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STRUCTURAL GEOLOGY OF NORTH AMERICA 



Cretaceous ) ( Evernden et al., 1957 ) . Presumably this should be the tenta- 

 tive geologic age assigned to the Idaho batholith. As far as the writer can 

 see there is nothing inconsistent geologically with such a conclusion. 



CONCLUSIONS 



The Idaho batholith is composite, with some of the smaller parts and 

 satellites of Late Cretaceous and early Tertiary age and some as young 

 as Miocene. The Batholith occurs at the junction area of great arcuate 

 segments of both the Laramide and Nevadan orogenic belts. It is similar 

 in size and composition to the batholiths of the Nevadan orogeny and 

 entirely dissimilar to the plutons of the Laramide belts. 



Having intruded the Permian volcanic sequence along its western 

 margin, it lies partly in the Pacific eugeosynclinal province. Its eastern 

 part intrudes miogeosynclinal sediments of the Rocky Mountan type. 

 It is strikingly discordant with the Laramide structures locally, but over- 

 all a fairly clear concordance prevails. This and extensive Paleocene con- 

 glomerates to the east, derived, presumably, from the roof rock of the 

 batholith, are the best evidence for a Cretaceous age. Isotope age deter- 

 minations indicate a Mid-Cretaceous date for the main and early com- 

 ponents of the great granitic mass. After cooling it formed a buttress 

 against which the Laramide folds and thrusts developed. Still later, 

 younger intrusions cut discordantly through older plutons and the 

 Laramide structures. 



