Page:Popular Science Monthly Volume 23.djvu/193

Rh fractured, are seen to have a peculiar undulated structure, which Sir D. Brewster pointed out, have been classed together as amethysts, a name often popularly restricted to the violet crystals, which owe their beautiful tint to the presence of oxide of manganese. Violet amethysts are not uncommon in the geodes occurring in volcanic rocks in many localities; but the finest are obtained from Siberia, Persia, India, and Ceylon; while Brazil yields white and yellow amethysts. The yellow and brown crystals known as cairngorms are varieties of rock-crystal or of crystallized quartz, if we restrict the term rock crystal to the clear, colorless specimens. The darker brown and black crystals, as well as those designated as cairngorms, may be grouped under the common name of smoky quartz. The dark-green quartz is called prase, and is colored by amphibole; there is also a lighter green species known as chrysoprase, tinted, it is said, by oxide of nickel; while oxide of iron probably gives color to the numerous red varieties. The common milk-white quartz, which is the ordinary quartz of veins and of quartz-rock, will be found, on microscopical examination, to be really transparent, but so full of minute cavities as to cause it to assume its milky opacity.

Quartz-rock, or massive quartz, is often found in mountainous masses, hundreds of feet in thickness. Many of the quartz schists and micaceous schists consist chiefly of quartz irregularly split up by thin leaflets of mica. Sandstone rocks, often consisting of little besides more or less rolled grains of quartz, will have been derived from the breaking up, under various denuding agencies, of rocks in which quartz has been the prevailing mineral. Veins of quartz have already been mentioned. These are very frequent in the old slate and schist rocks, sometimes forming broad and irregular bands; at others, mere threads traversing the other materials. Such veins will often present open spaces in which the quartz will be found regularly crystallized.

Flint and chert are forms of quartz usually occurring as concretions in limestone rocks; sometimes, however, as bands of considerable thickness. The black color so common to the flints of the chalk formation and to the chert nodules and bands in the mountain limestone is due to the presence of carbon. Hornstone is merely a variety of chert.

Chalcedony has been described as a mixture of crystalline and amorphous quartz; its tendency is to assume a botryoidal or stalactitic form; and its numerous variations of color and modes of occurrence have led to the adoption of different distinguishing names. Carnelians and sardes are only color distinctions of chalcedony; and the immense family of agates, including the onyx and sardonyx, is more or less composed of chalcedony, disposed in layers, regular or irregular, and combined with other forms of quartz, such as amethyst, jasper, etc. This latter name is applied to an aluminous variety of quartz: it is opaque, and has a less crystalline appearance than