Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/91

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1. Primitive Crystal. A rhomboidal tetrahedral prism with rhombic bases, the sides of the prism meet at angles of 92° 30′ and 87° 30′; the bases are inclined upon the edges of 92° 30′, so as to form with them angles of 55° and of 125°, fig. 1. The height of the prism is to the edges of the terminal faces, in the ratio of eight to seven. This prism divides in a direction parallel to all its planes, but much more easily longitudinally than on its terminal surfaces; this division takes place also with greater facility on two of the opposite sides than on the two others. It is likewise more readily effected, but at the same time less neatly, when the substance has undergone alteration.

2. Integrant Molecule. This slightly rhomboidal tetrahedral prism is besides divisible parallel to its axis, and to the greater diagonal of its rhombic terminal planes. I have not perceived any natural joints in the direction of the other diagonal. This second division shews that the integrand molecule of the laumonite is a trihedral prism, the exact half of the rhomboidal tetrahedral prism, fig. 2.

3. Fracture. Lamellar.



1. Specific gravity. Taken with a piece slightly changed, but still preserving in some degree its transparency 22,34. The variety