Hydration of anhydrite to form gypsum should result in an increase in volume if all of the calcium sulphate is retained within the system. In many secondary gypsum rocks former anhydrite is replaced on a volume for volume basis, and the additional volume of gypsum appears to be present as veins which cut the associated rocks. It is suggested that in such instances the hydration was caused by water that moved up under pressure from underlying formations due to relief of overburden load. The water made entry into the anhydrite-bearing rocks by hydraulic fracture, and the gypsum of the veins grew in the water-filled fractures while the overburden was supported on the water. It is possible that the fibrous structure which characterizes many gypsum veins is an induced feature, produced when the residual water escaped from the fractures and the overburden load was transferred to the gypsum of the veins.