PHYSIOGRAPHY. 145 



Quartz. 



ular or curved lamellar masses. Massive : composition 

 granular or columnar, and often impalpable; and then the 

 Fracture becomes conchoidal and splintery. Sometimes a 

 second composition produces indistinctly granular, or thick 

 amellar masses. Certain very thin columnar compositions, 

 f cut en cabochon parallel to the fibres, show an opalescent 

 ight. Pseudomorphic crystals, in the shape of cubes, oc- 

 tahedrons, derived from Fluor, in the shape of rhomboids 

 and hexagonal prisms, derived from Calcareous Spar, and 

 of lenticular forms, from Gypsum. Globular and tuberose 

 masses formed in vesicular cavities. Plates. Pebbles. 



1. There are several modes of occurrence among the crystals of Quartz, 

 hitherto confined to this species, which become evident on an inspec- 

 tion of the forms described above. The scalene triangular faces x and it 

 are the most remarkable in this respect. Their faces appear only to the 

 right, or only to the left of the faces s. Two individuals, differing in re- 

 gard to the right or left situation o( these faces, cannot be brought in any 

 such position, that all their faces become parallel ; and they are different, 

 therefore, like the right hand arid the left. This difference extends even 

 to the action of the individuals on light, as has been shown by Mr. 

 HERSCHEL. 



2. The species Quartz does n<jt by any means abound in varieties of 

 crystallization ; and yet, owing to the disproportionate size of the faces 

 of its forms, its crystals offer considerable diversity of appearance. The 

 chief perplexity among the vaiietiesof Quartz, arises from mechanical 

 composition, and the admixtures of different substances foreign to the 

 species. No less than thirteen different species are distinguished in the 

 Wemerian system, to which those of other systems more or less corres- 

 pond. Quartz contains most of its simple or crystallized varieties, and 

 may be said to represent the species most perfectly. It contains five 

 sub-species ; Jlmethyst, including violet-blue varieties ; Mock Crystal, 

 composed of the most perfectly crystallized, and some transparent, or 

 semi-transparent, massive varieties; Rose Quartz, confined to translu- 

 cent rose-red, and milk white, massive varieties; Prase, which is only 

 of a dark leek-green color ; and Common Quartz t at last, comprehends 

 all those varieties, not included in any of the preceding subspecies, 



VOL. II. 13 



