ORDER VII. RHOMBOHEDRAL QUARTZ. 325 



Globular and tuberose masses formed in vesicular 

 cavities. Plates. Pebbles. 



OBSERVATIONS. 



1. There are several modes of occurrence among the crystals 

 of rhombohedral Quartz, hitherto entirely confined to this 

 species, which becomes evident on an inspection of the 

 combinations quoted above, and depends chiefly upon their 

 hemi-rhombohedral and hemi-di-rhombohedral character. 

 The scalene six-sided pyramids are the most remarkable 

 forms in this respect. Their faces appear only to the right, 

 or only to the left of the faces of II. Two individuals, 

 differing in regard to the right or left situation of these 

 faces, cannot be brought in any such position that all their 

 faces become parallel, and they are different, therefore, like 

 the right hand and the left. This difference extends even 

 to the action of the individuals on light, as has been first 

 shewn by Mr HEBSCHEL. Dr BREWSTER found that 

 Amethyst, particularly that from Brazil, of various co- 

 lours, consists of thin films, shewing an opposite action upon 

 light. These films belong to individuals in a parallel posi- 

 tion, but differing from each other as to right and left. 



2. The species of rhombohedral Quartz is by no means 

 one of those which contain many different simple forms, 

 and various combinations depending upon them ; and yet 

 there are none in the whole order Gem which occur in so 

 many varieties, bearing at first sight so slight a degree of 

 resemblance to each other. This, however, depends upon 

 mechanical composition and the admixtures of different 

 substances foreign to the species. No less than thirteen 

 different species are distinguished in the Wernerian sys- 

 tem, to which those of other systems more or less corre- 

 spond. Quartz contains most of its simple or crystallised 

 varieties, and may be said to represent the species most per- 

 fectly. It contains five sub-species, Amethyst, including vio- 

 let-blue varieties ; Rock-crystal y composed of the most per- 

 fectly crystallised, and some transparent or semi-transparent 



