Several large calcareous concretions in Midway (Paleocene) claystone at San Antonio, Texas, are encrusted with a layer of gypsum 1.3 inches thick. It is proposed that this is secondary gypsum produced by the reaction between sulphuric acid and calcium carbonate. The sulphuric acid is a weathering product of iron sulfides present in the unweathered rock. The interface between the impermeable concretion and the surrounding impermeable claystone forms a zone of permeability along which the reacting solutions are trapped. It is proposed that, where exposed to the air, evaporation built up the concentration in solution of the reaction product (calcium sulphate) until precipitation of gypsum took place.