The planes of this modification replace the lateral solid angles of the primitive crystal, in the same manner as those of the second and ninth, but are still more inclined on the axis passing through those angles, fig. 155. Pl. 22. By fig. 156. the planes of this modification are shewn in a more advanced state, and by fig. 157. in combination with the secondary pyramid. Although the planes are of considerable size on many crystals, they are generally rough, or so much rounded, as hitherto to have prevented my obtaining a satisfactory admeasurement of their incidence on the planes of any other modification.
The crystals represented by figs. 158. and 159. were I believe found loose in a vein near the Land's End. Those of figs. 160. and 162. are from Huel Fanny. That of fig. 166. from Gunnis lake mine: it is about three quarters of an inch in length, and is perfect at both terminations. The crystals delineated by figs. 163, 164, 167, 168, 169, 170, and 171, are from Relistian mine. On the crystals, fig. 167. the planes of the first, sixth, and tenth modifications were evidently the consequences of a second deposition, as their natural joints with those of the fourth and ninth modifications are visible on every side.
This modification consists in a decrease along the edges of the two pyramids of the primitive crystal, by which each is replaced by a plane; fig. 172. Pl. 23. This plane, by a deeper replacement of those edges, would produce, it will be evident, another and more obtuse octahedron, fig. 173, the apices of which are visible in com-