The petrographic province of Central Montana is arbitrarily defined to include the Tertiary igneous rocks near the eastern flanks of the Rocky Mountains from the Yellowstone National Park on the south to the Canadian boundary on the north (Fig. 1).
Each mountain range is made up of one or more groups or subprovinces of igneous rock. The Yellowstone Park area contains at least three subprovinces, the Highwood Mountains four, and the Bearpaw Mountains about seven. The rocks of a subprovince may vary from mafic to felsic, but they are all related in mineral composition, and their chemical analyses fall near smooth variation curves. The mafic rocks of any subprovince differ from those of other subprovinces more than do the felsic rocks. The latter tend toward a granite of about the same composition for all the subprovinces. The mafic rocks of the various subprovinces range from ordinary basalts or gabbros to orthoclase gabbro, to plagioclase shonkinite, to shonkinite without plagioclase. In general, if the mafic rocks of a subprovince are rich in plagioclase all the rocks of that subprovince are rich in plagioclase whereas if the mafic rocks are poor or lacking in plagioclase all the rocks of that subprovince are poor or lacking in plagioclase.
The parent magmas of the various subprovinces are believed to be represented by the mafic rocks. The gradational character of these from gabbro to shonkinite leads to the conclusion that they have a common origin. They are believed to have been formed by the slow differentiation at great depth of a basaltic magma by the separation of plagioclase and hypersthene with some olivine.
From time to time parts of this changing magma were erupted toward the surface where they further differentiated to form the rocks we see. This latter differentiation was relatively rapid and was by crystal fractionation, modified in all the magmas except the shonkinitic magma by assimilation of granitic material.