566 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



enough to control mountain-building and of dates other than Eocene, Cre- 

 taceous, and Carboniferous as now defined. 



4. The Western Belt is a unit as regards the evidences of its behaviour 

 during mountain-building periods. The general orogenic periods include at 

 least the late Jurassic and the post-Laramie. In each case the degree of 

 deformation and metamorphism is comparable in all sections where detailed 

 studies have been made. 



The more local, late Miocene, late Oligocene, mid-Cretaceous, and pre- 

 Triassic crustal movements were also respectively of the same order whether 

 recognized in Alaska, British Columbia, or the American States. 



5. The western Belt is specially distinguished from the eastern Belt by the 

 extraordinary repetition, in the former belt, of heavy vulcanism in all thirteen 

 of the periods noted in the tables. Volcanic rocks are far from common in the 

 Eastern Belt and have everywhere but limited range. 



6. Finally, the Western Belt is to be considered a geological unit also 

 because of the steady relation of bedded rocks and batholiths from end to end 

 of the long mountain-chain. The late Jurassic invasion of the roots of the 

 mountains was almost as general a phenomenon as the post-Jurassic uncon- 

 formity or the late Carboniferous sedimentation. The proved late Miocene 

 intrusion of batholiths is more local, but future studies will doubtless increase 

 the number of exposed granites which are referable to the Tertiary. 



Batholithic intrusion is known in the Idaho portion of the Eastern Geosyn- 

 clinal Belt, and elsewhere has afforded bodies of huge size, but post-Cambrian 

 batholiths are comparatively rare in that belt. This contrast of the two belts 

 is partly to be related to the higher degree of orogenic crumpling, overthrust- 

 ing, and overfolding in the Western Belt as compared with the slighter disturb- 

 ances of the stronger rocks of the Eastern Belt. Yet the fact that, through 

 the entire width of the Cordillera, mountain-building has been controlled by 

 thrust from the Pacific basin is obviously of prime importance in this connec- 

 tion. 



The comparison of the two great belts into which the North American 

 Cordillera may be divided results in the view that the Cordilleran axes of geosyn- 

 clinal warpings and of orogenic foldings have remained largely parallel from 

 late pre-Cambrian time to the present. Obvious as may be the contrast of the 

 two belts in their respective complex histories, the interpretation of that con- 

 trast needs the steady attention of geologists for generations to come. It would 

 be out of place to attempt here a full discussion of the subject. It has been 

 touched upon rather to prepare the way for the following brief account of the 

 history of the Cordillera at the Forty-ninth Parallel. This legitimate field for 

 the present report could not be wisely entered without a preliminary survey 

 of the vast mountain-unit through which the International Boundary runs. 

 Since this has been the express purpose of the foregoing correlations, they 

 have not been treated in the full way their importance demands. 



