Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/396

 of the individuals that compose the species are afterwards found to be destitute.

Thus for instance M. Cordier, in a paper in which he has described with great perspicuity and accuracy a substance of a violet blue colour, which is met with either at Cape de Gat in Spain, among volcanic products, or near Nijar, also in Spain, in a granitic rock, gives this substance the name of Dichroïte, a word of Greek etymology implying double colour, because its crystals present a very deep blue when viewed in a direction parallel to their axis, while they appear of a brownish-yellow, when viewed in a direction perpendicular to this axis. But mica exhibits precisely the same phenomenon of refraction. I have a variety from Somma in very fine short hexahedral prisms, which, if viewed perpendicularly to their axis, are of a green, more or less deep in proportion to their thickness ; but when viewed parallel to that axis, through their sides, are of a very deep reddish orange yellow. A specimen in my collection, likewise from Somma, and perhaps unique for the beauty of the very bright, slightly greenish-yellow topazes it includes, contains small crystals of mica, in incomplete acute hexahedral pyramids, very transparent, and having the lustre of the hardest stones ; these have a slightly yellowish—red colour, refracted through the sides of the prisms, in consequence of which they are pretty constantly mistaken for very fine garnets. I have also some specimens of that quartz, which comes from Macedonia, and is known by the name of Leuco-sapphire, polished en cabochon, which, seen in one direction, are of a light bluish-grey, or nearly colourless, while in a direction perpendicular to the former, they have the fine blue of the deepest coloured sapphire. Dr. Wollaston, to whom mineralogy is daily under important obligations, has observed tourmaline's likewise possessing the same property. If a person, who has never seen the Dichroïte,