434 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



occasioned by the relief thereby afforded to the lateral creep of the deep- 

 seated rocks which has been transmitted from the overloaded and hence 

 compressed basement of the synclinal area, the primary cause of these 

 faults may after all be ascribed to compression. Besides, since the up- 

 thrusted block or area is one of relative weakness, naturally tending to 

 positive movement under lateral stress, it seems probable that suboceanic 

 spreading and continental creep contributed largely and in a similar 

 manner to the effectiveness of the displacement. 



Essentially the same principle may have operated in a folded area like 

 the Appalachian Valley, in which the sediment-loaded troughs sank by 

 gravitation and in which the planes of the resulting normal faults, by 

 rotation in the process of folding, finally assumed attitudes favoring 

 overthrusting. 



HORIZONTAL MOVEMENTS DUE TO COMPRESSIVE FORGES 



Discussion. — Horizontal movements due to shrinkage*^ of the litlio- 

 sphere may be assumed to have occurred even in the earliest geologic 

 periods. Judging from the prevalence of folding in all Archean and 

 Proterozoic rocks, it seems likely that such movements were then more 

 nearly equal in vigor in all parts of the surface than in subsequent eras. 

 Suess states this probability very definitely when he says "the folding 

 force was once active over the whole world, but is restricted at present 

 to particular regions."*^ It might be , suggested that during Archean 

 ages the oceanic basins were not yet definitely outlined and that the 

 waters covered the entire globe ; hence, that contraction effects were then 

 rather general in distribution. On the same and similar grounds may 

 we not reasonably infer that, as the rigidity of the earth increased and 

 the waters were gathered into the continually deepening hollows that 

 are now represented by our oceanic basins, the notable effects of shrinkage 

 became more confined in areal extent and on the whole proportion- 

 ally less ? 



From the beginning of the Cambrian to well into the Mesozoic folding 

 of the continental parts of the crust was almost entirely confined to 

 marginal tracts, advancing farther inland only in later periods. A 



^ The term "shrinkage" as employed here and elsewhere in this paper refers to move- 

 ments resulting in horizontal shortening of segments of the earth's surface, as is indi- 

 cated by the folding and overthrusting of stratified deposits, the tangential movements 

 of heavily loaded sheets of crystalline rocks, and the piling up of prisms of relatively 

 superficial masses of igneous or unstratified crystalline rocks, all in obedience to lateral 

 pressure. According to the theories of Hayford, Chamberlin, and Willis, such shortening 

 may occur without body shrinkage of the earth. Floating of the crust toward the 

 equator may have helped greatly in many cases. 



*i Suess : Pace of the earth, vol. 3, p. 4. 



