Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/258

 if we would methodically seek to obtain the nucleus. By following this plan, I have occasionally succeeded by the assistance of the pincers, or by sharply striking a piece of steel long enough to extend across the surface, with its edge placed on the quartz in the direction of its laminæ. That neither of these plans often succeeds, and I know of none more effectual, is to be attributed to the great brittleness of the substance, which renders it liable, even when struck in the direction of its natural joints, to present fragments wholly irregular, or in various degrees approaching the conchoidal form. Quartz may however, though with still greater difficulty, be split in two or three directions which are not parallel with the planes of the primitive rhomboid.

Zircone.

Fig. 1.

Several substances, not essentially differing in composition or in their crystalline form, are by Haüy arranged under the general term zircone. Their primitive crystal is described in the Tableau Comparatif as an obtuse octahedron with square bases admitting of regular fracture parallel with sections passing through the apices, and through the centers of the edges D. D. The jargoon of Ceylon does not admit of being split with the same ease as the hyacinth of France, of which I have obtained and possess regular cleavages in the directions mentioned by Haüy, and also parallel with sections that would divide the octahedron into four parts by passing along all the edges of both pyramids.

The fractures in the direction of the primitive planes were most difficultly obtained, and though numerous, are not sufficiently