The minerals in acid-insoluble residues, 2 microns and less in diameter, from 27 limestones and dolomites, ranging in age from Cambrian to Recent, both marine and non-marine in origin were determined qualitatively. Quartz is widespread and abundant. Illite is the dominant clay mineral in both marine and non-marine carbonate rocks. Kaolinite is most likely to occur in Mississippian and Pennsylvanian limestones, and may be more common in non-marine than in marine rocks. Montmorillonite was found to be the prominent clay mineral in the Bethany Falls (Pennsylvanian) and in one bed of the Morrison limestones.