QUARTZ: ITS VARIETIES AND FORMATION. 181 



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 as- 

 sume 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 be- 

 sides 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 concre- 

 tions in limestone rocks ; sometimes, however, as bands of consider- 

 able thickness. The black color so common to the flints of the chalk 

 formation and to the chert nodules and bands in the mountain lime- 

 stone 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. Carne- 

 lians 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 irregu- 

 lar, and combined with other forms of quartz, such as amethyst, jas- 

 per, etc. This latter name is applied to an aluminous variety of 

 quartz : it is opaque, and has a less crystalline appearance than ordi- 



