1. On the Optical Phenomena and Crystallization of Tourmaline, Titanium, and Quartz, within Mica, Amethyst, and Topaz

Extract

1. The first part was on the distribution of Tourmaline in Mica.—When liquids or gases have been confined in mica, they have often easily escaped and spread between the plates. Crystals, both of tourmaline and quartz, are found in mica, contemporaneous with it, and of considerable size. Such crystals generally have the faces of the hexagonal prism parallel to the laminæ of the mica.

But other crystals of tourmaline formed subsequently, and between the laminæ, are very different. They are hexagonal plates, with faces perpendicular to the axis of the prism. Some of the fluid which deposited them has penetrated between the laminæ, and there deposited hexagonal plates, often in circular groups round the cavity.