300 PAST CLIMATES AND CLIMAXES. 



mian deformation as a typical example for analysis; and the following account 

 is drawn from their discussion : 



Permian deformation. — ^This is regarded as typical in that it comprised a 

 sinking of the ocean basin, followed by a withdrawal of the epicontinental seas 

 and a raising of the continental platforms, together with arching or warping. 

 Pronounced arching and folding occurred chiefly aroimd the borders of the 

 North Atlantic. The essence of the movement is assumed to be a shrinkage 

 of the earth in which the ocean sectors shrank more than the continental ones, 

 as a consequence of their greater specific gravity, their loading by the ocean, 

 and the weight of sediments. The erosion of the land also lightened the con- 

 tinental sectors, and tended still further to disturb the balance : 



"A portion of the phenomena were direct expressions of deformation, others 

 were seemingly close sequences of deformation, while still others may be only 

 the more remote sequences of deformation in the form of changes of atmos- 

 pheric and hydrospheric constitution. If the solution of these problems lies 

 wholly in terrestrial causes, it seems at present most likely to be found in the 

 immediate and ulterior consequences of deformation, as realized in physio- 

 graphic changes, and in the constitution and working conditions of the hydro- 

 sphere and atmosphere." 



The circulation sequence. — The withdrawal of the epicontinental seas into 

 the ocean-basins must have restricted oceanic circulation in such a way that 

 the movement of warm water was reduced or interrupted. This must likewise 

 have affected atmospheric circulation, and resulted in the appearance or 

 extension of continental cUmates over large areas. It is obvious that circula- 

 tion must be caused by such inequalities as can be provided by polar tempera- 

 tures. Whether the higher temperatiu-es of the polar regions during various 

 geological periods were due to extension of circulation or to other causes, it is 

 clear that a restriction or interruption of the circulation during such a period 

 would lead to a general reduction of temperature. Conversely, the removal of 

 such restriction as a consequence of new extensions of the ocean and the sub- 

 mergence of land barriers would lead to a renewal of a more general circulation 

 of warm water and a corresponding increase of oceanic climates. 



The gradation sequence. — ^The Permian deformation was recorded in the 

 marked relief of regions that were sharply folded, and a less striking but still 

 appreciable rehef in the areas raised more gently. These changes increased 

 the gradients, and water as the chief agent of gradation began to work with 

 greater activity. The greater area of the land led to an increase in the number 

 and length of the streams. Many of the former streams were rejuvenated, 

 and erosion and deposit must have gone on much more rapidly than during the 

 Carbonic periods. This must have been reflected in an increased sedimenta- 

 tion in the epicontinental parts of the ocean. The general result must have 

 been shown in the universal occurrence of marked gradation and the corre- 

 sponding tendency toward a base level. 



The climatic consequences of gradation are naturally obscured by the greater 

 changes due directly to deformation or the resulting circulation. Still, the 

 wearing down of great folds, such as the Appalachian, and the filling of the 

 epicontinental areas could not have been without effect. Moreover, it seems 

 plausible to suppose that the aridity typical of the Permian was enhanced by 



