ORDER vii. RHOMBOHEDIiAL QUARTZ 829 



greatest frequency in nature. Those of common Quartz 

 enter into the regular mixture of various rocks, of 

 granite, gneiss, mica slate, topaz rock, &c. In others, 

 they occur in single crystals, and in grains, as, for instance, 

 in porphyry, and are frequently met with in vesicular ca- 

 vities, particularly of amygduloidal rocks. Here also are 

 found the finest varieties of Calcedony, Carnelian, of the 

 brown, and probably also the red Egyptian Jasper, the 

 agate bails, &c. Hornstone frequently forms globules in 

 compact limestone, and Flint globular and tuberose concre- 

 tions in chalk, often disposed in beds, and including petrifac- 

 tions. Many varieties occur in irregular nodules and large 

 massive concretions in various rocks. Thus, common 

 Quartz occurs in all those rocks, of which it forms besides 

 a regular ingredient ; hornstone and chrysoprase in ser- 

 pentine, fibrous Quartz and Cat's eye probably in some 

 shistose rock. Sometimes these masses are open in their 

 interior, and lined with crystals, and this seems to be the 

 case in the crystal vaults of the Alps, from whence the 

 largest and most transparent Rock-crystals have been 

 brought. Rhombohedral Quartz also forms beds by itself 

 of which we have examples in quartz-rock, and even in 

 the different kinds of sandstone ; they may be considered 

 either as produced by crystalline formation, or as conglo- 

 merated pebbles. Striped Jasper and Flinty slate form 

 particular beds ; in other beds, as those of Iron-ore, Iron- 

 pyrites, it likewise occurs, generally the varieties called 

 common Quartz, but also Prase, Hornstone, and Calce- 

 dony. It is very frequent in all kinds of veins. In these 

 localities are found Amethyst, several varieties of Rock- 

 crystals, Hornstone, Calcedony, particularly the blue one, 

 but chiefly common Quartz, constituting the greater part, 

 and sometimes the whole body of the vein. The agate 

 veins are among these ; they consist of different kinds of 

 rhombohedral Quartz, particularly Calcedony, and of un- 

 cleavable Quartz, alternating in various stripes with each 

 other. Rock-crystal, Amethyst, Flinty slate, but parti- 



