  Volterra's definition of dislocations in crystals distinguishes edge and
screw defects geometrically, according to whether the Burgers vector is
perpendicular or parallel to the defect. Here, we demonstrate a distinction
between screw and edge dislocations that enables a unified, purely topological
means of classification. Our construction relies on the construction of real or
virtual disclination-line pairs at the core of the dislocation in a smectic and
can be generalized to crystals with triply-periodic order. The connection
between topology and geometry is exploited.
