Precambrian rocks of the St. Francois Mountains consist mostly of silicic ash-flow tuff and intrusive granite. These rocks were formed during two igneous episodes occurring 1415 m.y. ago and 1320 m.y. ago, as determined by whole-rock Rb-Sr measurements. A major volcanic subsidence structure, the Taum Sauk Caldera, displaces rocks of the older group but is intruded along ring fractures by a granite of the younger group. These rocks may be the exposed part of a much more extensive Precambrian volcanic terrane, which is now buried under younger sedimentary rocks.