Precambrian igneous rocks in the St. Francois Mountains of southeastern Missouri consist of silicic volcanic rocks, mostly ash flow tuff, which crop out mostly in the western and southwestern part of the region, and epizonal granitic plutons which are mostly exposed in the northeastern part of the area. These relations suggest that the exposures in the northeast are of a somewhat deeper part of the crust and that rocks there are now exposed because of west, southwesterly tilting and beveling by erosion. In the eastern St. Francois Mountains the occurrence of a thick rhyolitic ash flow tuff, Grassy Mountain ignimbrite, and its distribution relative to a large, high-silica pluton, Butler Hill granite, suggest that the ash flow is the principal eruptive material of a caldera, the Butler Hill caldera, and that Butler Hill granite is the frozen magma chamber that was originally beneath it. Structural relations indicate that Butler Hill granite was emplaced into the volcanic rocks of the caldera during resurgent doming. Four small low-silica plutons, Silvermine, Stone, Knoblick, and Slabtown granites, are distributed peripherally to the Butler Hill granite and are interpreted as ring pluton emplaced along the ring fracture of the caldra or along arcuate intracaldera faults, after resurgent doming and emplacement of Butler Hill granite. More than 3 km of silicic volcanic rocks occupy a basinal structure in the western St. Francois Mountains. These rocks lie with angular unconformity upon up domed older rocks of the Butler Hill Caldera along an arcuate zone of megabreccia and disturbed structure on the western margin of the older caldera. This feature is interpreted as the topographic wall of a second caldera which lies to the west. The occurrence of the thick Taum Sauk rhyolite stratigraphically about 3 km higher than the rocks involved in the megabreccias may indicate that there was yet another caldera formed in this western part of the St. Francois Mountains.