Describes intraformational clastic dikes in Stephanian (Carboniferous) terrain of the Saint-Etienne basin and in Cretaceous terrain of the Fuveau basin, France, discusses the mechanics of formation of clastic dikes in general, and concludes that the intraformational dikes described were formed contemporaneously with sedimentation of the enclosing beds by the filling of fractures caused by the decrease in volume of the underlying coal bed.