Crystalloblastic anhydrite, present in some dolomitic parts of the St. Louis limestone (Mississippian, Meramecian) in southwestern Indiana, is exemplified by two different textural associations: (1) gneissic porphyroblastic anhydride in extensive lenticular beds, and (2) a later stage of porphyroblasts and veins formed penecontemporaneously with dolomite rhombs. Magnesium sulfate solutions acting upon calcium carbonate may have brought about the anhydride-dolomite association. Magnesium ions, necessary to form the sulfate solution, may have been released from the gypsum lattice upon conversion to anhydride or from some other sources. Textural characteristics of the gypsum indicate that it was formed by hydration from recrystallized anhydride. Although primary gypsum could not be recognized as such, much of the calcium sulfate was probably originally precipitated as gypsum. For gypsification to take place, space must be available for water in addition to space occupied by anhydride; this total volume (anhydride plus water) is greater than the amount of space occupied by the resultant gypsum. Distortion of rocks may follow gypsification of anhydride but is the result of directional forces exerted by crystals within gypsum veins rather than total volume expansion. Results of X-ray studies indicate that hemihydrate is a rare mineral constituent of the Indiana deposits.