Sixth Modification.
This modification, like the preceding, consists in a decrease on each of the solid angles, caused by the meeting of the two pyramids of the primitive form base to base; by which each is also replaced by two triangular planes placed on the edges formed by the meeting of the two pyramids, but inclining on the axis passing through the angles, more than those of the fifth modification, fig. 58. Pl. 18. The planes of this modification are shewn in combination with those of the primitive form by the dotted lines of fig. 59. and with those of the secondary octahedron by the lines within it.
Seventh Modification.
This modification consists in a decrease on each of the four solid angles caused by the meeting of the two pyramids of the primitive form, by which each is replaced by four triangular planes, placed on the faces of the primitive form, but inclining on the axis passing through the lateral solid angles, fig. 63. Pl. 18.
By fig. 64. the planes of this modification are shewn in a more advanced state, which renders the first figure in the series, that of 66, perfectly intelligible. Fig. 65. shews them in combination with the secondary pyramid, and will, without pursuing the series from fig. 66. to fig. 70. cause the latter figure to be at once understood.
I have not satisfactorily obtained the incidence of the planes of the primitive form with those of this modification.
The principal part of the crystals delineated in the series of this modification, in which the prism is long and the facet of the 7th modification is small, are from Relistian tin mine, and are about a line in thickness. I have not seen them from any other mine.
