Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/364

354 diagonal, shewn by the dotted lines c f and g b, and along the edges f g and e b, it will be divided into four parts, one of which will be represented by fig. 8, which is a right-angled triangular prism with inclined terminal faces: several of these are in my possession.

The fractures represented by figs. 5, 7, 8, prove the mechanical division of the crystals of this substance, in the direction of both diagonals; and what has before been said of that in a direction parallel with the faces of the prism, would suffice without further proof. If however evidence were wanting, the cleavage described by fig. 6, decides its practicability beyond a doubt. Having placed in the fire a macle represented by the dotted lines of that figure, and of about the same size, I afterwards obtained from it a nucleus similar to the fig. a b c d, represented within it, and of about the same size, with faces well defined and very brilliant; it is now in my collection. This nucleus, it will be seen, is of the same form as that of the macle described by fig. 208, Pl. 25, and resulted from a cleavage of fig. 6, (which is of the same form as fig. 209) in a direction parallel with each of its six larger faces; and, as hereafter will be shewn in describing the formation of those macles, consequently parallel with the faces of the prism.

Among the fragments obtained from crystals that had been placed in the fire, I found some quadrangular prisms having one terminal face similar to that of the upper one of fig. 4, but with indications of the lower terminal face in the opposed direction as represented by fig. 9.

On applying the goniometer to the face P and along the edge b c of fig. 9, I was somewhat surprised at finding that there is no perceptible difference between their incidence on each other and that of the plane P, and along the edge b c of a crystal similar to fig. 11;