Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/128



Fourth Modifications.



This modification, like the preceding, consists in a decrease on each solid angle of the primitive prism by an isosceles triangular plane, but which, instead of being placed, as in that modification, more on the lateral edges than on the terminal faces, inclines more on the terminal faces than on the lateral edges. On the crystal represented by fig. 42, the planes of this modification are perfectly brilliant.

Fifth Modification.

By this modification the terminal edges of the primitive prism are replaced by trapezoidal planes tending to form an octahedron, fig. 46. The succeeding figure shews the planes of this, in combination with those of the second modification, or the acute octahedron. The crystals described by figs. 46 and 47 are numerous, brilliant, and well defined: they rarely exhibit any lateral striæ, but are so minute as to render it impossible even to approximate the real admeasurement of the angles formed by the meeting of any two of their planes. An attentive examination induces the belief of their being somewhat more obtuse than the regular octahedron; and they are so delineated.