CHAPTER VI. PERMO-MESOZOIC 



TRANSITION FROM PALEOZOIC TO MESOZOIC! 



Theoretical considerations. There is no sharp plane of division to be 

 discovered between the Paleozoic and Mesozoic in Asia. In some dis- 

 tricts there was transition of sediment and of faunas; elsewhere, during 

 equivalent time, there is disturbance of physical or biological conditions. 

 In one place a decided change of sediment, in another an equally abrupt 

 change of fauna, in a third an unconformity of dip between beds that 

 differ in kind and in fossils : these may be taken as local dividing planes ; 

 but they do not range themselves into one horizon. They fall anywhere 

 within the later Carboniferous, or within the Permian, or Trias. It is 

 in the nature of things that a great transformation of features, climates, 

 and faunas, such as characterized the passage from the period of the Old 

 Life to that of the Middle Life, should take time, and that the evidences 

 of change should appear at different geologic moments in different places. 

 The better to understand the facts which are presently to be listed, we 

 may review briefly the theoretical sequence of events. 



The leading fact that distinguishes the early Mesozoic from the late 

 Paleozoic is the contrast in the extent of the lands, which, through with- 

 drawal of the epicontinental seas, became relatively very wide. This change 

 theoretically resulted from enlargement of the ocean basins by subsidence 

 and widening, which was associated with protuberance of the continents 

 and local deformation of rocks by folding.* 



There is no question about the leading fact, of which Asia, as well as 

 North America, offers a striking illustration. The wide seas of the late 

 Carboniferous shrank, in the Permian and Triassic, to embayments around 

 continental platforms. One effect of that shrinking was interference of 

 emerging lands with circulation of marine currents. Hence followed 

 climatic differentiation and contrasts of warm and cold currents in the seas 

 and sharper contrasts of climatic zones on lands. A further effect of these 

 changes was the development of new species and extinction of organisms 

 incident to altered conditions of habitat. Faunal change was evidently a 

 late effect, since it was sequential upon the others. Do these phenomena 

 afford any precise datum that might serve to distinguish a definite epoch, 



* Geology, Chamberlin and Salisbury, vol. n, p. 656. 



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