The crystals are of a similar length, though often considerably thicker than those described by Prof. Maskelyne (Phil. Mag. Jan. 1868), and his general description applies to those under consideration.

They are, however, ill-adapted for measurement, as single perfect crystals are rare. As a rule several crystals are longitudinally attached to one another, with slight radial divergence, in such a manner that it becomes difficult or impossible to discriminate between the reflections of their minute faces.