VOLUME XXII NUMBER 7 



THE 



JOURNAL OF GEOLOGY 



OCTOBER-NOVEMBER 19 14 



A SUMMARY OF THE OROGENIC EPOCHS IN THE 

 GEOLOGIC HISTORY OF NORTH AMERICA 



ELIOT BLACKWELDER 



University of Wisconsin 



INTRODUCTION 



Ever since the appearance of Dana's Manual of Geology^ students 

 of American geology have been familiar with the idea that the 

 earth's crust has from time to time suffered crumpling along 

 sinuous belts, many of which have a suggestive parallelism to the 

 margins of the ocean basins. Of these the folding of the rocks now 

 underlying the Appalachian Mountains serves as an example and 

 is undoubtedly one of the most widely known. Frequent notices 

 of such epochs of deformation appear in later works by American 

 geologists, and in recent years the general principle has been clearly 

 stated by Chamberlin.^ This principle, well supported by evidence 

 from many points of view, is that the lithosphere is not an abso- 

 lutely incompetent mass, but that it has sufficient powers of resist- 

 ance to the forces which tend to deform it to be able to accumulate 

 stresses for long periods of time without perceptible deformation. 

 These periods of accumulation are periods of quiescence, during 



^ T. C. Chamberlin, " Diastrophism the Ultimate Basis of Correlation," Jour. 

 Geo!., XVII (1909), 685-93. 



Vol. XXII, No. 7 633 



