ABSTRACTThe commercial underclays of Indiana are used extensively in the manufacture of heavy clay products in that State; the use of native underclays for making pottery and refractories has been comparatively limited. Field and laboratory data relating to those underclays now being utilized by the clay plants of the State are summarized. The data, for the most part, are based upon field observations and laboratory tests made by the writer. The underclays are discussed with reference to their stratigraphical occurrences, physical characteristics in the outcrop, and physical properties as determined in the laboratory by chemical analyses and fire tests. The data selected for each underclay horizon are representative of the average quality of the clay used commercially from that horizon. Brief mention is made also of the qualities of underclays in horizons other than those being utilized a t the present time.