Summary: 1.Microscopic examination of rock-gypsum reveals a wide range of structure, and many metamorphic types are represented. In the nodular masses there seems to be a dependence of grain and structure on the size of the mass. 2.Anhydrite appears as cores only in very large nodules, and would appear to originate under physical conditions established when concretionary growth reached a certain stage. In the main seam anhydrite is distributed without regard to the size and form of the bed, and in lamellae alternating with gypsum. 3.Microscopic evidence supports the view that the main seam chiefly originated by sedimentary deposition, possibly modified by some segregation during deposition; and that the nodular types are concretionary in origin. 4.The metamorphic characters are secondary effects of pressure originating in the partial or complete hydration of the anhydrite.