting and evolution of species must have led to a differen- 

 tiation of vegetation into a number of climaxes corresponding to the number 

 of climates. It now seems probable that there must have been a division dur- 

 ing the Permian into warm, cold, and dry climates, and each of these must 

 have had its proper climax. 



The deformation cycle. — The existence of a great cycle of deformation and 

 gradation and its basic importance in the physical history of the earth are 

 clearly recognized by Chamberlin and Salisbury (1906: 1: 539; 2: 657). 



"The existence of any land at all is dependent upon the inequalities of the 

 surface and of the density of the Uthosphere, for if it were perfectly spheroidal 

 and equidense,, the hydrosphere would cover it completely to a depth of about 

 2 miles. Not only are inequalities necessary to the existence of land, but 

 these inequahties must be renewed from time to time, or the land area would 

 soon, geologically speaking, be covered by the sea. This renewal has been 

 made again and again in geological history by movements that have increased 

 the inequalities in the surface of the lithosphere. With each such movement, 

 apparently, the oceans have withdrawn more completely within the basins 

 and the continents have stood forth more broadly and relatively higher, until 

 again worn down. This renewal of inequalities appears to have been, in its 

 great features, a periodic movement, recurring at long intervals. In the inter- 

 vening times the sea has crept out ovex the lower parts of the continents, 

 moving on steadily and slowly toward their complete submersion, which would 

 inevitably have been attained if no interruption had checked and reversed the 

 process. These are the great movements of the earth, and in them lies, we 

 believe, the soul of geologic history and the basis for its grand divisions. At 

 the same time, there have been numerous minor surface movements in almost 

 constant progress. While these two classes of movements have been associ- 

 ated, and are pei;haps due in the main to the same causes, they are sufficiently 

 different in some of their dynamic aspects to be separated in treatment. 



"It may be noted that during the Subcarboniferous and Carboniferous 

 periods, in eastern America at least, a stage of approximate base-level at least 

 seems to have been developed over some considerable portion of the territory, 

 as shown by the configuration of the surface upon which the deposits of these 

 periods encroached, and there is reason to believe that this condition was a 

 rather general one. So far as can be judged by available evidence, this con- 

 ception may be extended to all the continents; indeed, this conception is 

 almost necessarily involved in the wide transgression of the seas of these 

 periods. This conception involves almost necessarily, as its essential pre- 

 requisite, the further conception of a protracted period of relative quiescence, 

 for in such a period only could base-leveling be accomplished. It is presumed 

 that during this period of quiescence, the energies that were to actuate the 

 subsequent deformation were accumulating stresses preparatory to actual 



