654 ELIOT BLACKWELDER 



When the earHer and later orogenic epochs are compared, there 

 appears a suggestive sameness running through the whole series. 

 Although we are still unable to express geologic chronology in terms 

 of years, an estimate expressed in terms of epochs or thickness of 

 strata suggests that the crumplings were no more frequent in the 

 Paleozoic era than in later periods; nor do the facts seem to imply 

 that the earlier disturbances were more intense or more widespread. 

 In fact, if we should take the available data without making 

 allowance for the greater loss of record pertaining to the earlier 

 periods, we might reach the improbable conclusion that they had 

 been more widespread, more severe, and more numerous since the 

 Triassic than before. 



THE CAUSES OF OROGENIC EPOCHS 



It is beyond the purpose of this paper to discuss the origin of 

 earth folding. 



From the very nature of its effects, the cause of orogenic epochs 

 must be sought in tangential compression affecting the rocks of the 

 earth's outer shell. As to the origin of that force there is, however, 

 no unanimity. It constitutes a problem for which several hypoth- 

 eses have been offered. These are explained and critically analyzed 

 in the writings of Dana,^ Reade,^ Chamberlin,^ Willis,'* Taylor,^ 

 Leith,^ and others. 



' J. D. Dana, Manual of Geology, 4th ed. (1895). 



^ T. Mellard Reade, Origin of Mountain Ranges, p. 125. 



3 T. C. Chamberlin and R. D. Salisbury, Geology, I (1905). 



'^ Bailey Willis, "Research in China," Carnegie Institution, II (1907). 



5F. B. Taylor, "Origin of the Earth's Plan," Bull. Geol. Soc. Am., XXI (1910). 



* C. K. Leith, Structural Geology (19 13). 



