On curved laminae,graded layers,convection currents and dynamic crystal sorting in the Ploumanac'h (brittany) subalkaline granite

Curved, layered objects in a subalkaline, potassic, intrusive granite are analyzed: biotite laminae and cyclic reverse size and modal layers are encountered in an annular zone; both cyclic and intermittent layers are observed in rhythmic arrangement. Truncation, the bifurcation curvature of layers with ladle shape, represents the action of magmatic currents, as would convection cells or fluid-dynamic fingers. Emphasis is put on the Bagnold effect related to such currents, which would explain the reverse size-grading of layers; this kind of size-sorting (hydrodynamic) occurs in highly viscous granitic magma. Simple convection seems more likely than diffusive convection.