Siliceous pseudomorphs of gypsum. (a) Siliceous moulds of gypsum showing that (1) gypsum crystals were cast by silica deposits, then (2) embedded in a silicified mass (gogotte) before residual gypsum was dissolved. Sample from altered/dissolved gypsum layers in the vicinity of outcrop in ancient gypsum quarries of the Plateau d’Avron, Neuilly-Plaisance (Seine-Saint-Denis). (b) Gogotte-like siliceous concretions with mamillary protrusions; (1) silex nectique (‘float-stone’) in gypsum deposit, (2) silex melinite (brown flint) interspersed in magnesium-rich clays, with geotropism indicated by overlapping concretionary growths. Extracted from Cayeux (1929).