Probably everyone who has considered the arrangement of the constituents of an aggregate composed of radially grouped fibres has realised that the individual fibres are conical or pyramidal in form, their apices meeting in the centre of the aggregate. In a coarse example of this mode of erystallisation, such as the calcite concretions in the magnesian limestone of Durham, this conical form of the constituent crystals is very easily seen; the cones cleave across, where composed of a cleavable mineral, showing that the particles composing them have the same orientation within the limits of the cone, and that each cone, in fact, is a crystal, bounded ultimately by contact with its neighbours.