DIASTROPHISM AND THE FORMATIVE PROCESSES. VII 

 PERIODICITY OF PALEOZOIC OROGENIC MOVEMENTS 



ROLLIN T. CHAMBERLIN 



University of Chicago 



INTRODUCTION 



Within the last few years there has been a growing beHef among 

 American geologists that diastrophism, in its greater manifestations, 

 has been periodic, rather than continuous. According to this view 

 the earth has passed through periods of diastrophic activity alter- 

 nating with periods of relative quiescence.^ During the more pro- 

 tracted of the periods of quiescence the earth's surface is believed 

 to have remained stable sufficiently long to allow base-leveling to 

 reach an advanced stage, and this is held to imply stabihty, for, 

 without stability in the outer portion of the earth, mature base- 

 leveKng is scarcely possible. During such a time of base-leveling, 

 stresses within the body of the earth must be accumulating steadily, 

 but because of the high rigidity now apparently demonstrated 

 these stresses show little outward manifestation of their presence 

 for a long time, until finally, according to this view, the increasing 

 internal stresses reach such an intensity that the mass of the earth 

 can resist no longer but yields, and a period of active deformation 

 succeeds the state of crustal inactivity. The deformation once 

 inaugurated continues till the stresses are essentially eased. Then 

 another period of quiescence sets in, to be followed in turn, after a 

 long interval during which new stresses have developed, by another 

 deformative outbreak. 



Such diastrophic movements are held to be in themselves major 

 events in the earth's history; and, in addition, they are the direct 



' T. C. Chamberlin and R. D. Salisbury, Geology, III (1906), 192, 193; T. C. 

 Chamberlin, "Diastrophism as the Ultimate Basis of Correlation," Jotir. Geo/., XVII 

 (1909), 685-93; Charles Schuchert, " Paleogeography of North America," Bull. 

 Geol. Soc. Am. XX (1910), 427-606; Bailey Willis, "Principles of Paleogeography," 

 Science, XXI, No. 790 (1910), 246-49; E. O. Ulrich, "Revision of the Paleozoic 

 System," Bull. Geol. Soc. Am., XXII (191 1), 281-680. 



315 



