358 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



EXPLANATION 



BEARPAW SHALE 

 (UPPER CRETACEOUS! 



JUDITH RIVER FORMATION 

 (UPPER CRETACEOUS) 



CLAGGETT SHALE, EAGLE SANDSTONE AND 



COLORADO GROUP 



(UPPER CRETACEOUS) 



KOOTENAI ANO ELLIS FORMATIONS 

 ( LOWER CRETACEOUS AND JURASSIC) 



MISSION CANTON LIMESTONE TO FLATHEAO SANDSTONE 

 (CARBONIFEROUS TO CAMBRIAN) 



METAMORPHIC ROCKS (PRECAMBRIAN) 

 AND ALKALIC IGNEOUS ROCKS (TERTIART) 



NORMAL FAULT 

 U, UPTHROWN SIDE 0, DOWNTHROWN SIDE 



Thrust fault 

 t, overthrust side 



Fig. 23.5. Geologic map of Little Rocky Mountains, Montana. Reproduced from Knechtel, 1944. 



An approximate parallelism of the volcanic groups with the major 

 faults and flexures of central Montana has been pointed out by Thom 

 (1923), but it is evident from inspection of the Tectonic Map of the 

 United States that the major faults are clearly not the loci of the mag- 

 matic activity. However, the subparallel alignment and contemporaneity 

 of origin lead Thom to view all the faults, flexures, and igneous rocks as 

 tied to the regional deforming forces of the Laramide orogeny. 



Petrology 



The igneous rocks range from rhyolites to basalts in one category and 

 from shonkinites through nepheline syenites to syenites — rocks that are 

 rich in potash and soda and almost devoid of plagioclase — in another. 

 The rocks of each mountain group fall into one or more eruptive stages; 

 and the rocks of each stage have peculiar mineral and chemical features, 

 although they commonly range from highly mafic to highly felsic. Each 



