Ichthyopthalmite

In the improved state of Mineralogy since this work began; what was generally considered as Zeolite has been divided into many species ; of one of them, the Ichthyopthalmite, or Fishes-eye-stone, only one variety was for Some time known, and considered as exclusively foreign; other varieties having been since found, it is discovered that some minerals hitherto thought to belong to the other Zeolites, belong to it: among them are two or three British ones; that I have figured as Stilbite from Stroutian, (tab. 258,) is one of them. The upper figure in the present plate shews the primitive crystal upon carbonate of Strontian with carbonate of Lime; it is from Stroutian, and serves to illustrate the former plate (258;) and to make it clearer, I have added an outline of the crystal upon that plate: the rounded sides there mentioned are composed of one primitive and two secondary faces confounded together by striæ. The lower figure exhibits crystals of an entirely different form; in them all the edges of the primitive are replaced,—the vertical ones by wide faces; the terminal planes are also considerably larger than the sides, so that the crystals have the general form of acutely rhomboidal tables, such as often accompany the fine foreign specimens of prismatic Stilbite, and then shew such beautiful white pearly surfaces and translucent edges. The colour of these crystals is remarkable, and nearly the same as that of the red Stilbite, figured at tab. 260, but rather more inclined to purple, by which a few crystals that occur upon that specimen may by a careful examination be distinguished; they are both from the same rock: the few white crystals about this are Stilbite. Stilbite, although often very brilliant, may be distinguished from Ichthyopthalmite by its inferior lustre and transparency.