ORDER VII. KHOMBOHEDIIAL QUARTZ. 327 



colours of these has given occasion to the distinction of 

 common Caicedony and of Cornelian, the former of which 

 comprehends greyish colours, or in general such as do not 

 possess bright tints of colours, while the latter refers to red 

 varieties. Common Carnelian moreover occurs in globular 

 and irregular tuberose shapes ; fibrous Carnelian is found 

 in reniform masses, and generally shews very distinctly the 

 above mentioned composition. The rhombohedron-like 

 crystals of a smalt blue colour from Transylvania, are also 

 enumerated among the varieties of common Caicedony, 

 probably because there exist reniform varieties of Caice- 

 dony possessing the same colour, though they are more 

 nearly related to common Quartz. Common Quartz also 

 occurs in massive varieties, shewing columnar composition. 

 If these be thin, parallel, strongly coherent, and more or 

 less bent, Fibrous Quartz a particular species is formed, and 

 Cafs eye, another species of those systems, if they are 

 nearly impalpable, and almost solely to be observed in the 

 opalescent light, which they exhibit when cut with a con- 

 vex surface. Cat's eye is generally greenish-grey, but 

 there are varieties of various yellowish, red, and brown co- 

 lours, all of them inclining to grey, and sometimes even 

 nearly black. It possesses a small conchoidal fracture, and 

 is more or less translucent. If several of the preceding va- 

 rieties are distinctly coloured by some foreign mineral sub- 

 stance, or intimately mixed with it, various other pretend- 

 ed species are formed. Chrysoprase is a variety of common 

 Quartz, consisting of small granular particles of composi- 

 tion, coloured apple-green by oxide of nickel ; Plasma is a 

 variety of Caicedony, coloured leek-green, and almost grass- 

 green, by some substance, which is not exactly ascertained. 

 Heliotrope, likewise a variety of Caicedony, but mixed and 

 coloured by green-earth, containing blood-red spots of Jas- 

 per. The brownish-red colour of the commonly so called 

 Hyacinth from Compostella is produced by an admixture 

 of oxide of iron. If the same thing takes place in com- 

 pound varieties, the individuals of which are still recogniz- 



