THE OROGENIC EPOCHS IN NORTH AMERICA 651 



is closest, the zone is likely to be narrower than where the folds are 

 open and wavelike. If this is generally true, it may mean that the 

 narrowest places are narrow because they have been the most 

 compressed. 



The crumpled strips produced at each epoch overlap more or 

 less, but rarely coincide. Thus the Mesabian orogeny affected the 

 north side of Lake Superior while the Penokean affected the south 

 side. Again, the Laramide and Nevadian zones are largely distinct, 

 although they overlap considerably. Successive foldings in the 

 same general region were usually roughly parallel to each other and 

 resulted in an ever-widening belt of crumpled strata. In North 

 America we have at least three such composite zones, the Cordilleras 

 in the west, the Atlantic system in the east, and the ancient sys- 

 tems of the Canadian shield where nearly all structures trend east 

 by north. 



Although overlapping of the limits is common, it is rare that the 

 locus of greatest intensity occurred twice in the same place. The 

 Sierra Nevada region, intensely folded in the Jurassic, was but little 

 affected in the Miocene, although the Coast Range to the west and 

 parts of Nevada to the east both suffered folding at that time. 

 Intense crumpling produces a thickening of the deformed strip and 

 therefore increases its competency to resist further thrusts and to 

 transmit them to regions beyond. It seems probable, also, that the 

 intrusion of great batholiths such as accompanied the Nevadian 

 orogeny must notably strengthen the mass within which they 

 congeal, and thus render it less liable to yield to compression in 

 later periods. 



Many students of the subject have shown that some of the dis- 

 turbances were inter-continental. For example, there appears to 

 have been some crumpling in northwestern Europe, corresponding 

 approximately in time to the Taconic orogeny in eastern North 

 America. The Brunswickian likewise has its counterpart in the 

 mid-Devonian deformation in Scotland. The Arkansan seems to 

 correspond in time to the Armorican orogeny of western Europe, 

 the Nevadian to the folding of the eastern interior ranges of Asia, 

 and the Antillean almost unquestionably to the rearing of the great 

 Alpine system of Eurasia. These striking correspondences strongly 



