CENTRAL MONTANA ROCKIES 



359 



stage is separated from the other by intervals during which few or no 

 eruptions occurred, but instead, extensive erosion. Chapter 33 deals 

 with the origin of the igneous rocks in this province and should be re- 

 ferred to for a discussion of the igneous and tectonic provinces of the 

 western United States. 



In each of the stages a rock near the mafic end is believed to repre- 

 sent the primary magma. This rock ranges from an ordinary basalt to 

 orthoclase basalt to plagioclase shonkinite to shonkinite rich in potash 

 and lacking plagioclase. The gradational character of the eruptive stages 

 and their close association in time and space indicate a common origin 

 (Larsen, 1940). Two periods of magmatic differentiation are required: 

 first, a deep-seated differentiation to yield the primary magmas of the 

 individual eruptive stages, and second, a shallower differentiation of the 

 primary magmas which were probably derived from a basaltic magma by 

 the removal of crystals of calcic plagioclase and hypersthene in depth. 

 | The relative flatness of the sedimentary rocks into which and through 

 I which the magmas have moved indicates that the magmas have not been 

 i disturbed by orogenic forces; therefore they could have differentiated 

 I j during the long, quiet interval which seems necessary. The second period 

 of magmatic differentiation by crystal settling was characterized, in most 

 stages, by assimilation of siliceous material. The amount of assimilated 

 material was especially large in the Crazy and Little Relt Mountains 

 where syenites were followed by granites. 



The Shonkin Sag laccolith, one of nine in the Highwood Mountains, is 

 worth special mention. It has long been held as a classic example of 

 magmatic differentiation in place, but the theory has been questioned 

 and one of multiple intrusions proposed (Rarksdale, 1937). More re- 

 cently, Hurlbut and Griggs ( 1939 ) contend that the first theory has the 

 greatest merit. In describing the laccoliths of the Highwood Mountains 

 they point out, first, that they are broad, sill-like bodies and not the 

 domed-shaped ones that Gilbert ( 1877 ) pictured in the Henry Mountains 

 of Utah, and second, that the peripheral contacts are not simple wedges 

 of intrusive rock, but a complex of multiple sills, crumpled strata, and 

 small normal and reverse faults. Examine Fig. 23.6. 



1000 200O FT. 



Sy 



Sh __, c _ K^x. 



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-r 





















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of Sandstone • • 













a : r 



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 »rt.; 





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ion 



no, 

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So 



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ble Shonk,nit«)( 

 of Sandstone 



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Fig. 23.6. Upper section: the Shonkin Sag laccolith, K is Cretaceous sandstone, Sh is shonkinite, 

 Tr is transition rock, and Sy is syenite. After Hurlbut, 1939. 



Middle section: detail of eastern termination of Shonkin Sag laccolith, K, Cretaceous strata, 

 Sh is shonkinite, Sy is syenite, Ph is phonolite, Nos. 1 to 5 are sills of shonkinite porphyry. After 

 Hurlbut, 1939. 



Lower section: diagrammatic section of Boxelder laccolith. After Pecora, 1941. 



The main body of the Shonkin Sag laccolith is made up of three hori- 

 zontal layers, an upper one and a lower one of shonkinite, and an inter- 

 mediate one of syenite. This is true of all the laccoliths in the group; the 

 larger the pluton, the greater the amount of syenite. According to the 



