The northern end of the Big Belt Range of west-central Montana is underlain chiefly by upper Cretaceous potash-rich basalts which were extruded unconformably on Cretaceous sediments and folded with them into northwesterly trending anticlines and synclines. The western border of the igneous rocks is marked by an imbricate zone of Mississippian to upper Cretaceous rocks. The Lewis overthrust, carrying the Belt series over the upper Cretaceous strata, bounds the fault zone on the west.
The trachybasaltic breccias, agglomerates, and flows, 3200 feet thick, are intruded by many chonoliths, sills, and dikes ranging from gabbro to quartz monzonite. Some of the trachybasalts have abundant primary analcime phenocrysts. All evidence favors crystal settling as the cause for the magmatic differentiation within this petrographic province.