Issues

COVER EXPLANATION.—Cathodoluminescent image of thin section through calcite cement crystals, from cavity in Permian patch reef, Aberford, Leeds, Yorkshire, U.K. Calcite shows intrinsic blue luminescence (manganese-free) with hairline orange and red luminescent zones (manganese bearing}; pore on left is non-luminescent. Change in cement crystal habit can be reconstructed from its luminescent zones; crystal core formed from equant rhombohedron {10¯14}, middle section combines equant rhombohedron with scalenohedral {21¯34} sectors, and the youngest cement formed from acute rhombohedron {10¯11}(Miller-Bravais hkil indexing). See Dickson, this issue.