Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/127



Second Modification.

This modification consists in a decrease on the terminal edges of the primitive prism, by which each is replaced by a trapezoidal plane, inclining more on the lateral than on the terminal faces; and, as will be obvious on consulting the planes of this modifications numbered 2.2, on figs. 23 and 24, tending to produce a very acute octohedron. On the crystals described by figs. 13, 14, 15, and 16, the lateral primitive planes have wholly disappeared The lines on fig. 9 shew the striæ, which, on the larger crystals, are not only visible, but deep.

Third Modification.



The third modification consists in a decrease on the solid angles of the primitive prism, by which each is replaced by an isosceles triangular plane, inclining more on the lateral edge than on the terminal face, fig. 17. The length of the crystals delineated by figs. 23 and 24, which shew the approach to the acute octahedron, would, if the apices of the crystals had been complete, have equalled at least four times their breadth; they are deeply striated in the direction shewn on fig. 9. I possess a crystal delineated by fig. 24, on which the planes 2.2, are perfectly brilliant and well defined. Fig. 31 shews the combination of the planes of this with those of the preceding modifications and of the primitive prism. On the crystals described by the four last figures in the series of this modification, two of the four planes of the second modification have disappeared on each pyramid, giving to their common base and to their terminal faces the form of triangular planes.