360 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



theory of separate injections, the syenite magma was injected into a 

 partially solidified shonkinite; but according to the theory of magmatic 

 differentiation in place, the syenite was formed by the settling of heavy 

 minerals out of the shonkinite magma and the rising of leucite crystals. 

 The minor injections of the shonkinite in the syenite at the lower contact 

 of the syenite are explained as due to surges of magma incident to 

 deformation of the magma chamber. 



The Boxelder laccolith of the Bearpaw Mountains is also an instruc- 

 tive example of differentiation in place (Pecora, 1941) and the lower 

 cross section of Fig. 23.6 has been prepared to show the relations. 



STRUCTURES OF THE NORTHERN GREAT PLAINS 



East of the zone of flexures and domes and north of the Black Hills 

 is a long, asymmetrical, gentle fold known both as the Cedar Creek anti- 

 cline and the Baker-Glendive anticline. See Fig. 23.1. Between it and 



the Porcupine dome is the shallow Sheep Mountain syncline. All are 

 Laramide structures. They are so gentle, however, that they hardly 

 deserve inclusion in any belt of Laramide orogeny. The very low Bow- 

 doin dome northeast of the Bearpaw Mountains is in the same class. 

 The Cedar Creek anticline has produced commercial gas from the Upper 

 Cretaceous strata in several local domes along it, and deep wells have 

 shown the presence of the Lower and Upper Mississippian strata there, 

 and consequently the extension eastward of the Big Snowy trough (De 

 Wolf and West, 1939). One reached the Precambrian at a depth of 

 9680 feet, having passed through 3920 feet of Upper Cretaceous strata, 

 220 feet of Lower Cretaceous, 1450 feet of Jurassic and Triassic, and 

 4090 feet of Poleozoic (Seager, 1942). Oil was found in a local dome, 

 the Pine field, on the anticline in 1952 in Ordovician and Silurian strata. 

 Several other small anticlines and domes in the setting of the major 

 structures previously described, have been drilled and produce oil. The 

 Charles evaporite sequence is a prominent productive zone. 





